Vivarium resistance heater. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 02, 2016, 09:18:29 pm
content: Some thoughts on an idea for a vivarium heater ! I have a heat sink that can handle an output heat of 100 watts, my thoughts are thermostat controlled resistance heater using power resistors. I was thinking using 12 Volts dc from a converted atx psu, to power two 5.6R 50 watt resistors in parallel. Does this sound viable ? Heating would be thermostat controllable, so cycled, not on continuous. Any thoughts on this idea appreciated. I have a fan mounted on the heat sink, thoughts are running that slow to circulate the heat. What do you think for simple heater ? Thanks for any advice.

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 03, 2016, 08:41:37 am
content: I know this is a very simple approach to a 12 volt heater, but it would be useful to have some understanding how high power resistors would suffice in an application like this.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: May 03, 2016, 09:06:31 am
content: Sure, why not? After all, the whole purpose of a resistor is to dissipate power as heat, isn't it?I once made a lead melting pot using several "cement" type wirewound power resistors embedded in salt, surrounding a porcelain ceramic bowl. Powered from a Variac, worked fine.I think the major issue would be proper thermal coupling to your heatsink. If you get the more expensive metal-encased power resistors it would probably be easier than using the kind I used.I suppose your ATX supply will work OK for this. If you use your 2 resistors in parallel you will have a resistance of 2.8 ohms. So the current will be I = V/R or 12/2.8 = about 4.28 amps and the power dissipated will be P=I2R or a bit over 51 Watts total, well within the power handling capability of the resistors as long as they are well-coupled to the heatsink.

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 03, 2016, 10:15:23 am
content: Thanks for your reply. So I would get 100 watts from 2 x 5.6 ohm metal aluminium clad resistors ? Does that sound correct. And a total current draw of just over 8 Amps for the 2 x parallel resistors ? Thanks again.

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 03, 2016, 10:25:56 am
content: So thinking about this... I really need 1.4 ohms from 2 x power resistors in parallel to get 100 watts of heating power ? Does that sound right, I think it is. Thanks again.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: May 03, 2016, 11:36:34 am
content: Er... Ohm's Law. V = IR and P = I2R, or P = V2/R. Resistors in series simply add together for total resistance, resistors in parallel go as 1/Rfinal=1/R1+ 1/R2+ ...So, working backwards... if you have a 12 volt supply and you want to dissipate 100 watts from it, you calculate 100W = 122/R and solve for R.R = 144/100 = 1.44 Ohms.So you could put4of your 5.6 ohm resistors in parallel to get 1.4 Ohms. The current will then be I = V/R = 12/1.4 = about 8.6 Amps. Can your ATX psu supply that much on its 12 volt output bus without straining? Each resistor will dissipate 1/4 of the total power, or about 25 Watts each, well within their 50W ratings _if_ they are properly heatsinked. (Heatsunk?)On the other hand, if you only have two of the 5.6 ohm resistors and you still want to dissipate 100 watts, then you need more voltage. P=V2/R, so 100W = V2/2.8 so V2=100*2.8= 280 and sqrt(280)= about 17 volts. The current will then be I=V/R = 17/2.8 = a bit over 6 amps. BUT... each resistor now dissipates a full 50 Watts, right at its rating. So you'll need to pay attention to the heatsinking if you want the resistors to last.

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 03, 2016, 11:48:40 am
content: Thanks for helping me here, I thought it was the way you have exlained it. I do have converted atx suppls up to about 25 Amps and a couple of converted server supplys rated at 100 Amps each. So should be fine for 100 watts on the 12 Volts rail. Thanks again for your help, much appreciated.