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Old 14th November 2011, 01:56 PM   #1
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Default Tube Power Supply Dummy Load

I need to load up a power supply design to see what the output will be under load. Any ideas on a safe way to do it? Planning on using a 240v 200ma High side and 9v 4 amp low side Power Transformer. Also plan on solid state rectifiers and a 6v DC Regulator.

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Old 14th November 2011, 03:17 PM   #2
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I generally use wire wound power resistors. About 200 watts, but 100 or even 50 watts will do. Just use basic ohms law to get the resistance needed (R=EI). Adjustable resistors are handy. If working with clip leads, please be careful. I also have an electronic load that uses transistors should I feel like being hi-tek.
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Old 14th November 2011, 03:32 PM   #3
DF96 is online now DF96  England
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Incandescent light bulbs? Slight snag is that they change resistance with temperature, so watch out for initial surges.
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Old 14th November 2011, 03:49 PM   #4
TheGimp is offline TheGimp  United States
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I built a FET CCS dummy load with a pot for the current set resistor. I can load a power supply up to 500V or up to 200mA, but not both at the same time as my heat sink is not big enough. It is good for about 50W as is.
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Old 15th November 2011, 12:52 AM   #5
g(f(e)) is offline g(f(e))  United States
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I picked up a big wire wound resistor with a slider on it at the Ham fest awhile back. It works great. I set the resistance based on the current for the circuit of course, as HollowState mentioned. Ham fests are a good source for these kinds of items.
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Old 15th November 2011, 01:37 AM   #6
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Well only thing I have is a few low ohm 5 watt wire wound resistors to try. I do like the idea of a light bulb but don't have access to any 220-240V range bulbs state side. Can I parallel several together to get more load? I don't see it realistically drawing more than 80-90ma.

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Old 15th November 2011, 01:42 AM   #7
Pano is offline Pano  United States
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Two in series is what you want if using 120V bulbs. I'd say about 20W bulbs.
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Old 15th November 2011, 01:45 AM   #8
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Thanks, as soon as Edcor Transformer shows up I'll be giving it a try.
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Old 16th November 2011, 04:06 PM   #9
jrenkin is offline jrenkin  United States
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I am working on the same issue. My concern with light bulbs is initial current draw. I need roughly 400v and 150mA, or 2700 ohms, 50w. This would be 4 x 25w bulbs perhaps.
The problem is, that is under operating conditions and the cold measured resistance of each bulb is only 48 ohm, so initial current draw when turned on is 2 amps! Sure to blowy poor rectifier tube and transformer probably.
Am I figuring something wrong? Or should I just go buy appropriate resistors and do it safely...
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Old 16th November 2011, 05:52 PM   #10
DF96 is online now DF96  England
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You can reduce the problem by using extra bulbs. If two in series are needed at full power, try three or four in series. They won't glow so much so won't change resistance so much. And/or use some ordinary resistance in series too e.g. half and half. Any PSU ought to be able to withstand a temporary overload of twice the normal current for a few seconds.
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