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Old 15th September 2010, 03:53 AM   #1
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Default Computer power supply

I have a few computer power supplies, a couple of which have outputs rated at +12 Volts at 20 or more Amperes. I was thinking this could work great for powering 12 Volt gear. But most 12V gear actually wants 13.8 Volts.

So I found a control to raise the voltage. When I got to about 13.4 Volts, the overvoltage protection circuit cuts in and shuts down the unit.

Can anyone direct me to a schematic diagram of a typical unit so I can change that circuit to allow 13.8 Volts? I only need a few percent. increase.
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Old 15th September 2010, 05:50 AM   #2
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There are some, but most likely won't be as yours is... Check for LM, there should be few OP amps, that are looking at output voltage for error, and 13.4v is enough, even 12 would be
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Old 15th September 2010, 06:38 AM   #3
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I raised the output as high as I could but the thing still cuts out. I put it aside and tried another one. This one works better but has lousy regulation. There was no control pot so I had to bridge resistors to get the voltage where I want it.

Yes I need 13.8V for ham radio stuff in order to get full output power. I think it's wrong but that's how they do it.
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Old 17th September 2010, 12:24 PM   #4
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Quote:
Originally Posted by bob91343 View Post
I have a few computer power supplies...
You might try two supplies, one +12 and one +5 in series (can't use just one supply due to common ground), then turn the supplies DOWN until you get to 13.8v
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Old 17th September 2010, 12:34 PM   #5
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Quote:
Originally Posted by bob91343 View Post
But most 12V gear actually wants 13.8 Volts.
Yeah, 13.8V is common in automotive equipment, but if it isn't working at 13.4 then there isn't much chance that an extra 0.4 volts will make a difference. Turning the voltage up will only increase the likelihood of the supply shutting down due to excessive current draw, if that is what is going on. It certainly won't make an appreciable difference in transmit output power, if that is what you're concerned about. Have you got a ham licence? Usually you have to learn some electronics to get one.

w

Last edited by wakibaki; 17th September 2010 at 12:41 PM.
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Old 17th September 2010, 04:39 PM   #6
macboy is offline macboy  Canada
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This page has several schematics. The overvoltage protection typically has several voltage dividers, one for each voltage, which reduces each to a certain level. If any of the (normalized) voltages exceeds a specific threshold, it triggers the protection. To increase the threshold on the +12V, you just need to tinker with its voltage divider.
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Old 17th September 2010, 06:16 PM   #7
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macboy, thanks! I will look through those diagrams to see if something comes close to what I have.

As for the extra 0.4 Volts, sure I agree that the power won't drop that much. But it seems that you can't get specified performance from ham gear unless you run it at 13.8 V. I tested a Yaesu rig recently that wouldn't put out rated power at anything less. I don't like Yaesu but I imagine the other brands are similar.

I did test this poorly regulated one on two meters (one of my very rare appearances on that awful band) and it worked well. But I am thinking more in terms of running my TS-440S with it. That radio draws serious current, so I need a good power supply.

My main concern here has to do with noise. Will I hear QRN from the switching circuits, or are these units well enough designed to avert that? After all, the receiver sensitivity runs to well under one microvolt.
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Old 17th September 2010, 06:24 PM   #8
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Another idea if you want to use two power supplies would be to take the output of each across the +12V and +5V, giving you 7V from each supply. Two of those in series minus the little bit that computer power supplies tend to run under would probably be at almost exactly 13.8V.
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Old 17th September 2010, 07:20 PM   #9
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I don't know if it would work to charge the 5 Volt sources. That's what would happen with the bucking of the 12 and 5 to get 7. I'd have to try it. But two power supplies would be defeating the purpose of saving space over an analog power supply.

Anyway, there are some options here and I will play with it when the mood strikes. Actually I have several power supplies here I don't use, each capable of driving my rig, so it's not that I need more. I just hate to see these units go to waste when they are really rather capable.
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Old 17th September 2010, 08:19 PM   #10
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You have a Kenwood transceiver like that, presumably an antenna or two and you won't shell out for a decent PSU or a battery and charger?

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