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power supply load changes

power supply load changes

(OP)
Hi,

I'm testing a switching supply that's 45watts at 12 vdc. I put it on a series of power resistors to test it. Set the resistors to 3.2 ohms. After running for a while the resistors got real hot. I turned it off and checked the resistance. It had dropped to .4 ohms due to the heat. How can I set the load so that it doesn't wonder around? I can guess but there's no guarantee I'll hit the right resistance.

Thanks

RE: power supply load changes

I've never known a bulk resistor to go down in resistance as it heats up. Of course, I would expect any resistor to get hot trying to burn off 45W...

Dan - Owner
http://www.Hi-TecDesigns.com

RE: power supply load changes

You can buy (or hire) electronic loads which can be configured to provide constant power or constant current depending on what you are wishing to control. Or you could make your own if you have a little electronics knowledge.

The cheaper - much cheaper - alternative would be to use a larger wattage resistor attached to a suitable heatsink so that it doesn't undergo such a large rise in temperature, in which case the temperature coefficient will be of less significance. A resistance which falls as temperature increases (i.e a negative tempco) is unusual among the common resistor types. What are you using?

RE: power supply load changes

I've turned carbon film resistors cherry red for hours with no change in resistance and melted the ceramic coating off power resistors with no change. Then I'm sure you have been far more creative than I was. Six or more 10W resistors in series or parallel would handle it with no need for a heat sink.

RE: power supply load changes

If accurately hitting 3.75A is not so crucial, I'd put a 50W automotive headlight lamp and a 0.33Ω 5W resistor in series. You'll be close. A 0.33Ω 10W resistor would stay even cooler. Either is less than two bucks.

You'll also have extra light to work by.

Best to you,

Goober Dave

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RE: power supply load changes

(OP)
Thanks Guys. Used a variac. Works good. But now I wonder how long should I test it?

RE: power supply load changes

What's the objective of the test?

Mike Halloran
Pembroke Pines, FL, USA

RE: power supply load changes

PFF - you can't use a Variac as a d.c. load! Variacs are variable transformers, typically for mains use. You will very likely cook the copper wire insulation and ruin a perfectly good Variac if you use it as a resistor. What ItSmoked has pictured is a very large wirewound variable resistor - not the same thing at all.

RE: power supply load changes

(OP)
Have been using a large variac for about two weeks now. Doesn't work real well but it does allow fine tuning. The problem I'm running into is that I can't get the unit to fail. It shut down once from over heating. After it cooled down a bit it started right up again. It's becoming difficult to collect any data to analyze. Been running it in an oven at 180F. The PS heats up to 229F runs for hours this way. Like I said so far it only shut down once. Suppose I could find an oven that gets hotter but than how do you adjust your data for the excessive heating? Suppose I can get it to fail regularly at temps over 220F at variable time intervals. Like: Shuts down after 55 minutes at 222F then again after 60 minutes at 224F and so on. I'd like to be able to adjust the shut down times to a normal operating temp of 158F then generate a weibull dist and find out how long these PS will last. Any idea how to do this? Thanks.

RE: power supply load changes

But is the shutdown an actual failure? If you can recover operation, then that's not a failure and has no reason to be put in a Weibull dist. Is the shutdown even a circuit problem, as opposed to an intentional shutdown to prevent damage to the supply? Seems unlikely, since 224ºC is awfully high for most applications.

TTFN
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