Resistance heater.

[sureshot]

Looking for some help if possible to building a resistance heater. Would i be right in thinking that four 12 ohm resistors in parallel will give me 3 ohms and with 12 volts an output power heater of 200 watts if my four resistors are 50 watts each. Any help appreciated, my issue is thinking my power circuit is 3 ohms so only drawing 4 amps total, so would i be getting the 48 watts power output from each resistor, or am i only going to get 48 watts from the total of the four 12 ohm resistors. Thanks for any help.

 

[KrisBlueNZ]

Four 12Ω resistors in parallel will give a combined resistance of 3Ω.

If you connect 12V across them, each resistor will have 12V across 12Ω so 1A will flow and it will dissipate 12W.

Four of them like this will dissipate a total of 48W.

 

[sureshot]

Thanks for your help Kris. So i need to lower the resistance to get the resistors power up really then, so resistors of 0.75 ohms should be ok for four 50 watt resistors i think, maths not my strong point.

 

[(*steve*)]

The formula you may want is

P = V²/R

or

R = V²/P

for 200W at 12V, you need a resistor value of 0.72 ohms

for 50W at 12V, the resistors would be 2.88 ohms

So a single 200W 0.75 ohm resistor, or four 2.88 ohm 50W resistors in parallel, or four 50W 0.18 ohm resistors in series, or four 50W 0.75 ohm resistors in series/parallel

in any arrangement the resistance measured across the ends should measure 0.72 ohms (or 0.75 ohms)

 

[sureshot]

Thanks for your help Kris. So i need to lower the resistance to get the resistors power up really then, so resistors of 0.75 ohms should be ok for four 50 watt resistors i think, maths not my strong point.

So my working out is 4 x 0.75 ohm 50 watt resistors with 12 volts across the four parallel resistors dissipates 192 watts and draws 16 amps if i have it right looking to achieve 200 watts.

 

[sureshot]

Ah ok thank you Kris and Steve think i have it now, i need four 50 watt resistors at 2.88 to make the 0.72 value in parallel and get the 200 watts i need. Many tanks for your help.

 

[(*steve*)]

Four 0..75 ohm resistors in parallel would have a total resistance of

1/(1/0.75 + 1/0.75 + 1/0.75 + 1/0.75) = 0.1875 ohms.

The current at 12V would be

12/0.1875 = 64A

And therefore the power would be

64 * 12 = 768W

four 0.75 ohm resistors would need to be connected in series/parallel. That is you connect two pairs of resistors in parallel, then connect these in series.

Or yes, four 2.88 ohm resistors in parallel

But there are a couple of other things to note:

1) a 50W resistor dissipating 50W is right at its limit.
2) 0.75 ohms may be easier to get than 2.88 ohms.

Depending on what you're actually trying to achieve, you may be better off looking at what's available (also cost and availability) before trying to arrange resistors in a cost-effective manner to dissipate 200W.

For resistors operating at their limiti of dissipation, you will need to connect them to a heatsink to ensure they don't go overtemp (and their max surface temperatures can be *really* high! If your temperature requirements are not extreme, it may be better to use resistors rated at a total of say 300W so you don't risk overstressing them.

Are you using these with a thermostat to maintain a fixed temperature, or is it for something else?

 

[sureshot]

Oh yes i see what you mean, at the max rating its stressing things a bit, Yes my diy bathroom heater as its a tiny room and 200 watts would heat it up in there, i could wire up to a thermostat, its just an idea and no mains voltage in the bathroom only lead in feeding 12 volts. Learned a bit about resistors in different configurations though.

 

[KrisBlueNZ]

sureshot, yes you were right in post #6.

 

[(*steve*)]

Some examples are...

seven 5 ohm 50W resistors in parallel gives 202W of heat from 12V and the resistors are rated at 350W

seven 5 ohm 30W resistors in parallel gives 202W of heat from 12V and the resistors are rated at 210W (but check out the price! *gasp*)

six 4.7 ohm 50W resistors in parallel gives 184W of heat from 12V and the resistors are rated at 300W

six 0.5 ohm 50W resistors in series parallel gives 192W of heat from 12V and the resistors are rated at 300W

five 0.15 ohm 50W resistors in series gives 192W of heat from 12V and the resistors are rated at 250W

Oh, a heater. I would suggest a fan heater If you want to do this, connect the resistors to a finned heatsink and use a fan to blow the warm air around (similar to something from a computer motherboard). If you're doing it without a thermostat, I would recommend something where the resistors are not running to their maximum capacity

 

[Anon_LG]

If you're powering this of batteries they are not going to last long if you're powering it of a car battery. Based on 64 amps across resistors:

50Ah (large car battery)/64 A= 0.7 Hours = 42 minutes

(hey look 42)

Am I right?

 

[(*steve*)]

Looks about right to me.

And using a rule of thumb that you shouldn't more than 50% discharge the battery, you get about 20 minutes.

edit: Now I'm awake... 16 (ish) A, not 64 A so 4 times as long

 

[KrisBlueNZ]

I don't think the OP mentioned anything about batteries...

 

[sureshot]

Sorry on late reply internet connection problems, thanks for all your help guys, plan is a converted atx or large linear transformer, using a large car amplifier aluminium chassis as the heater with 4 x 50 watt power resistors running from 12 volts , current drain expected about 16 amps, i will post back results.
Thanks Kris (-;