80A 5V Supply Problem - Page 1
Forum section: electronics / projects

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: March 01, 2014, 09:27:40 pm
content: 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?

author: Rerouter
author_rank: Super Contributor
timestamp: March 01, 2014, 09:36:12 pm
content: 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 * Requally at 55A your dissipating just over 300W into your (250W) Resistor?

author: calexanian
author_rank: Super Contributor
timestamp: March 01, 2014, 09:49:05 pm
content: 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.

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: March 01, 2014, 10:04:43 pm
content: Quote from: Rerouter on March 01, 2014, 09:36:12 pmNo, it was actually more like 51.

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: March 01, 2014, 10:06:41 pm
content: Quote from: calexanian on March 01, 2014, 09:49:05 pmI 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.

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: March 01, 2014, 10:08:57 pm
content: Quote from: Rerouter on March 01, 2014, 09:36:12 pmI 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.

author: c4757p
author_rank: Super Contributor
timestamp: March 01, 2014, 10:09:35 pm
content: 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...

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: March 01, 2014, 10:13:41 pm
content: Quote from: Rerouter on March 01, 2014, 09:36:12 pmSorry, the supply was tweaked to about 5.1V. I should have said that I was only going to be using at max 55A.

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: March 01, 2014, 10:15:44 pm
content: Quote from: c4757p on March 01, 2014, 10:09:35 pmYes, and that's why I am monitoring the test and only allowing a short period of time. I have seem them blow before.

author: Zero999
author_rank: Super Contributor
timestamp: March 01, 2014, 11:23:32 pm
content: Is it mounted on an appropriately sized heat sink?

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: March 01, 2014, 11:26:33 pm
content: Quote from: Hero999 on March 01, 2014, 11:23:32 pmNo, 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.

author: SeanB
author_rank: Super Contributor
timestamp: March 02, 2014, 07:51:12 am
content: 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.

author: G7PSK
author_rank: Super Contributor
timestamp: March 02, 2014, 10:53:30 am
content: 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.

author: nctnico
author_rank: Super Contributor
timestamp: March 02, 2014, 11:21:21 am
content: Quote from: Terabyte2007 on March 01, 2014, 11:26:33 pmThere 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.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: March 02, 2014, 11:31:35 am
content: QuoteThen chucking it in oil may not be such a good idea either.

author: Psi
author_rank: Super Contributor
timestamp: March 02, 2014, 11:33:33 am
content: 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)

author: VK5RC
author_rank: Unknown
timestamp: March 02, 2014, 11:54:03 am
content: 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?