I have a resistance heater than I would like to run off a regulated 5 volt DC power supply I have. After testing the heater with a VARIAC to find the proper temp, I found that 3.8 volts @ 7.5 amps gives the desired results. I would like to use a resistor to drop the voltage seen at the heater by 1.2 volts (5-3.8). This is just for testing, I'm aware of the inefficiency and the fact that the resistance of the heater varies with temperature. I came up with .16 ohms @ 20-25 watt power rating. This is for something quick and dirty, does not need to be TERRIBLY accurate at this point. The other problem is, if my numbers are right, where can I find a resistor like that? Other simple options to wire it?

 

U can always use a Power MOS to drop the the voltage.

 

U can always use a Power MOS to drop the the voltage.

I'm sure that would work although it is overshooting my abilities, I work as a machinist so, being a project, the electrical aspect is definitely not a strength of mine. What about wiring multiple resistors in parallel to divide the current up and give me my resistance? I'm not concerned with doing it the "best" way, as long as it works... If this could work, what would be the most practical amount/size/rating of the resistors? And of course, the fewest....

 

See...the fewest..u said it.

It most economical way is to use a power MOSFET.

If u can wire resistors, u can wire a MOSFET

 

You can run down to Radio Shack and buy (6) 1 ohm, 10 watt resistors. Put them in parallel for .166 ohms. That's quick, dirty, and within 4% of where you want to go. Besides, a pile of 1 ohm, high wattage resistors can come in handy.

 

I get the same resistance (0.16 ohms) but a different power dissipation:

7.5 x 7.5 x 1.6 = 9.0 Watts. You should probably use a resistor array with a higher wattage rating if you are going to run the thing for more than a minute or two.

#12's idea makes a lot of sense.

 

It most economical way is to use a power MOSFET.

Just wondering what you're suggesting exactly. Would you just choose a MOSFET that has the right Rdson with 5v applied to the gate?

I agree that a single-component solution (2, when you include heat sinking) is more economical and elegant than a resistor bank, but a MOSFET isn't single component if you have to control the gate.

 

See...the fewest..u said it.

It most economical way is to use a power MOSFET.

If u can wire resistors, u can wire a MOSFET

Tell you what, do you have a particular vendor and part in mind? I would certainly give it a shot if I knew where to start. I sure don't want to have to carry a 60 lb. VARIAC around to demonstrate my device .....appreciate the suggestions as well, if you don't know then ask somebody who does....

 

You can run down to Radio Shack and buy (6) 1 ohm, 10 watt resistors. Put them in parallel for .166 ohms. That's quick, dirty, and within 4% of where you want to go. Besides, a pile of 1 ohm, high wattage resistors can come in handy.

Funny, I stumbled on a parallel resistor calculator online and that's what I came up with..... I think I already have two if I'm not mistaken....

 

I get the same resistance (0.16 ohms) but a different power dissipation:

7.5 x 7.5 x 1.6 = 9.0 Watts. You should probably use a resistor array with a higher wattage rating if you are going to run the thing for more than a minute or two.

#12's idea makes a lot of sense.

Yeah, excuse me, i didn't mention I doubled the value just to play it safe, because I do plan to run it for 5-10 minutes at a time.

 

Hang on a minute. You mentioned using a variac to dial in the right conditions but then you said you want to use a 5V DC supply. Maybe I'm crazy but isn't the variac an AC device, meaning you dialed in 3.8v AC at 7.5 amps, not 3.8V DC? You'll want to be sure you measured rms voltage. And how did you determine the amperage?

I'm not saying you did anything wrong, I'm just hoping to prevent a surprise.

 

Just wondering what you're suggesting exactly. Would you just choose a MOSFET that has the right Rdson with 5v applied to the gate?

I agree that a single-component solution (2, when you include heat sinking) is more economical and elegant than a resistor bank, but a MOSFET isn't single component if you have to control the gate.

Absolutely appreciate the help from everybody, like I said though, this is just for a quick and dirty solution, for now, for a prototype.... I am taking notes though because I may most definitely need a proper "elegant" solution in the future....

 

Hang on a minute. You mentioned using a variac to dial in the right conditions but then you said you want to use a 5V DC supply. Maybe I'm crazy but isn't the variac an AC device, meaning you dialed in 3.8v AC at 7.5 amps, not 3.8V DC? You'll want to be sure you measured rms voltage. And how did you determine the amperage?

I'm not saying you did anything wrong, I'm just hoping to prevent a surprise.

gotcha. well from what I understand the type of voltage, AC or DC, supplied to this type of resistance heater does not matter, but for testing I was applying AC voltage through the VARIAC. Unless I'm missing a "surprise" like you said lol, I would imagine the heater would function the same with an equal DC voltage applied (it is rated as a DC heater to begin with). I know for a fact major cartridge heater companies recommend testing for power needs, etc. using the same method, with an AC VARIAC... I checked the current draw with a DVOM in series at stable voltage... 3.8 volts SHOULD be 3.8 volts regardless of alternating or direct with a simple heater like this unless you are thinking about something that I may be missing?

 

It's OK to test a heater with AC voltage, and the whole point of RMS meters is so they will display an equivalent DC voltage. The MOSFET approch will not work well with AC but the resistors will work just the same with AC or DC.

I'm surprised you haven't already been to Radio Shack and solved this for about $5.

 

... the type of voltage, AC or DC, supplied to this type of resistance heater does not matter

Correct, the only concern is the measurement of true rms voltage instead of peak or something else. And since you may want to do other testing or tweaking with your variac, go with the resistor bank - which will work great for AC or DC - and forget we ever said anything about a MOSFET, which is strictly a DC solution.

 

It's OK to test a heater with AC voltage, and the whole point of RMS meters is so they will display an equivalent DC voltage. The MOSFET approch will not work well with AC but the resistors will work just the same with AC or DC.

I'm surprised you haven't already been to Radio Shack and solved this for about $5.

I plan to tomorrow....learned a few things as well which never hurts.