Resistor temperature

I already asked it in this topic: USB "charger doctor" (volt+current display)
But I guess only people interested in a charger doctor read that so therefor I’ll try to ask it again in a separate topic.

I bought THIS load resistor and I want to use it with the charger doctor to discharge a powerbank and measure how many mah it puts out.
But I measure a temp of 160 degrees celcius after only a couple of minutes. I have now mounted it on a piece of computer heatsink and used some arctic silver, but the heat isn’t transfered fast enough to the board and therefor the heatsink isn’t that effective. If I let a small computer fan blow at it the temp drops to 130/140 (temps measured with an IR meter so could be somewhat off)

I assume these temps are normal, but I’m wondering, is it safe to let it run for extended periods of time?

5 ohm wire wound resistors usually get quite toasty 1A = 5 watts dumped thru resistor as heat, 2A = 10 watts

as long as you can pull the heat away it should be fine…wonder what the power rating of those resistors are

Pics of the heatsink? Might be able to epoxy a finned heatsink over both resistors (say a cpu heatsink) then push air over it…the epoxy will wick heat away ok but don’t expect it to be “cool” to the touch…these things are load resistors…getting hot is what they are designed to do

Where I work they have a HUGE resistor bank capable of dumping over 1500 KWatts (1.5 Mega Watts) X2, used for load rating our backup generators…they get HOT HOT HOT HOT HOT (I also believe they are oil bathed too)!!!

yup. they are power resistors, they are designed for that. They are made out of ceramic. You are discharging your batteries with a resistor, which has to get rid of that energy somehow, so it does it as heat.

This type of power resistors often has a working temperature around 300C, but it will probably be hard on the circuit board if you run it much above 100C.

To really use this type of power resistor it must be mounted some distance above the circuit board and with lots of air around it (not with another power resistor next to it).

Here is my really ugly and probably bad solution

Insulation tape is there to avoid a short between the resistors (already had it happen while handling it and trying different methods, even touching it with my fingers made the powerbank shut down)
Had now way of fixing it to the heatsink so therefore I uses some of those “straps” you will find sometimes wrapped around cables to keep them together.
Heatsink only get’s to 60/70 degree so it’s not doing a very good job. At first I only had some arctic silver under the board, so I tried a few blobs between the resistors and the board, in the hope to drop the temps a little bit more, but it doesn’t seem to do anything.

I have a couple of other heatsinks so I can try to attach them to the resistors itself.
Will have to buy some epoxy then.

And yeah, since they have to lose their energy somehow I assumed the temps are normal, but since I want to discharge a 15000mAh powerbank to see how much usefull energy comes out of it, I expect it to run for a while and was wondering if it would be wise.

At 2A you get 20 watts of power lost as heat.

Each 5 ohm resistor only gets 1A.

The two 5 ohms in parallel is 2.5 ohm with 2A

I knew my math was screwed up…jeez

I even plugged it into an online ohms law calculator…daggum

wait…I did it again

5V 1A 5Ω resistor = 5 watts, 5V 2A 5Ω resistor parallel = 2.5Ω = 10 watts??

Unless I am punching it in wrong

Power (watts) = Voltage (E) * Current (I)

We didn’t know the resistance…all we knew was volts out and current

P = E2/R
P = I2 * R
P = E * I

You got me second guessing myself… :~

Made in Mexico/China? Our new HV Wire wound resistor have bad coatings that will short out. Your fan is best bet if the heat concerns you. Those USB connectors should be good for 1 amp but 2 amp may be pushing if any quality issues.

From your link; heat, smell, smoke is normal lol. Careful with the twistys which could short both resistors to heat sink. That green pc board in not a good heat conductor, normally the resistors would be raised .25 -.5” off board.

I can’t make out your pic but 1-18650 will discharge 8 watts @ 2 amps max rating with initial 4 volts or so with 2-4ohm resistors in parallel.

Well, after a month I’m finally able to use this the way I want.
Already had some heatsink and a old graphic card fan, ordered some epoxy and a 5V fan and this is the result.

Measuring the temps was a bit more difficult but on a 2A load:
After 10 minutes with the fans off temps are 80 degrees celcius and rising. Heatsinks are not comfortable to hold (around 50/60 degrees)

With fans on, after 30 minutes the highest reading I could get was 62 degrees celcius. Heatsinks are cold to the touch.

Design: 2/10
Functionality: 8.5 /10
Peace of mind stuff won’t catch fire: 10+

I still won’t run it without some supervision, but at least I’m not worried anymore.

shorts across the wire wound resistors! That green coating/insulator is cheap sauce. The one black fan should be more than enough cooling with direct airflow. Me, myself and I would dump the heat sinks to rule out accidental shorts.

…looks awesome :slight_smile:

Do you have any potting between the resistors and the copper of the heatsink?

The aluminum heatsink is on the back side of the PCB…really not doing that much good

You want direct thermal contact with the resistor

You can easily get away with the aluminum heatsink flipped over replacing the copper fan mounted heatsink, run a bead of high temp gasket maker on the top of the green resistors, then lightly place the heatsink, let it harden, then run some more gooply goop on both sides of the original bead to build up and taper (give more surface area) the potting pulls the heat away

Either way…as I learned in the Marines “If it’s stupid but works, it ain’t stupid” :wink:

Hi Johnny,

if you can use a soldering iron maybe you could replace the resistors with something like this resistor . Then it/they could be mounted directly onto the heatsink.

Well, I guess if I would remove the heatsinks I have a higher chance of ripping the insulation. It’s all glued together and I guess it won’t come off easily. I guess the epoxy also kind of helps insulating it from the heatsink? Or is it conductive?

The alu heatsink indeed does not do much. At the very first instance, it was just to have some solid underground because I did not want to leave it on a desk or something while running, and hoped it would cool a bit. This was after a couple of second testing and noticing it became HOT.
After a couple of minutes the alu heatsink does heat up to uncomfortable levels and with the fan it does not so it might help a degree or 2. And if not, I guess it doesnt do it any harm either.
For the rest, the copper heatsink is just epoxied to the resistors. No other fillings or anything.

As for your last sentence. Definitely true.

I do have a soldering iron and some soldering capabilities, but with the usb connector and the switch to select 1 or 2 amps, don’t know if I could pull it off. Would have to wreck this whole thing to try it. But will remember it if I ever need another one or a new one. Maybe I could try to build something myself then