80A 5V Supply Problem

[Terabyte2007]

Ok, this is my scenario, I have an 80A 5V DC supply which I am only using around 55 Amps. I went and purchased new 8 Gauge low resistance power cable from my local electronics shop. I am using high quality spade lugs to bolt to +/- output which are connected to the 8 gauge cable and then to my test load which is a 250W Dale .1 Ohm 1% resistor. I have got only .00137 Ohms of resistance in the cables, checked with my 4-wire Kelvin on my Agilent DMM. The power dissipation is mostly through the Dale load resistor, but I am getting quite a bit of heat in the cables still. By my calculations, I should have around 4.932V across the load, and it is about 5V across the load. The rest dropped over the cable. Now, I realize that their is still a fair bit of current flowing through the wire, but my power dissipation over the 6 feet of cable at .00137 Ohms should be relatively low. I think, but am not certain the heat is conducting back through my load which is damn hot because the temperature of the cables seem to be hotter towards the load. So, one would speculate that the heat from the load is traveling back up the cable but it's pretty warm even at the supply side of the cable.

I only ran the test for about 10 minutes, that's all my load could handle because it was running at full potential.

My question, Could some of this heat be radiating back up the cable that far?

[Rerouter]

at that resistance both of your cables are dissipating 4.2W and even on 8 gauge wire will have a very noticeable heating effect,

P = I^2 * R

equally at 55A your dissipating just over 300W into your (250W) Resistor?

[calexanian]

If you are really concerned about the wire resistance, find some welding ground wire. 1 gauge is typical and most welding supply shops carry it. I just ordered some 4 gauge online for an induction heater and it was not all that expensive. Also good jumper cables can be a donor.

[Terabyte2007]

at that resistance both of your cables are dissipating 4.2W and even on 8 gauge wire will have a very noticeable heating effect,

P = I^2 * R

equally at 55A your dissipating just over 300W into your (250W) Resistor?

No, it was actually more like 51.

[Terabyte2007]

If you are really concerned about the wire resistance, find some welding ground wire. 1 gauge is typical and most welding supply shops carry it. I just ordered some 4 gauge online for an induction heater and it was not all that expensive. Also good jumper cables can be a donor.

I was trying to get away with a smaller gauge wire, but it looks like I will have to use something like 1 or 4 gauge welders.

[Terabyte2007]

at that resistance both of your cables are dissipating 4.2W and even on 8 gauge wire will have a very noticeable heating effect,

P = I^2 * R

equally at 55A your dissipating just over 300W into your (250W) Resistor?

I knew I was going to get some heat, I was hoping to minimize it. I was also cheating and trying to get away with a smaller gauge wire. It looks like it will work, it just gets warmer than I would like.

[c4757p]

Does your resistor look like this?



They're not as overload-friendly as their bulk might suggest... quite capable of blowing their insides halfway across the room...

[Terabyte2007]

at that resistance both of your cables are dissipating 4.2W and even on 8 gauge wire will have a very noticeable heating effect,

P = I^2 * R

equally at 55A your dissipating just over 300W into your (250W) Resistor?

Sorry, the supply was tweaked to about 5.1V. I should have said that I was only going to be using at max 55A.

[Terabyte2007]

Does your resistor look like this?



They're not as overload-friendly as their bulk might suggest... quite capable of blowing their insides halfway across the room...

Yes, and that's why I am monitoring the test and only allowing a short period of time. I have seem them blow before.

[Zero999]

Is it mounted on an appropriately sized heat sink?

[Terabyte2007]

Is it mounted on an appropriately sized heat sink?

No, it is actually not. That's why I was speculating heat transfer back up the cables adding to the power dissipation through the cables. The load is only a test simulation.

[SeanB]

Drop the resistor into a bucket of engine oil to cool it better.  You probably would be better off doubling up the leads as well to get them running cooler. Do not be tempted to use jumper leads, many are very thick but the cabling inside is only 2mm at best. Welding cable works, though I have a reel of 00 gauge PTFE wire I use at really high current, it is rated for 30 seconds at 8kA, and makes really good jump leads, though the clamps tend to burn. I use the reel as a convenient 50kg foot stool.

[G7PSK]

I would not bother with the oil its far too messy use some water at 5 volts even tap water is not going to conduct very well if you have it de-ionized water is better otherwise put a tiny drop of washing up liquid into the water, you don't even need a bucket a jar will do as it will take a long time to boil the water and you will never get hotter than a 100 deg C.

[nctnico]

Is it mounted on an appropriately sized heat sink?

No, it is actually not.

There is your problem. Mount it on a big heatsink preferably one with a fan. Dissipating 250W into a 100W resistor is not a good idea. Better use several in parallel.

[HackedFridgeMagnet]

Dissipating 250W into a 100W resistor is not a good idea.

Then chucking it in oil may not be such a good idea either.

[Psi]

high current cable is more or less a solid block of copper, it will conduct heat really well even over long distances. (just takes a while to get there)

[VK5RC]

I suspect it is heat transfer up the cable, if you have a couple of temp probes I would measure the temp of the cable say 2cm from the resistor  and  mid way along the cable, the temp profiles will give it away.
Those power resistors, don't they have to be derated as they get hotter, so if working they often can only handle a percentage of their rating power?