# Lowering 12V DC to 4VDC using resistors?

Discussion in 'The Projects Forum' started by teddy77, Apr 30, 2012.

1. ### teddy77 Thread Starter New Member

Apr 17, 2012
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0
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....

2. ### strantor AAC Fanatic!

Oct 3, 2010
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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.

3. ### mcgyvr AAC Fanatic!

Oct 15, 2009
5,360
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Whats the heater for? veggie oil?

4. ### teddy77 Thread Starter New Member

Apr 17, 2012
21
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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?

5. ### #12 Expert

Nov 30, 2010
18,078
9,616
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.

6. ### teddy77 Thread Starter New Member

Apr 17, 2012
21
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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.

7. ### teddy77 Thread Starter New Member

Apr 17, 2012
21
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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....

8. ### #12 Expert

Nov 30, 2010
18,078
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How's this for quick?

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9. ### teddy77 Thread Starter New Member

Apr 17, 2012
21
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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....

Apr 5, 2008
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11. ### teddy77 Thread Starter New Member

Apr 17, 2012
21
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15A, \$20 bucks. Sounds perfect. Thanks.

12. ### iONic AAC Fanatic!

Nov 16, 2007
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What is the AH rating of your Battery? At 7.5A draw you may not last long before a recharge is due.

13. ### strantor AAC Fanatic!

Oct 3, 2010
4,985
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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...