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Electric Heater and FeCl3 Questions: Lots of em

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DigiTan

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Okay I've got a few power source and temperature questions regarding a project I'm working on for this winter. Basically, I want to build an immersion heater to heat up a small etch tank (250ml-750ml) for my PCBs. Keeping with electrical safety, I wanted to ask you guys a few questions before I got started.
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(1) My first question is about the power transformer I'm using to step the mains voltage down from 120VRMS to 6 Volts. The transformer has the rating "4.5A" imprinted on the secondary. Is this 4.5A the absolute max rating? And is this RMS current? Also, with the center tap at ground and the two secondaries at 6 Volts, is the max power 4.5A*6V or 4.5A*12V? And if the secondary rating is 4.5Amps, what fuse do you recommend for the primary?

(2) Next, I'm trying to calculate how much power the heater needs to dissipate to heat the Ferric Chloride in a reasonable amount of time. Before I can do this, I need to know what temperature the FeCl3 needs to be. MG Chemicals suggested heating it to no greater than 130°F (55°C), and I was thinking 110°F (43°C) to 120°F (49°C) sounded about right. Is that pretty reasonable?

(3) Also, I'm trying to Google for the specific heat of Ferric Chloride, but so far I'm not getting much. I need to factor this in for the heater design, so any information would be helpful. If it helps, the MSDS is here.

(4) Finally, I have a question on possible heating elements. I was thinking of using several power resistors in a parallel or series fashion--isolated from the FeCl3 by Pyrex glass. Can anyone recommend any good threads, or web articles on using resistors as heaters?
 
FeCl3's specific heat is probably very close to water.

My recommendation? I just used a $15 halogen floodlight pointing into it. That heats it up nicely and no contact with acid required. Early on I didn't position it well and it melted the side of the plastic tub. I give it more clearance and keep the light away from the sides now.

120F is fine. As the temp gets higher some resists will soften and there is a greater risk of losing fine lines or creating pinholes. And emissions of acidic vapor increase. I recommend you get a cartridge respirator good for acid gas when you work over it. Don't wear good clothes either.
 
DigiTan said:
Okay I've got a few power source and temperature questions regarding a project I'm working on for this winter. Basically, I want to build an immersion heater.
Have you tried pouring etchant into a large yogurt container, closing it and putting it in the microwave for at least a minute or two? :lol:

I don't know how well it works, but it is a thought.

The transformer has the rating "4.5A" imprinted on the secondary. Is this 4.5A the absolute max rating?
more than likely, yes.
Until you get an answer from someone with transformer experience, assume that the answer is yes.

Also, with the center tap at ground and the two secondaries at 6 Volts, is the max power 4.5A*6V or 4.5A*12V?
For the same reason above, go with the worst case scenario. 4.5A * 6V.

And if the secondary rating is 4.5Amps, what fuse do you recommend for the primary?
My own fuse recommendation is to pick one that satisfies the absolute minimum current requirements. That way, if you give too much current, the fuse can blow instead of the rest of your circuit.

(4) Finally, I have a question on possible heating elements. I was thinking of using several power resistors in a parallel or series fashion--isolated from the FeCl3 by Pyrex glass. Can anyone recommend any good threads, or web articles on using resistors as heaters?

I assume the easiest way to go is to ignore ohms law, and use low value resistors with lower wattage ratings than what is required. and then I could be wrong.
 
Thanks, guys. I'm using this for an automated circuit etcher, so I'll probably stick with the power resistors as the heat source so I can control the etch times with high precision. Aside from the UV lamps & ballasts, I'm looking to build everything from discrete parts.


For the power resistors, I was planning to use P = V²/R for resistor selection. If the max RMS power I can get out the transformer is 27W (from 4.5A*6V), and my secondary has 6-volt outputs, it implies the heating resistor should have at least a 1.34Ω equivalent resistance.

I'm estimating it will take just under 22,000 Joules of energy to heat a 750ml tank of water to 120°F (49°C) from room temperature. So (ignoring heat losses), the <27W heater would have to work for over 13 minutes (800secs). I might drop the tank size to 500ml or less to lower the power requirement. I'm guessing FeCl3 has a lower specific heat than water, so that might lower it some more if that's the case.


Anyway, I had another question on the fuses. Are typical fuse ratings for RMS current, peak current, or some other quantity?
 
My experience with transformers is limited but I'll offer the following:

A. The transformer probably can deliver something in excess of 4.5 amps for some period of time given the proper input voltage/current and load (intentional or otherwise). The absolute maximum is determined by the transformer construction/design. It can be determined by measurement or by contacting the manufacturer.

B. The transformer will deliver 4.5 amps or less depending on a number of things - that are all a part of a more detailed rating of the transformer. Those details might include ambient temperature, time, duty cycle, among other things. Quite often a transformer won't see a continuous load at the maximum but rather some moments at peak - some at something less than peak -and it is rated accordingly. Some applications require that a component such as a transformer deliver the rated power continuously and under some very harsh conditions.

It might be safe to say that many electronic devices, active and passive follow similar rules. Quite often the "rules" or operating conditions aren't particularly explicit - other times the manufacturer cites a standard that is quite detailed.
 
DigiTan said:
Anyway, I had another question on the fuses. Are typical fuse ratings for RMS current, peak current, or some other quantity?

Fuse is for RMS current. But really a fuse's behavior is usually not tightly predictable. A 3 amp slo-blow fuse will take 4, even maybe 5 amps for some number of seconds.
 
Hmm...I double-checked the imprint on that transformer and it's saying the current is actually 0.45A instead of 4.5A. I thought the leads looked too small.

Anyway, does anyone know of any web articles or schematics for electric heaters. I've already get a design in mind, but I want to check what other people are doing.

So far, I'm planning to using wire-wound power resistors and connecting them to metal rods to transfer the heat. Compared to nichrome wire, that tseems a little crued though, some I'm trying to find designed that used nichrome or tungsten heating coils (a soldering iron, a hair dryer anything). My main concern is how to control the current since their resistance will probably be like, in the 1 ohm range.
 
I'd say your main concern should be how to avoid getting any acid on those devices or their leads while maintaing good thermal conductivity with the acid. The aquarium heaters are nichrome wire inside a glass tube (probably wanna use pyrex here). The thermal transfer in fairly poor but they're only designed for mild warming.

You need to choose a resistance that matches the voltage of your supply. Perhaps you should look at 12v sources, plenty of ways to get high current 12v power.
 
Have you considered cartridge or band heating elements? I do not recall which materials are compatible with ferric chloride but if you can't purchase a cartridge that can be directly immersed you might put it in a sheath of compatible material. A band heater could be flat or round but the operating priciple is the same - you attach it to the container or tank. These are so common in industry and the cost is fairly low, relatively speaking. www.thermalcorp.com might help show you some things.

Regarding the resistors you might just go with a tab mounted type - looks similar to a TO-220 case for a transistor or IC. They are modest in cost. Caddock is the only mfr I am familiar with because of non-inductive properties but I am sure there are many others (Digikey, Mouser and others probably have plenty of them). You could select the value you want.
 
Okay, after looking at all the options, I think I'm going to try an recreate something like the nichrome heaters used in the aquariums. Because of power comsumption, I was going to power everything from the AC mains, and since I done have any adapters to supply the 4+ Amps I need for this kind of heat, it looks like I'll be doing it from scratch.

I'll check out the cartridge heaters. Aparantly, you (the buyer) can specify the resistivity you need, and that should give me just what I need to control the wattage. I'll have to borrow some time on our university equipment for the low-resistance measurements, but that shouldn't be much trouble. That only leaves the problem of locating glass tubes for insulation.
 
Etching tank

DigiTan, I think your best bet is to use a glass-enclosed aquarium heater.

A quick google for, "aquarium heater," yielded the following from a major retailer (that I do not, however, recommend doing business with):

"A direct-reading, thermometer-type scale makes presetting temperatures easy
Completely waterproof and totally submersible
Double-sealed and needs no rubber or plastic cap over the adjustment stem
Compact design
Available in 6-, 10-, 12- and 14-inch lengths
Choose from the following wattages: 50, 100, 150, 200, 300 watts."

You will have to check for absence of exposed metal and adequate sealing for FeCl, but these are made for permanent immersion in WATER, so are at least well sealed. I would think a visit to your local aquarium supply store would allow you to inspect the type of seal. 300 watts is certainly way more than you need for an etching tank.

I made an etching tank many, many years ago out of bare epoxy board and epoxy adhesive and it worked fine. I chose to make it thin to minimize etching fluid volume, vertical to promote fluid circulation and agitation using an aquarium air pump and porous air distributor to speed up the process, and applied a surplus, flat, flexible, red rubber area heater to the flat side of the tank. It worked fine except that I could not see through the tank wall to watch etching progress. You have to leave lots of headroom and shield the top to avoid spatter that can stain and damage almost anything in the area.

If I was to do it today, I would fabricate a tank out of 3/16" or 1/4" clear plastic sheet, using adhesive specifically made for the material, and use a flat rubber heater element operating directly on 115 VAC with a thermostat mounted on the outside of the tank in series with the element. I don't think 120 degrees F would be a problem, but I would check with the plastic supplier. I believe that acrylic would be very satisfactory and not very expensive. With this approach you will have to settle for slow, gentle heating because plastic is a poor heat conductor. If you are in a hurry, you could preheat the etching fluid in a glass vessel and just use the tank heater to maintain temperature during etching.

If you use a porous air distribution element (which I highly recommend), be sure to test a sample of the type you want to use by soaking it in etching fluid (it is not "etchant") because some will soften and break up in FeCl.

I don't think you can use any type of metal in contact with the FeCl, which is the reason I recommend glass-enclosed aquarium heaters.

I think you could use the same basic layout with extra space at each end to make room for vertically oriented aquarium heaters, keeping the feed-through seals and wire completely out of the etching fluid.

Actually, at work, we just used a pyrex baking pan sitting on a flat glass secretary's leg warmer panel for years. Worked fine, but messy, bulky, and inefficient

Have fun.

awright.
 
Yeah, aquarium heaters were the first option I checked out. Unfortunantly, I'm "between jobs" as they sometimes say, I thought the heaters from the retail shops were kind of overpriced for what they do. Anyway, it's still one option I've got open.

The air pumps though I can do, if fact, I've got a couple used one on hand, along with a few feet of clear tubing. The design is looking pretty spill-proof so far, so I'm hoping it'll get some good agitation without getting messy. I found a plastic case, to hold the PCB. It's a little small (a little larger than a VHS cassette, but it will work for now--plus the top can swing open to work as a splash guard.

Back to the heater, if I can't find an aquarium or cartridge heater for cheap, I have a few tungsten coils from a discarded hair dryer ("One man's trash..." you know the rest). Anyway, the project's come as long way in the past few weeks and I might have some photos ready later..
 
circuit board etching tank

I'm having a little difficulty visualizing how you will use the bare heating element from a hair dryer. (Actually, while it doesn't matter much, I believe that the hair drier heating element will be Nichrome or similar resistance alloy, not tungsten.)

Regarding using resistors vs. Nichrome: the important factor is the temperature of the heating element/resistor (which will not be the same as the temperature of the fluid due to the thermal resistance between the element and the fluid and the rate of heat transfer). Tungsten has a very high tolerance to temperature, although it will burn up in the presence of oxygen, as anybody who has accidentally cracked a light bulb knows. Nichrome can glow red hot without damage, although it gets brittle and fragile. Temperature ratings of resistors vary all over the map, depending upon the materials they are made of. If you can transfer the heat to the fluid efficiently enough to hold the temperature down to below the damage threshold of the resistor, any resistor will do. But how do you do that? Putting the heating element in a pyrex tube surrounded by gas will result in a high thermal resistance and poor heat transfer. Filling the tube with a heat transfer fluid or gel, like silicone fluid or grease (or even better, thermal grease for transistor mounting) will greatly enhance heat transfer, but now you have to worry about sealing the tube and accommodating thermal expansion of the fluid, not to mention the mess.

Here's a thought. How about using a linear Tungsten-Halogen Quartz lamp directly immersed in the fluid at the bottom of the tank? You will obviously have to use it at way below its rated voltage/power, but that's easy. Your main problem will be passing the quartz tube through the tank walls with a good fluid seal having adequate temperature rating, but that should be doable. I'd try leaving clearance annulus of about 1/8 inch around the tube and filling the annulus with a high temperature silicone seal like Loctite 59630 (Grainger Stock No. 5E223, $7.15 for 80 ml tube), rated for use up to 600 degrees F. I'd use a T-H lamp with welded flying leads instead of the common button-ends that require a spring-loaded lamp holder. Look at this listing from C&H Sales, Pasadena, CA:

**broken link removed**

"QUARTZ INFRARED HEATING ELEMENTS GTE SYLVANIA All have quartz glass bodies with metal tabs at ends. Have wire leads for electrical connections. Leads are approx. 1-1/2" long. Quartz glass bodies are all 0.4" nom. dia. 1000 watts 230 - 250 Rated Volts 10 3/4" Length. Stock #HE9550 $3.95"

Of course, this will get incandescent at rated voltage, but at 115 volts would dissipate a nominal 250 watts (actually much more than than that due to the temperature coefficient of tungsten - its resistance will be much lower than calculated from the rated power and voltage due to the low operating temperature). Use it on 115 volts with a dimmer or Variac to limit its temperature to a safe level if left on for a long period and cycle it on and off with a thermostat mounted on the tank. C&H also has a large selection of adjustable thermostats. I'd use two thermostats for redundancy in case one fails. You want to operate at a low enough power to avoid boiling at the quartz surface.

I know nothing about the compatibility of quartz with FeCl, but would imagine it is fine. Ask GE or Sylvania or take a chance.

Have fun.

awright
 
i would suggest you to fint a heating element from an electric stove from a junkjard or so. the have few meters in there and you can trim a nice good heating element for your needs. just wrap it around the box and aply power (use foil betwean plastic and heatwire to prevent plastic from melting).
 
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