Hi,

I'm sure this is a pretty simple problem for most on these forums, but I'm having some trouble figuring out the correct sizing, etc. Basically what I want to do is run a particular heating element with a 12 volt DC power supply. To get the proper temperature, I used a variable power supply and found that 3.8 volts @ 7.5 amps gives the desired result. What I want to do is use a simple, cheap method (resistors) to lower the voltage at the heating element to the desired 3.8 volts using a 12 volt power supply since the element will be run off a car battery since I do not want to tote around a 40 lb. Variac when I want to use it. I'm having trouble with two things: 1)what exact value resistors I would need and where to place them in the circuit and 2)if there is any way I can wire it so the resistors do not get overly hot since from, from what I understand, dropping the voltage so much will require a lot of heat dissipation. Any help/alternate suggestions would be greatly appreciated....

 

using resistors is fail. You are going to dissipate exactly twice as much heat in the resistors than in the heater. That's waste. you want 1/3 of the power from 12V, so apply 12V to the heater via PWM with a duty cycle of 33%. You can do this with a MOSFET and a 555 I believe.

 

Whats the heater for? veggie oil?

 

using resistors is fail. You are going to dissipate exactly twice as much heat in the resistors than in the heater. That's waste. you want 1/3 of the power from 12V, so apply 12V to the heater via PWM with a duty cycle of 33%. You can do this with a MOSFET and a 555 I believe.

That's what I was afraid of....I would imagine the large current draw of the load makes it impractical (?).......PWM makes total sense, do you happen to have any links as to schematics for such a circuit?

 

There is also the fact that the battery voltage will change as it discharges.
As in, how acurate does it need to be? If you do anything about monitoring the temperature and compensating for voltage changes, this gets complicated rather quickly.

Another approach would be to make a 3.8 volt switching voltage regulator. Pretty much a PWM circuit like strantor said but with a voltage regulating aspect.

 

That's what I was afraid of....I would imagine the large current draw of the load makes it impractical (?).......PWM makes total sense, do you happen to have any links as to schematics for such a circuit?

actually, better yet, are these available pre-made or adjustable (duty cycle?) which would be great. especially in small sizes since space is a concern.

 

There is also the fact that the battery voltage will change as it discharges.
As in, how acurate does it need to be? If you do anything about monitoring the temperature and compensating for voltage changes, this gets complicated rather quickly.

Another approach would be to make a 3.8 volt switching voltage regulator. Pretty much a PWM circuit like strantor said but with a voltage regulating aspect.

I understand completely, actually this is STRICTLY for a prototype, I just want to get something "fast and dirty" for testing and I will eventually consult an electrical engineer since I do plan on marketing the entire "device" of which this is an element....

 

How's this for quick?

 

I understand completely, actually this is STRICTLY for a prototype, I just want to get something "fast and dirty" for testing and I will eventually consult an electrical engineer since I do plan on marketing the entire "device" of which this is an element....

I agree, I thinking a "switching" method is the way to go. Since these methods use a duty cycle, there would be no heat generated either correct? Other than the heating element, of course....

 

Hello,

SgtWookie made a simple PWM ciruit some time ago:
http://forum.allaboutcircuits.com/showthread.php?t=9016

You will need a more powerfull mosfet in the ciruit.

Bertus

 

How's this for quick?

15A, $20 bucks. Sounds perfect. Thanks.

 

What is the AH rating of your Battery? At 7.5A draw you may not last long before a recharge is due.

 

I understand completely, actually this is STRICTLY for a prototype, I just want to get something "fast and dirty" for testing and I will eventually consult an electrical engineer since I do plan on marketing the entire "device" of which this is an element....

I agree, I thinking a "switching" method is the way to go. Since these methods use a duty cycle, there would be no heat generated either correct? Other than the heating element, of course....

Did you just agree with yourself? LOL, well as long as all of our personalities and all of your personalities all on the same page...