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Author Topic: Computer power supply w/ mobile ham radio  (Read 2823 times)
KC2MLF
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« on: February 08, 2004, 02:58:44 PM »

I've got all these spare computer power supplies lying around, and I'm wondering...
Is it OK to use one of them for a power supply for a mobile (radio shack HTX-10) radio? The specifications of the radio says it takes 12-16V @ 5.0 A. The power supply I have puts out about 15 volts at 8 amps. Would this work? Or would it be an extremely bad idea? Has anyone ever tried this before?
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K0RFD
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« Reply #1 on: February 08, 2004, 03:31:24 PM »

There was an article in QST about using computer power supplies.  The article is in the Members-Only section at http://www.arrl.org/members-only/tis/info/pdf/0205035.pdf

Although the author of the article thought it was a good idea, there have been at least two follow-up articles in QST by other hams since the original article was published that say it ISN'T such a good idea.

Computer power supplies are pretty dirty, they require a load on the 5V output to fire up, they really have insufficient capacity on their 12V outputs, they need to have their grounds isolated from one another if you try to use the high-current 5V supplies in series, and on and on.


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K3UD
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« Reply #2 on: February 08, 2004, 03:56:30 PM »

Check with Steve, WB2WIK concerning using computer supplies to power 100 W ham rigs. Steve recently had what amounted to an op-ed piece in the technical correspondance section of QST commenting on the article already mentioned to you. In general it is not a good idea, as Steve's piece will show.


73
George
K3UD
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KZ1X
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« Reply #3 on: February 08, 2004, 04:14:07 PM »

You can get a 15 amp 13.8V supply for $25 on eBay that will work perfectly.  Don't mess around with computer supplies that can damage your rig.  Been there, done that.
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K1CJS
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« Reply #4 on: February 08, 2004, 06:44:49 PM »

First, computer power supplies being dirty?  Not necessarily so.  The newer ones are clean--they have to be to avoid corruption of data, memory and the processor.

Second, unless you daisy chain three supplies, where are you going to get the fifteen volts?  Most modern computer supplies provide + 12 volts and - 5 volts.  Any other source in the supply would not have the current capacity to do any good for radio

Third, the + 12 volt supply is just that, 12 volts.  Not 13.8, the level most '12 volt' rigs are designed to do their best at.  Yes, there is a pot adjustment in the supply to adjust that voltage, but at most you'll probably get 12.5 volts out of it.  Not quite nominal for a 12 volt rig, and if you start to draw serious current, the voltage level drops.

You would have to have at least a three hundred watt supply to even attempt the changeover with any success, but it is possible to do so--to run a low power rig, no more than 25 watts.

I have a three hundred fifty watt computer power supply that I have redone for just that reason.  It sits on the 'spares' shelf in case it is needed.  I had to actually get onto the pc board (most are pretty crowded) and change the resistor setup (the values of a couple) to get 13.8 volts out of it.  The most it can SAFELY supply is 12 amps, and not continuous duty, either.

I used it to run a fifty watt rig--at low transmit power only-- for six months before I got a better supply, one made for powering our type rigs.

To conclude, it would be OK as an 'emergency type' supply, but don't rely on one such for everyday use.  Without extensive reconstruction, it won't last too long, and just may take your rig with it if and when it goes.

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K0IPG
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« Reply #5 on: February 09, 2004, 12:03:47 AM »

Well, with no one else liking the concept, I guess I'm going to have to speak up and say that it's not that bad of idea.

Folks, we're not powering a pacemaker, here. Absolute worse case, the supply explodes in a big ball of smoke, and you have to build a new one. Because of the way switching supplies are designed, the only damage that could come to the rig is if, for some reason, the PMW IC went crazy and starting switching the transistors fast enough that the supply outputs more than 13.8v. In this case, almost EVERY switcher has a crow-bar circuit to disable the supply in an overvoltage condition (and if it doesn't, you could easily build one - a zener diode and an SCR is all it takes).

As far as the supply's ability to supply enough voltage: The supply is easily adjusted to increase the 12v output to 13.8v. The supply is regulated by a PWM IC that uses an error amplifier to measure the output voltage, and adjust transistor switching speed accordingly. If you want to increase voltage, just decrease the amount of sampling voltage being fed back into the IC (lower voltage at the IC means the supply will want to increase voltage output to meet the reference voltage). In every computer PSU I've seen, the output voltage sample is taken with a voltage dividing resistor network (one in series, one to ground), and connected to pin "1" of the extremely common TL494 (also marked "3900") PWM IC.

As far as the supply's ability to source enough current: The rating on the sticker isn't exact. There are very few components that determine the supply's maximum current capacity. The switching transistors themselves, the power transformer, and the half-wave rectifiers on the transformer secondary. There's not much you can do about the power transformer's size, but you can certainly replace the switching transistors with larger ones (they're just plain NPN's capable of high switching speeds). Half-wave rectifiers are even easier - usually a 30A rectifier is used for the 5v line, and a 15A is used for the 12v line (to save money, of course). Simply switch the two around (you won't be using the 5v output, except for regulation) and you'll have higher current capacity on the 12v line.

Although there are some problems with the use of computer PSU's, they aren't nearly as bad as some make them out to be. Get ahold of that QST article and give it a try. It explains everything in fairly good detail, and in the end, you'll have a supply that will work just fine. I've been using a converted "145w" supply (it turns out they were VERY conservative in rating it) with a 100w HF rig for about a year and a half now, with absolutely no problems. I also own a repeater, powered by a converted computer supply. Our region's packet radio node is powered by a converted PSU. And all of these work without any problems. Any problems to be cited with the use of converted PSU's are problems present in any homebrew power supply (switching or linear). With a little common sense and care, you'll be able to make a converted computer supply work just fine.

73
Dan K0IPG
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TOASTY
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« Reply #6 on: February 09, 2004, 04:21:12 PM »

i put 3 in series and get 15.4v from it.  works fine for me, doesnt even get warm when i tx at 50w on the 2m mobile hooked up to it.
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WB2WIK
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« Reply #7 on: February 09, 2004, 06:12:33 PM »

Good chat here.

I thought I might add one comment, though: While hams may think that a PC power supply automatically has a "crowbar" (overvoltage protection) circuit, that's certainly not the case.  In fact, few PC power supplies actually use a crowbar circuit; what many do use, however, is a management chip which monitors the primary bus output(s) and shuts the power supply down if some predetermined voltage level is exceeded.  Most of those chips are not electronically alterable or programmable, the preset voltage threshhold is determined within the chip with no way to alter the threshhold.

As such, if you have a power supply that you'd like to "crank up" to 13.8 or 14 volts, there's a pretty good chance you won't get it there for more than one reason.  In a sampling of current-generation ATX12V* standard 350-to-600 Watt power supplies I've tested, only one out of seven could be adjusted above 12.600 volts without shutting down. (Reason: The management chip assumes >12.6V is excessive overshoot, and won't even let the power supply turn completely on.)

While most FM transceivers will work quite well, and maybe even meet specified parameters, when operated at "only" 12 volts DC (as opposed to 13.8V), a number of multi-mode (SSB-CW-etc) rigs, including many HF transceivers, won't.  My own Kenwood TS-850S is one such example.

WB2WIK/6

(*The current standard for P4-class computer power supplies.)
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K0IPG
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Posts: 252




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« Reply #8 on: February 09, 2004, 06:45:46 PM »

Steve makes a good point about the voltage adjustment. Most of the older AT-style power supplies (which are more suitable for conversion, anyways) use a LM393 or 339 comparator to shut the supply down in over voltage condition. Depending on how it's configured, you can either disconnect Vcc from the IC, or you can ground the +12v sampling pin.

I'd suggest avoiding the newer ATX-style supplies. A good quality, high-wattage AT-style supply is much more suitable for conversion.
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