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Author Topic: Power Supply  (Read 313 times)
KF6FOD
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Posts: 7




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« on: August 18, 2004, 07:17:02 PM »

Can you hook-up two power supplies together ?
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WB2WIK
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« Reply #1 on: August 18, 2004, 07:42:54 PM »

Sure you can, but you might damage them when you turn them on!

The answer to this is, "It depends on exactly what the two power supplies are, and whether you're trying to connect them in series or in parallel."

WB2WIK/6
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KF6FOD
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« Reply #2 on: August 18, 2004, 07:53:27 PM »

Sorry, I should have added this in the original question....can you hook-up two simular power supplies together in parallel to handle a device that requires greater amperage ?  Thanks to everyone in advance.
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WIRELESS
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« Reply #3 on: August 18, 2004, 07:55:00 PM »

You can usually hook power supplies up in series if they have a floating negative not connected to chassis ground.  I hooked these kinds of supplies in parallel too and they usually will "sync" voltages but they also may not and could smoke.
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KA5N
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« Reply #4 on: August 18, 2004, 08:14:52 PM »

The problem with paralleling power supplies is that each supply checks (compares) the output voltage and regulates to that voltage to maintain near constant terminal voltage.  If you simply hook two outputs together one supply will provide most of the current (it will regulate to a higher voltage than the other).  Another problem is that the output voltage of one supply will be fed INTO the other and may cause major distruction.  Ideally one would have two identical supplies with exactly the same output voltage set point.  The use of low forward drop diodes in each supplies' output would tend to equalize the load and prevent meltdown.  For a heavy load that would mean some BIG diodes.  
Another method would be to use low ohmage resistors in series with each supply to equalize the load.  The same sort of system is used on many 20-50 amp supplies with multiple series pass transistors (see any 20, 35 or 50 amp Astron). This method requires that the supplies have to be carefully matched for output voltage.  
The other problem with suppling high current at low voltage is the drop in the cabling.  It doesn't take much resistance to drop one or more volts at 50 amps drain.  A good system would sense the voltage at the load terminals to offset cabling losses.
In a word, it is possible to parallel suppllies but more likely than not you will have big problems accomplishing it.  Trade your two smaller supplies for one big one.
Allen
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KF6FOD
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« Reply #5 on: August 18, 2004, 08:23:25 PM »

Thanks Allen...I'm going to take your advice and just get a bigger supply...much easier to deal with.
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N6AJR
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« Reply #6 on: August 18, 2004, 09:46:28 PM »

some of the larger astrons have a port in the back you hook up when syncronizing 2  PS together, and some high current linears ( the als 500 by ameritron) has a link in the plug which lets you feed it with 2 seperate supplies . each supply supports on side of the final amp... it works..
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WB2WIK
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« Reply #7 on: August 18, 2004, 10:10:32 PM »

Before you give up, you might want to read this, anyway:

I designed power supplies for about 20 years, for a living and to feed my family and hobbies (!) and it's really impossible to say whether you can parallel power supplies without knowing exactly what those power supplies are.  If they are identical, and have the ability to remote sense (as most decent power supplies do), you can usually parallel them without worry by simply using remote sensing and a pair of OR'ing diodes, which are very inexpensive.

What, exactly, are those two power supplies?  Do you have their schematics?

WB2WIK/6
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N8FVJ
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« Reply #8 on: August 19, 2004, 10:14:56 AM »

Use high current isolation diodes, one on each power supply + terminal. The diodes have a one (1) volt drop, so adjust your power supplies to 14.8 volts output or live with 12.8 volts output.
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WB2WIK
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« Reply #9 on: August 19, 2004, 11:34:27 AM »

N8FVJ, the diode isolation suggestion is a good one but doesn't guarantee much except that the output of one power supply won't back into the other one.  As far as "load sharing" goes (that is, the output currents from both supplies being about equal, and providing about equal power to the load), the diodes do absolutely nothing in this regard.

It's not only possible, but pretty likely that if two separate power supplies are used, even with isolation diodes (a diode OR gate), they will *not* load share, and one power supply might have to provide 90% of the load current while the other one loafs along at 10%.  Or maybe even 100%/0%.  This depends on lots of factors, including very critical adjustment of each PSU output voltage, measured *after* the OR gate, and under load.

With critical adjustment and a good diode OR gate, using diodes that thermally track each other pretty well (they should be mounted on the same heat sink, at least, and be of the same type diode), this "passive load sharing" arrangement is probably good for sharing within about 20% or so.  To get it closer than that, you really need active load sharing, which is additional circuitry not contained in most amateur power supplies -- although it is usually contained in "N+1" redundant power supplies used in higher-end computers.  The circuitry's not expensive, just rarely used in stand-alone power supplies.

WB2WIK/6

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AA4PB
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« Reply #10 on: August 19, 2004, 04:51:39 PM »

The diodes act like switches, connecting the load to one supply or the other. Unless you have some method of keeping the two supplies at *exactly* the same output voltage, all of the load will be supplied by the one with the higher voltage.

Diodes work with batteries because batteries have a higher internal resistance (i.e. not regulated). The higher voltage one will drop until it exactly matches the other and the loads are balanced.

There are commercial power supplies that are designed with interconnecing regulators so that the voltages track each other. That's not available on the typical "ham" type power supply.
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