PCB Etching Tank, Temperature Control & Bubbly Bubbles

[Jugurtha]

Hello,

We have been asked to present a project involving PIC microcontrollers and sensors.

I have for my part chosen to build an Etching Tank for PCBs with temperature control and fluid circulation. I would have loved to do a spray etching tank with some sort of peristaltic pump, but due the time constraints, I don't think I will. Maybe once I deliver this one, I'll look into the other.

My motivation for this project is to hit two birds with the same rock: It is an assignement and, more importantly, it is something which will hopefully be used by the Lab technicians in the Faculty to develop students' PCBs.

This is a sort of a logbook I'll update as I get this done. I'll write down what I have in mind. Also, I didn't post this in the "Homework" section, since I think this is beyond "just" homework, and I'm willing to construct another copy for myself.

As I say, in order to ask a question, one must know something. I have done my research and have a couple of questions. If you'd like to chime in, that would be nice.

Also I have found similar projects, but none very detailed as to their construction, except maybe for these two:

https://www.instructables.com/id/Printed-Circuit-Board-PCB-Sprayer-Machine/

and

https://www.recontech.co.uk/index.php/the-scieng/diy-pcb-etch-tank/40-diy-pcb-etch-tank-introduction





-Fluid Circulation:

This will be achieved using an aquarium pump. If the fluid is not circulated, the reaction etchant/copper would cause local (partial) saturation which will impede the reaction's speed.

Hence, whether it's manually agitating a vessel, spray etching or using bubbles to circulate the fluid, this would improve the speed and cut the reaction time.


-Temperature Control:

I'll go Bang-Bang (hysteresis) control for the temperature. Etchants have an optimal temperature at which the reaction's speed is maximal.

Here's a link of a graph that shows different speed reactions for different etchants:

**broken link removed**

(The page **broken link removed**)



From the readings I have done, 40°C to 42°C would be a good temperature. The page from which the image has been taken mentions that they didn't monitor above this temperature, as there were fumes. So I'll take that into account.

For the Bang-Bang control, I'll set two thresholds: One slightly below the "optimal temperature" and the other slightly above it. There will obviously be a heating element (Aquarium Heater) used and a Temperature Sensor.

If the temperature is below the lower threshold, the heater is turned ON.
Temperature rises, once it reaches the higher threshold, the heater is turned OFF.
Temperature falls, once it reaches the lower threshold, the heater is turned ON... etc.

I will make some room between the thresholds. By this I mean that it takes the fluid some time to reach a certain temperature, and this changes with respect to the ethant used.

I'm thinking of adding flexibility and add controls to set the temperature, maybe they'll just want to heat water in that tank to a specific temperature.

I'm also thinking of adding some sort of display (maybe LCD to display set point and actual temperature).



-Heating Element (mentioned above): An aquarium heater would do. From my readings, I have found that several "aquarium heaters" come with a physical limitation as to the temperature they could go up to. I've read that you could remove this limitation to make them go above that temperature.

-Sensing Element: Sensor. I'll be using an LM135 since I saw it on a nearby store's website and I liked its characteristics.

I would like the sensor to be immersed and I am looking for a way to do that without harming the sensor, since the environment is aggressive. I found an Instructable to make it waterproof, but it uses a cap made of ... copper, and it's an etching tank for PCBs, so you see how funny is this


Since the output is analog, I'll go with a PIC which has internal ADC (Analog to Digital Converter).


Also, since I wouldn't like the heater to be ON when there's no (or not enough) fluid in the tank; I'm thinking of adding a level sensor and some sort of alarm that would bypass the heater and force it to OFF. Just a security measure. It's low on the priorities list, but if I'm ahead of schedule, I'll think about it.



-Air Pump: Small one, with a small flexible tube with holes in it to make fine bubbles. Fine bubbles are better in my opinion.

-The Tank: It will be Plexiglass, I think. I asked for the price, and someone told me about 45 US Dollars for 1m². It will have some valves to drain the etchant to a container, and a lid to protect from splashes.



So my questions are:


1-How do you (cheaply) make a sensor resist a fairly aggressive/harsh chemical environment?

2-Is there a better, widely available sensor than the LM135, and cheap enough for this particular application and in the same range of accuracy?

3-For the level sensing, what would you advise for such an application (not expensive)

4-Which PIC microcontroller would you go with for this ? Have in mind that I'll need internal ADC, maybe two (if I include sensing level).

EDIT: The PIC16F88 seems appealing to me (Small: 18 pin, it has got ADC. It seems a good replacement for the 84 )




The main point is not to go too expensive for something that isn't really worth expensive stuff ... Feel free to make comments about any question I didn't ask.



I include some research about Etchants here:

************************Etchants:*********************

-Ferric chloride: dark brown, toxic, "disposal problems".(40%). ("Never use steel spoons to take some"). "Dispose of it using Sodium carbonate (or Bicarbonate), the result is a sludge of copper carbonate powder and a solution of sodium chloride (salt). You can pour the solution in the drain and throw the sludge to the garbage.", ETO (Pommie, recommended Ammonium Persulfate. Not so great according to Hans - 2008).

-Sodium persulfate (Recommended).

-Acid cupric chloride, acid copper(II) chloride, cupric chloride: Can be regenerated without throwing away the solution. Widely used in industry. (Recommended)

-Ammonium persulfate.

-Ammonia.

-25-50% Nitric acid.

-Hydrochloric acid and hydrogen peroxide.


-Tom Gootee: 1 part Muriatic acid (hydrochloric acid 28%) to 2 parts Hydrogen peroxide (3%).




*************Influence of temperature**********************

**broken link removed**

Temperature above 42°C would cause too much fumes.



-PCB face down: Copper that reacted will "fall down" and not cause "local saturation".


Parameters:

-Tank capacity.
-Heater wattage.
-Etchant used.
-Ambient temperature.

These four determine how long it takes to reach the optimal temperature. And different temperatures for different etchants, maybe?

An approximation: With Ammonium persulfate in a 1L tank with a 50watts heater, it reaches 130°F

(About 54°C) in 5 to 7 minutes, depending the ambient temperature.

https://www.electro-tech-online.com/attachments/tank-1-jpg.10316/


I may seem to go overhead, but bear in mind that simplicity follows complexity and seldom precedes it (Who said that?)



As always, gentlemen, thank you for your time.

 

[Mr RB]

That sounds like a really fun, useful and exciting project and I wish you well with it (and will enjoy seeing the results).

If I can make some suggestions;
1. make the tank thin and vertical. This gives benefits for double sided etching, and full visibility of both sides while etching. That REALLY helps.
2. you can set the temperature sensor in a hard plastic tube filled with epoxy, this should be etchant proof. Keep the sensor close to the surface (end of the tube) to reduce the thermal time lag.
3. get the sensor close to the heater, directly above it, again to reduce thermal lag.
4. rather than circulate nasty etchant, why not use a LARGE aquarium bubble stone (they are cheap and non-metallic) at the bottom of the tall thin vertical tank. Coupled wiht a large aquarium pump this will make enough bubble activity to mix and circulate etchant and clear waste from the 2 PCB surfaces. You can get long thin bubble stones.
5. With a large tank you will need a decent sized heater especially with a vertical tank.
6. Consider adding a settable timer, with count down time and beepy alarm.
7. Also take into account loading and unloading of the boards in a tank full of nasty hot etchant. Like take a bit of time to design a good PCB holder and maybe some way to lift it out of the etchant, and clip above so the board(s) can drip into the tank for a minute before you move the board to rinse it, or some rinsing tank next to the etch tank as part of the whole system.
8. Ive worked with heated acid and FC tanks in industry, and things around them were always corroded. Spare a thought for some type of sealable lid when the tank is not in use, or even when it is in use. That might be as simple as a top plastic plate with a thin rubber gasket held down sealed by gravity. It might even be part of the PCB holder assembly.

 

[Jugurtha]

That sounds like a really fun, useful and exciting project and I wish you well with it (and will enjoy seeing the results).

Thanks, I'll probably take pictures of it and will document steps, etc...

1. make the tank thin and vertical. This gives benefits for double sided etching, and full visibility of both sides while etching. That REALLY helps.

Exactly, I was thinking of making a 40cm (15.74 inches)high, 30cm (11.81 inches) wide and 3cm (1.181 inches)deep tank. The capacity will thus be of 3.6 litres. I wanted to make it a bit high so that when they fill it with say 2 litres, there would still be some room and it won't be filled to its top. Avoiding problems, etc... And, to make it possible to develop big boards.

2. you can set the temperature sensor in a hard plastic tube filled with epoxy, this should be etchant proof. Keep the sensor close to the surface (end of the tube) to reduce the thermal time lag.

That's very neat, thank you very much.

3. get the sensor close to the heater, directly above it, again to reduce thermal lag.

I was thinking of putting the heater on the side. I don't think I'll go "immersed heater", to avoid problems. But if I find a very thin and a "totally" etchant proof heater, I'll think about putting it in the bottom. Just above the tube from the air pump with holes in it, so that the heated etchant circulates uniformly, and I don't get severe temperature gradient.

4. rather than circulate nasty etchant, why not use a LARGE aquarium bubble stone (they are cheap and non-metallic) at the bottom of the tall thin vertical tank. Coupled wiht a large aquarium pump this will make enough bubble activity to mix and circulate etchant and clear waste from the 2 PCB surfaces. You can get long thin bubble stones.

Yeap, that's what the long tube with holes in it is for. Although if I find cheap and very thin bubble stones, it would be more interesting (esthetically speaking).

5. With a large tank you will need a decent sized heater especially with a vertical tank.

Thanks for the reminder, I think 150 watts would do. If I don't find them, I think I'll go Scorched Earth Approach and put two on both sides. Makes me think of Angelina Jolie in Lara Croft: Tomb Raider. Guns on the hips It might seem overkill, but I think that it is still good.

6. Consider adding a settable timer, with count down time and beepy alarm.

Yeap. Thinking to put that in the LCD.

7. Also take into account loading and unloading of the boards in a tank full of nasty hot etchant. Like take a bit of time to design a good PCB holder and maybe some way to lift it out of the etchant, and clip above so the board(s) can drip into the tank for a minute before you move the board to rinse it, or some rinsing tank next to the etch tank as part of the whole system.

8. Ive worked with heated acid and FC tanks in industry, and things around them were always corroded. Spare a thought for some type of sealable lid when the tank is not in use, or even when it is in use. That might be as simple as a top plastic plate with a thin rubber gasket held down sealed by gravity. It might even be part of the PCB holder assembly.

You're right, I saw that in some videos where they had neat compartment filled with water to rinse the PCB. They used the lid to hold the PCB, so I'm thinking to do something similar. I think of having two etchant immune cables with clips on them dangling from the lid, I would place the PCB by clipping it, and then let it dangle in the etchant as I put the lid (It's attached to the lid). Then once the etching is done, I'll just pick up the lid (the PCB is attached to it) and put it in the rinsing compartment filled with water.

Great remarks, Mr RB.. Thank you very much

 

[JimB]

Nice project, well defined.

I agree with most of Mr RBs suggestions, but would like to comment on this one:

2. you can set the temperature sensor in a hard plastic tube filled with epoxy, this should be etchant proof. Keep the sensor close to the surface (end of the tube) to reduce the thermal time lag.

Plastic and epoxy will have significant thermal resistance which may slow the response and be a problem.
I don't think the temperature will change very quickly in any case, but why make things worse.
My first thought was to use a temperature sensor with a stainless steel body, OK I realise that we are talking bigger money here, but if you can find one cheap, say a PT100 type, then consider using it.

JimB

 

[Jugurtha]

Hey Jim,


I provided a link in my original post (a similar project)

https://www.recontech.co.uk/index.php/the-scieng/diy-pcb-etch-tank/40-diy-pcb-etch-tank-introduction

He's using an external (non immersed) thermometer with display.. Take a look at the probe (which is immersed).

I've tracked down the thermometer, it's Coralife .. But it's just a generic device made in China. You find almost identical designs with different labels, so it's just ordering a different label/design from probably the same factory in China, and marketing it.

My first thought was to use a temperature sensor with a stainless steel body, OK I realise that we are talking bigger money here, but if you can find one cheap, say a PT100 type, then consider using it.

You know what ? Come to think about it, why not get inspired by the PT100. I've used them in my second year chemistry lab experiments, they had temperature display and they're great.

My question might seem naïve to you, but won't the stainless steel be "eaten" by the etchant ?

Probes vendors claim their stainless steel probes resist this kind of environment, but websites talking about PCB etching say that etchants eat any metal (even stainless steel, they say).

I must say that sometimes, I pour some chemical on a fork/spoon/knife (don't ask) and there's a reaction. The spoon is more brilliant, which might be good news if I wanted it to be brilliant. But if I want my probe to be chemically inert in a certain environment, that's not good. It means it will wear out. How long ? I don't know. I don't want to say "probably a long time".


But if it's resistant... I'm thinking why not make one ? However instead of Plastic+Epoxy, it will be Stainless Steel tube+Epoxy. I'll put the sensor in the SS tube and fill it with resin.
On the Thermal-Lag thing:

I see it from two different angles:

1-It can be dealt with in software: By this I mean that I can include this lag in software and adjust the thresholds. If the sensor reads 41°C, then it's probably X°C in the fluid. So I'll adjust. However, this might be complicated as I must take into account the direction of change, increasing or decreasing ? If the sensor reads 41°C and the temperature was increasing, then it's 42°C.

If it reads the same value (41°C) and it was decreasing, then it's 40°C for example. So this adds a bit to it.

The other way of looking at it is:

The overall thermal-lag will be reduced, due to the fact that the body now is metal. It won't be instantaneous, but for this application, it won't be too shabby.

Even if it's 15 seconds between the fluid reaching a certain temperature, and this temperature being sensed.. How much would the temperature in a 2L tank change in 15 seconds (knowing that the heater is maybe 150W). Not /that/ much, I guess.

Ideally (in an isolated environment) 150W is about 35 calories/second.

1 calorie increases 1g of water's temperature by 1°C. I'll assume the same temperature coefficient for the sake of simplicity: 2 litres tank and 35 calories/s means a rate of change of about 0.01791 °C/s.

In 15 seconds, the temperature would change by 0.26°C which is not bad for this application. I don't need something "very" accurate.

If we make the scenario worse, perturbations, and water being different than the etchant, I guess even 1°C is still fine with me. (Even if it's huge, but for this application...)

Heck, it might even not reach that temperature if it's cold and the thermal dissipation is greater than what the heater can deliver

To recap my question:

-What's your opinion on stainless steel probes (I'm really seeing it, I think it would be beautiful: Stainless steel tube+ epoxy .. Even if not for this application. I'll probably do it for another one).

Thanks,

 

[Mr RB]

I would not put stainless or any metal in the etchant.

The thermal lag of a sensor in plastic will be fine, I've done lots of temperature control projects using a TO-92 package temp sensor glued to things with epoxy or encased in epoxy. Because it will be surrounded by the liquid which is a good heat conductor it won't matter that much that the temp sensor is encased with 1mm to 3mm plastic around it. The thermal time lag will be a few seconds at most which will work fine with a on-off setpoint type temperature controller as the heater will take a couple seconds to raise the water temp directly above it by 0.3'C or so, so the result with the plastic sensor near the heater both immersed would cycle on and off for 5 to 10 seconds, and local liquid temp ripple will be maybe 0.5'C to 1'C, but total etchant temp ripple (since it is mixed and distributed by the bubbling) will probably be much less than 0.1'C.

In other words, don't sweat the thermal lag, just don't do something silly like put the sensor on one side of the tank and heater 40cm away.

Aquarium immersible heaters are usually glass test-tube like shapes, so you could semi-immerse it at least if you don't fully immerse.

Maybe if you are already using plastic tubes and epoxy you could make your own immersible heaters? Just put some power resistors in series and epoxy them into plastic tubes, then semi-immerse. You could run it from a low voltage PSU like a PC PSU.

 

[JimB]

I would not put stainless or any metal in the etchant.

Just something which seemed obvious to me without having given it a lot of thought.
There are of course many varieties of stainless steel, when I get back home in a couple of weeks, if I can remember, I will put some 316 stainless steel in ferric chloride solution and see what happens.

The thermal lag of a sensor in plastic will be fine

If you have experience, I will go along with your judgement.

JimB

 

[Reloadron]

Great project!

-What's your opinion on stainless steel probes (I'm really seeing it, I think it would be beautiful: Stainless steel tube+ epoxy .. Even if not for this application. I'll probably do it for another one).

You may find this link helpful. Scroll down to the chemical section.

Years ago I had the misfortune of getting involved with a project in the plating department areas. Large tank after tank of every nasty substance in the world. Most requiring tight temperature control and agitation.

Among the things used for thermowell sensor protection that worked overall well was teflon tubes also called a PFA Thermowell. Anytime we look to protect a temperature sensor there is a tradeoff in thermal conductivity. Sometimes temperature lag can be a good thing serving as a hysteresis allowing for simple (and cheap) On / Off temperature control.

Agitation be it from air bubbles or another source is very important. I was never a fan in most cases of placing a temperature sensor in close proximity to the tank heating source. You want thermal uniformity in the tank and placing the sensor too close to the source of heat can create problems. If we heat a tank with good agitation with a heater to a set point once that point is acheived and the heater begins to cycle we should be able to move a temperature probe throughout the working zone of the tank and see little temperature change. No thermal inversion layers.

Anyway in your case I would give some thought to placing the sensor inside a Teflon type flexible tube. Also experiment with agitation to acheive thermal uniformity.

Ron

 

[Jugurtha]

Great ideas in there, I'm trying to keep the cost to a minimum and the materials to ones widely available.

You may find this link helpful. Scroll down to the chemical section.

Years ago I had the misfortune of getting involved with a project in the plating department areas. Large tank after tank of every nasty substance in the world. Most requiring tight temperature control and agitation.

Among the things used for thermowell sensor protection that worked overall well was teflon tubes also called a PFA Thermowell. Anytime we look to protect a temperature sensor there is a tradeoff in thermal conductivity. Sometimes temperature lag can be a good thing serving as a hysteresis allowing for simple (and cheap) On / Off temperature control.

Agitation be it from air bubbles or another source is very important. I was never a fan in most cases of placing a temperature sensor in close proximity to the tank heating source. You want thermal uniformity in the tank and placing the sensor too close to the source of heat can create problems. If we heat a tank with good agitation with a heater to a set point once that point is acheived and the heater begins to cycle we should be able to move a temperature probe throughout the working zone of the tank and see little temperature change. No thermal inversion layers.

Anyway in your case I would give some thought to placing the sensor inside a Teflon type flexible tube. Also experiment with agitation to acheive thermal uniformity.

Hey Ron,

I've found something called "composite pipe" or "multi layer composite pipe".. "PE/AL/PE" It's some sort of sandwich pipe, polyethylene on the outside and inside and aluminum in-between. It looks very nice, actually it looks great. I've tried to set it on fire with my lighter and it didn't want to, the air inside the tube got very hot, but the tube didn't catch fire.

So I cut a small tube, I put my pinky finger inside and lit fire on the outside just to the opposite side of it. It didn't take long to my finger to curse me and use funny words. About 12 seconds.

They use it for water, and gas and other things. I have just found it after talking to a guy in a drug-store. I have looked it up, and it seems very rugged and doesn't react with other stuff. I checked it out, and it should be okay with etchant. Although at the section level, there is aluminum appearing, so I'll have to cover that or it will cause problems when immersed.

Here's a picture (although mine is white)

View attachment 62662

And here's a link explaining a bit (plumbing)

https://plumbing-leaks-faucets.knoji.com/what-is-composite-pipe/

What do you think about it ?

 

[Reloadron]

Hi Jugurtha,

Long as it offers good thermal transfer and is impervious to the chemical in use. In the interest of keeping cost down and doing a project of this type to learn as well as come away with a useful tool will take some trial and error. Also, I should point out that as to sensor placement I am not at all in disagreement with Roman's thinking. When I say "too close" I mean not directly beside or above the chosen heating element.

Have you given any thought to the element aside from aquarium heaters? A Google of submersible 12 volt (or 24 volt) heating elements brings up some interesting results. Again, anything exposed to the solution needs to get along with the solution.

Ron

 

[Mr RB]

Ho Reloadron, yep that's a good point about the agitation. I should have stated that if putting the sensor above the heater the liquid should be well agitated, but again if there is an effective bubbler going that should be fine.

The PIC ADC and a LM135 should give about 0.5'C per ADC count, and the on/off setpoint control will regulate temp at around 1 ADC count. So set to 40'C the temperature will be 40.0'C to 40.5'C above the heater (at the sensor). Assuming some good bubbling is going on the temperature won't be much different elsewhere in the tank apart from being a bit cooler at the far extremes of tank compared to where the heater is. I think the temperature is not that critical for this application, so even if it is 40.5'C near the heater and 39.0'C near a tank corner it won't make much difference to the etching.

Jugurtha; I'm not keen on the metal pipe even if it has a polyethylene coating. There's no real need for the wall strength of metal pipe and any metal that might eventually come into contact with the etchant can corrode and probably contaminate your etchant. You can get plastic pipe as used in electrical conduit or even something like a plastic pen.

 

[Jugurtha]

Great insights everyone, thank you very much. Now that I have the "main" layout, and know what I won't be doing, I'll be on it.

I'll update the thread once I get some work done.(And some stuff ruined )

 

[Jugurtha]

Update:

I bought:

-PIC16F88 (about 4 USD)

-Silicone: ABRO 999 Gasket Maker **broken link removed**


-Epoxy: Alteco 3 Ton Quick Epoxy **broken link removed**

The PE/AL/PE is dismissed after Mr RB's suggestion. I tried to mimic what will be done and I put a wire in a pen tube and filled it with silicone. I left it to "take". I'll be trying the Epoxy tomorrow (This stuff stinks, and it does stink in a "mean" way.. You could swear it does it on purpose).

The thing is the stuff I bought isn't like what I see in videos (epoxy is black and hardener is transparent, silicone is grey), their main use here is as adhesive (especially engines and stuff) .. The Epoxy came with a hardener.. Anyway, I checked that they resist high temperatures (Silicone until almost 400°) and that they don't harm sensors (it's written there).

I'll be testing with different tubings.

-I've discovered the LM35 and the TMP36. I know the LM35 is available and not expensive (Less than 2.5 USD). And it seems very interesting (accuracy and the fact it's calibrated in °C instead of °K).

-I'm doing some research on Transformerless Power Supplies: I'm thinking that it would be a shame to use a big thing since the micro-controller is the only one using low voltage (Plus the sensor), it wouldn't be beautiful to have a power supply in the traditional sens "just" for the uC. I'm trying to assess the influence of this supply on the sensor (would it make the reading less accurate, and stuff) and vice-versa.

I'm reading the data-sheets,

-Microchip's "Tips'n'Tricks" : https://www.electro-tech-online.com/custompdfs/2012/03/41283A.pdf

-Application Note 945 : ww1.microchip.com/downloads/en/appnotes/00954A.pdf

-Application Note TB008 : https://www.electro-tech-online.com/custompdfs/2012/03/91008b.pdf

Although if I find it complicated, I'll do a small power supply with transformer and Graëtz bridge, etc...

 

[alec_t]

I'm doing some research on Transformerless Power Supplies

For safety's sake DON'T go there. You need mains isolation, so a small transformer is recommended. One about an inch cubed would suffice (if the sensor current is only a few mA).

 

[Jugurtha]

Thanks for the heads up, hopefully I've read some posts of yours and I've read your signature many times (it's kind of a reminder), so thanks.

Where are these transformerless power supplies used in, then ? In applications where "safety" isn't necessary ? Like when the user is Homer Simpson and you don't care about him ?

Or it's just that they are a proof of concept without any real practical (for now) application ?

 

[Reloadron]

I strongly agree with Alec in that a transformerless power supply is not a good idea to consider for this or most projects. Power supplies of that design are generally used in low current applications especially mass produced low current applications. Their main advantage is cost in something mass produced. Your project involves a tank full of conductive solution which to my thinking in the best interest of safety calls for mains power isolation. Additionally here in the US any circuit involving conductive liquid would also have mains power delivered through a GFCI protected circuit.

Have you given much if any thought to the heater? While you don't need much for the temperatures you desire even a 12 volt 12 watt immersible heater element of sorts would draw about an amp when fully on. The heater you choose will be a key player in determining the power supply.

Ron

 

[alec_t]

Where are these transformerless power supplies used in, then ?

In appliances which are double-insulated and wholly contained so that there is no possibility of anyone touching a possibly 'live' part. For example, only yesterday I was dismantling a failed IR security lamp. That had a transformerless supply in it.

 

[Jugurtha]

In appliances which are double-insulated and wholly contained so that there is no possibility of anyone touching a possibly 'live' part.

All right, so I can transformerless if I build a case but nevertheless, for safety's sake and for the fact I won't probably build a case right now, I'll use a transfo.

Have you given much if any thought to the heater? While you don't need much for the temperatures you desire even a 12 volt 12 watt immersible heater element of sorts would draw about an amp when fully on. The heater you choose will be a key player in determining the power supply.

Heaters and pumps I saw in aquariums were directly plugged in mains. I was thinking of making them apart from the circuit. My thought was to plug them in mains directly, cut the cable, use a relay driven by the micro-controller to complete the circuit.

So that there will be a low voltage, "logic part" of the circuit and a "high voltage", "power part".

They get the power directly from mains but the loop isn't completed until the relay is activated. Is there anything inherently wrong (safety related, technical feasibility or otherwise) with this approach ?

Is there something I'm not seeing clearly, I'm asking this since you seem to insist on the Heater aspect and it ought to be important and I have the feeling I'm not seeing the big picture here. The tree that hides the forest.

 

[Reloadron]

Most of the aquarium heaters I have seen include a thermostat and are designed to heat the tank to between 19 and 30 degrees C. and include thermostat. I may be way wrong on all of this. My early etching experiments were kitchen stove experiments. While heat is not absolutely necessary and only speeds the etching process my current understanding is temperatures in the 35 to 45 degree C range are used. I am just not sure if what an aquarium heater will give you what you want. Hopefully another member much more familiar than I am will weigh in on this. I am just not sure if even an aquarium heater, even designed for a large aquarium would get you where you want to be? Your concept is fine as to turning it on and off, I just question if it is the best choice. I don't want to give bum advice so hope another member chimes in with the heating.

Ron

 

[Jugurtha]

Most of the aquarium heaters I have seen include a thermostat and are designed to heat the tank to between 19 and 30 degrees C. and include thermostat. I may be way wrong on all of this. My early etching experiments were kitchen stove experiments. While heat is not absolutely necessary and only speeds the etching process my current understanding is temperatures in the 35 to 45 degree C range are used. I am just not sure if what an aquarium heater will give you what you want.

Indeed, you are right. As stated in my original post, I have seen some projects that raised this issue (heaters don't go that high), and in which they "modified" the heaters by removing the temperature limitations circuitry (typically a thermostat).

I intend to do the same, or get a barbaric heater not designed for boring fish.

Something like those you find in barbershops where they put them in water to heat it up so it's warm and nice for shaving.

As there will be something I'll need to think about, which is driving the relay:

The PIC will be using 5V probably. A Typical relay will need 12V. So I think it's either:

1- Get 12V from the transformer for the relay and use a regulator (7805) for to get the PIC's 5V from the 12V.

2- Get 5V from the transformer ( and then use some sort of voltage multiplier (Cockroft-Walton comes in mind).

Either solution makes the project even less elegant than it already is. Solution 1 makes me irk due to the huge loss and Solution 2 makes me irk due to the huge part count (unless there's an IC for that).

I'm just thinking out loud and just deflorating the subject. I won't bother the forum without thinking more deeply about it and having looked at various solutions to come with better, more specific questions.

Again, thanks for taking the time..