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Old 14th December 2012, 09:17 AM   #11
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Just a Pyramid 52 amp power supply, looking for a 100 amp no luck yet.

I have 2 8D batteries I was thinking of running in parallel. That would be about 3600 cranking amps.

Why would I damage the resitors?

Or one of these would do inline with my power supply?

http://www.ebay.com/itm/Northstar-SM...ff6891&vxp=mtr
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Last edited by seankane; 14th December 2012 at 09:21 AM.
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Old 14th December 2012, 09:23 AM   #12
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Damage from driving too much power into them. With only a 52 amp supply, you won't be able to drive more than about 600w into them. They could easily take that without fans.

You should realize that the dummy loads will become VERY hot when driven for long periods of time. They can even melt the solder off of the terminals. I'd suggest using high-temp wire. No matter what type of wire you use, you should prevent the insulation from touching the resistors. When you wrap the wire onto the lugs, wrap it twice to prevent it from pulling out if the solder melts.

Batteries are OK for short term testing but they become a pain after a while. Add to that the fact that they're only good for a limited period of time and have to be replaced. They also don't start to supply significant current until the voltage has dropped to about 12v. A good power supply is much better. If you can't find a large supply, try to find three 5v power supplies pulled from servers.

Last edited by Perry Babin; 14th December 2012 at 09:26 AM.
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Old 14th December 2012, 09:27 AM   #13
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OK makes sense. If I run the 2 8D batteries inline with my power supply, what do you think that would gain me as far as wattage I would be able to drive.

The ones I have are about 1500 CA each so 3000 CA

With 4 300 watt resistors in parallel for a 1 ohm load shouldnt they handle the 1200 watts?

Would my supply keep up with the batteries or would I have to use a battery charger.
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Old 14th December 2012, 09:30 AM   #14
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32 x 3,9 R /50w Dale, 2 x Papst 88x88mm

4 x 8R
2 x 4R
1 x 2R

Click the image to open in full size.

Click the image to open in full size.

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Old 14th December 2012, 09:41 AM   #15
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In parallel, they would easily be able to take 1200w. If you test with music and use fans to keep them cool, I'd expect that they would easily take 2000w.

The power supply would be stressed with any load that's greater than its rated output unless you set the voltage on it below 12v. Above 12v, it will supply 100% of the current until the voltage drops to below about 12.5v. At that point it's trying to supply the amp AND charge the batteries.

A battery charger would help but when set to the higher current modes, they typically produce a lot of hum (not really important when using dummy loads) and can cause the voltage to spike to near 20v.

I mentioned the 5v supplies. The attached photo shows one example. They are connected in series and set for equal output totaling 13.5v.
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File Type: jpg eqswitchingdcpowersupplies.jpg (97.6 KB, 91 views)
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Old 14th December 2012, 09:44 AM   #16
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Any ideas where to find these supplies? I have no idea what they are called or how to look them up?
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Old 14th December 2012, 09:48 AM   #17
AndrewT is offline AndrewT  Scotland
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A test load should remain at known resistance.

If your dummy load needs to be at that known test load resistance, then it must be cool.
The wire must be cool.

That is very different from using a big sink to get rid of the heat from an overheated wire.
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Old 14th December 2012, 10:04 AM   #18
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I can't find any right now. These were pulled from mainframe computers. They still show up regularly on eBay.

One of the cheapest options may be to get a rack and 4 of the Pyramid supplies that you have now. You'd likely have to use two mains circuits to use all 4 to their limit.
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Old 14th December 2012, 10:33 AM   #19
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Quote:
Originally Posted by Perry Babin View Post

I mentioned the 5v supplies. The attached photo shows one example. They are connected in series and set for equal output totaling 13.5v.
I have the same:

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

With pot in both PSU change to 2x6 = 12V DC.

Click the image to open in full size.

Regards zeoN_Rider
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Old 14th December 2012, 10:41 AM   #20
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The mainframe supplies are generally rated for 100+ amps of current but it's the same principle.

If 12v is enough, you can use a standard computer power supply. The larger supplies like the CMPSU-1200AX can supply a bit over 100 amps from the 12v rails. Some can supply over 150 amps at 12v.
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