AC power question

[large_ghostman]

i normally work with DC 3-12 volts, but i need to work with AC, as a outline i need a heater of anywhere around 100-200W per unit (unit being each heater) i can live with 75W, its needs to be small and in a perfect world no more than 24V AC. but i would do 220V if i can make it safe enough.
i thought about it then i came across mums hair straighteners! the ceramic plates are plenty hot enough if i use 2-3 and 2-3 would give me good options for quick warm up then switch x number off and just pulse 1-2 to keep temp stable. as my dad says robert would then be my mums brother! no idea what he means but he says it alot in stuff like this!
any any way, i got thinking and found these on ebay https://www.ebay.co.uk/itm/12111761...eName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649
they are replacement elements for the hair thing, but i had no idea what voltage they use so i messaged the guy on dads account and they just sent a message with a number for tech support, i rang it and the guy was really vague when i asked what voltage they ran at and said in the end he thought about 12V now that sounds iffy to me but i am not sure if watts working out is same for AC as DC (i do know well i think i know they are AC). so i called a repair center for hair stuff and the man said that they were 240V and controlled by pwm with a PIC!!! now again that sounds a bit dubious to me as i didnt think pwm was easy with AC due to zero crossing blah blah blah. so my question is what voltage could i run them at (if line voltage then i can use ceramic sleeve for safety) i have a 24V ex weller soldering iron transformer i could use for 24V AC. i am thinking 160 ohm at 240V is going to get very very hot very very quick and i will have trouble not burning it out
thoughts and guidance please

 

[MrAl]

Hi,

For resistive elements the current, voltage and power are basically the same. If you know the RMS voltage then you can calculate the power P=V/R just as in DC.
It's only where there is a phase shift that you have to consider the phase of the current, and this occurs with reactive elements like inductors and capacitors.

But there are also heating elements that are not resistive. These kinds of elements usually have a label written something like:
120 volts AC only.

So you have to be a little careful there to make sure they can run on either AC or DC if you plan to use DC.

 

[large_ghostman]

i am pretty sure they are AC but i think they must be more than 24V or they wouldnt get hot enough to straighten hair

 

[MrAl]

Hi,

Ok if they are AC only then you have to be a little more careful about calculating the power.

If they each run at 24vac and have a power of 100 watts, then the power should scale appropriately though so at 12vac they probably have a power of 25 watts.

 

[large_ghostman]

they are rated at 160 Ohms or you have a 70 Ohm option, thats all the info given except they are for hair straighteners and get very hot, i am thinking i will have to go with line voltage and maybe sandwich them between cpu heat sinks with fans on top so the heat isnt so harsh and just pulse them on and off as needed

 

[large_ghostman]

or i could go with some hong kong soldering iron elements (cheap) **broken link removed** like these ones, they are 24V and i would have thought (but i will check) that my transformer would be able to handle at least 2 of these, the up side is they are 24V the down side is i will have to get creative with say a cpu heat sink with fan on to get useful heat from them to heat inside of incubator, lets face it a soldering iron dosnt heat the room you solder in! but they do get hot so if i spread the heat over a heat sink and then blow air over the fins i might get useful heat from it

 

[Reloadron]

to heat inside of incubator

Hi there Little Ghostman. So would that be the objective, to heat an incubator? When I was a kid, other kids were hatching chicken eggs using only a light bulb in a box.

So what are you exactly trying to do and what temperatures do you need?

Ron

 

[dr pepper]

You can control ac with pwm, but I dont think yer mam's hair strighteners will go to that extent.
If theres a pic in there then it'll use phase angle control, the pic waits for mains zero, then waits a time delay depending on the power level, then turns on a triac, so you get less and less of the mains cycle as the power level is decreased.

A bulb as ron says is a good heat source, you can still control a bulb easy enough.

If you want to prototype a 240v triac based dimming circuit then use your 24vac source and use to start with a 24v bulb, I did this when I prototyped a pic dimmer, well I couldnt find a 24v buld so I use 2 12's in series.

 

[ClydeCrashKop]

As my dad says robert would then be my mums brother! no idea what he means but he says it alot in stuff like this!

I thought "Bob's your uncle" was a peculiar expression when a friend was using it so I looked it up.
https://en.wikipedia.org/wiki/Bob's_your_uncle
...And Bob's your uncle is an expression of unknown origin, commonly used in Britain and Commonwealth nations. Typically, someone says it to conclude a set of simple instructions, similar to the French expression "et Voila!" or the American slang expressions "...and that's that," or "...and there you go!"
'Bob's your uncle' is an exclamation that is used when 'everything is all right' and the simple means of obtaining the successful result is explained. For example: 'left over right; right over left, and Bob's your uncle - a reef knot'. It is sometimes elaborately phrased Robert is your mother's brother or similar for comic effect. With his customary whimsical humour, P.G. Wodehouse extended it to "Robert's your father's nearest male relative".

 

[large_ghostman]

thanks guy's i didnt think there were any replies as it didnt flag!! so a quick catch up, yes its for an incubator, dad has used bulbs before but a major problem with them is they fail alot so you add back ups with a LDR to switch on when one fails, another problem is at the size of incubator i am building (old under counter fridge and around 450 eggs) you need 1-2 100W bulbs, and 100W bulbs are hard to get around here, plus because its a competition with dad i want smaller because i want more room for eggs and therefore better chance of hatch rate! so space and failings being major factors i am looking at other ways, temps are normally quoted as 37.7C for chickens and 35.5 for geese etc etc but reading dads papers from uni and his own notes from when he built them for the farm i need 101 F or for chickens 99.5F -101.8 F, working in C is ok and the normal way for most commercial incubators but dad is a expert on bird embryology and his hatch rates prove that working in F is way better but harder,
anyway the hair things are small and get very hot i doubt i need phase angle just kind of from x temp to x temp heater on say 7 seconds off 3 (i will experiment to get the times this is just an example) then from y temp to y temp 4 seconds on 3 off untill Z temp (near correct temp) where i will say do on 2 seconds off 5 seconds, if i do the experiments i should get pretty close to hardly any fluctuation in temp, so i was thinking zerocross triac driver and just on/off in bursts.
also i have a neat idea how to even out the temp in different parts of incubator (other thread). the soldering iron elements that are 24V give good temp but i think would need a heat sink with fan to get a useful amount out slowly. also the kind of heating elements i have mentioned dont fail as much and putting a backup in takes less space. i know for this comp dad is going the bulb route just to get his working quicker! and i want mine to be better
also the other reason is a bulb would ruin one of my other plans

 

[large_ghostman]

got a message from a repair guy that repairs GDH hair straighteners, he suggests for the temps i want/need that i try to fire only on positive cycles and for only one cycle (20ms) every 4-5 seconds! also gave a big warning not to underestimate just how quick and how hot these things get, so is looking like a big heat sink with fins like a cpu one is going to be the way to go and only firing for very short bursts, so zero cross driver? and timer interrupt?? start with 1-2 cycles every couple of seconds and monitor heat sink temp and take it from there. i am going to the soldering iron (24v AC )elements first, i really doubt they will do it even with a heat sink and fan but at least it will give a guide just how much i need to pump into the heat sink to get a useful temp out of it with a pwm controlled fan, and its safer to start with

 

[Reloadron]

Well for an incubator I would use maybe a few heat lamps and a lamp dimmer. Using a dimmer and running the lamps at less than their rated power should ensure longevity. If you want some control then I would think about simple On / Off control. I believe most incubators I have seen also have a small fan or two for some airflow.

Ron

 

[NorthGuy]

You can use water heater elements, or electric cooktop elements. If you get a 2kW @230V element, it'll give you 20W @ 24V. This is gentle enough not to cause a fire and about the power you need to heat your incubators with slightly less than 100% duty cycle.

 

[large_ghostman]

Well for an incubator I would use maybe a few heat lamps and a lamp dimmer. Using a dimmer and running the lamps at less than their rated power should ensure longevity. If you want some control then I would think about simple On / Off control. I believe most incubators I have seen also have a small fan or two for some airflow.

Ron

due to dads research (professional) my incubator will have 12 fans at the front 2 in the bottom 1 at the top for venting and 1 in the humidity chamber i am in competition with a pro dads commercial incubators he built at the farm had hatch rates way higher than anything you can but off the shelf, maybe dosnt matter for chickens but dad had to hatch some very very rare birds for conservation projects to stop some breeds/species from dying out, the comp between me and dad is a fun one that will take 6-12 months to complete, but its important that i win! bulbs will not cut it against the master lol

 

[large_ghostman]

You can use water heater elements, or electric cooktop elements. If you get a 2kW @230V element, it'll give you 20W @ 24V. This is gentle enough not to cause a fire and about the power you need to heat your incubators with slightly less than 100% duty cycle.

i need around 100W general and 200W at times to get enough heat to compensate for heat loss when doors open etc, the comp is for hatch rate, dads best so far is a steady 97% of all fertile eggs hatching! average hatch rate at farm was 94% of fertile and 84% of all eggs wich is absolutely top notch in commercial terms.

 

[NorthGuy]

i need around 100W general and 200W at times to get enough heat to compensate for heat loss when doors open etc, the comp is for hatch rate, dads best so far is a steady 97% of all fertile eggs hatching! average hatch rate at farm was 94% of fertile and 84% of all eggs wich is absolutely top notch in commercial terms.

The good thing about big resistive heaters is that you can easily change the output by varying the voltage. For example, to get 200W out of 2kW heater you need to decrease voltage by square root of (2000/200 = 10) = 3.16. If they're designed for 230W, you run them at 73V. You can build a transformer, which will produce 73V. You can use one transformer for all incubators, although it's going to be a big one. Big heating elements are very reliable.

You could run them directly from 230V, but if your electronics fails, they will start heating full force and may cause a fire.

It is very easy to regulate a heating element with AC supply. You just switch it with about 0.5Hz frequency. You detect zero crossing. You switch once per 2 seconds. If your AC is 50Hz, you will get 200 half cycles during 2 seconds. You turn the heater on at the beginning of the 2 second interval, count the number of cycles that you want it to be on, then switch it off until the end of the period. Since you're always switching at zero crossing when there's no voltage or current, it's very easy on your switching element.

By varying the number of periods the heater is on, you vary the heat output. Say, to get 100W out of 200W element, you keep it on for 100 half-cycles and off for 100 half-cycles. If you want 80W, you keep it on for 80 half-cycles then off for 120 half-cycles. You will get nice even heat, much better than the standard heating. Your only task is to match the heat output to the input from your sensors.

 

[MikeMl]

...You could run them directly from 230V, but if your electronics fails, they will start heating full force and may cause a fire...

To say nothing of cooking the chickens.