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Author Topic: Power Supply Transformers Voltages?  (Read 519 times)
KC2MMI
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Posts: 623




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« on: September 01, 2004, 01:07:23 AM »

Someone jog my memory?

If I want a power transformer for a 12v nominal DC supply (meaning, 13.8 to 14.4 after regulator losses, and a solid 14.4 so it can also charge a deep cycle battery) and I get a power transformer rated 12VAC on the secondary, that 12VAC will translate into about 12*1.4= 16.8VDC after a bridge rectifier?
Or about 16.4 after the rectifier losses as well?
And something like 14.9 max then, assuming something like a 1.5V drop needed for a common adjustable regulator to work?

I've got an old high-capacity homebuilt power supply that uses an LM317 with eight 150W bootstrap transistors, that was originally built with a transformer chosen because it was "available, period" and good enough at the time. It saturates and folds back down too close to 13V to make a good charger, or high capacity supply, and I'd like to address that now.

Assuming of course, the new transformer is rated 12V on the secondary at the full load I'll be designing for. Not intending to pull more than 20A, but designing for 30A.
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W8JI
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« Reply #1 on: September 01, 2004, 06:40:48 AM »

You seem to have the basic idea but are low on diode drop and forgetting transformer ESR.

Diodes are NOT stable in forward bias. At low current they drop less voltage than high current, the drop comes off the peak voltage, and if it a bridge remember it is two diodes in series. If it is a grounded CT you are wasting transfomer size, but there is only one diode in series with the secondary.

As for voltage, transformer ESR causes regulation issues. Even a very stout transformer in a capacitor input supply sags 10% or so on peaks and some can be 30%!

This all has to be added in, along with the required headroom for the regulator pass transistors to function properly (normally a few volts headroom is required at full load).

73 Tom
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KZ1X
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« Reply #2 on: September 01, 2004, 09:46:01 AM »

Another 'trick' is to use a second, independent power supply to power the regulator circuitry of the high current supply.  I do that on one of my 75 amp units, works great.  In this way, you need less 'overhead' to run the regulator.  My pass outputs are old-fashioned 2N3771 TO-3 jobs on a fan-cooled heat sink, and I run 6 in parallel with emitter equalizing resistors.

On my 250 amp 13.6V supply, I actually regulate the PRIMARY, with a souped-up chopper-type lamp dimmer.  The chop frequency was adjusted so the inductive mass of the HUGE transformer secondary removes the 'buzz.'  The output is a FWB into a half-Farad, which more than makes up for the regulation 'latency'.  Neat!
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KZ1X
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« Reply #3 on: September 01, 2004, 09:46:18 AM »

Another 'trick' is to use a second, independent power supply to power the regulator circuitry of the high current supply.  I do that on one of my 75 amp units, works great.  In this way, you need less 'overhead' to run the regulator.  My pass outputs are old-fashioned 2N3771 TO-3 jobs on a fan-cooled heat sink, and I run 6 in parallel with emitter equalizing resistors.

On my 250 amp 13.6V supply, I actually regulate the PRIMARY, with a souped-up chopper-type lamp dimmer.  The chop frequency was adjusted so the inductive mass of the HUGE transformer secondary removes the 'buzz.'  The output is a FWB into a half-Farad, which more than makes up for the regulation 'latency'.  Neat!
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WB6MMV
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« Reply #4 on: September 01, 2004, 10:49:00 AM »

If you are planning to replace the original power transformer and still want to use the LM317 and associated pass transistors, then I would suggest finding a transformer with a 16-17 volt secondary with at least a  20 amp rating.  This should give you plenty of "headroom" for the secondary voltage to sag when you load it down.  As Tom has pointed out, there will be diode drop and transformer ESR which all affect the basic ability of the supply to regulate effectively.  As also pointed out, you can use a transformer with two secondaries; one handling the power and one to power the regulator circuitry.  This is the approach that the Astron power supplies use.

I have designed supplies using transformers with secondaries as low as 15 volts, but to get good regulation its easier to add some "headroom" and find one with 16-17 volts.  Fair radio,Nebraska surplus sales and RF sales have transformers that fit these categories. At 16-17 volts you can use a single secondary to power the regulation circuitry and the power section.

 Besides checking the voltage and current ratings of the transformer, one easy way to look for these types of transformers is to check their weight.  A good transformer in this category should weight at least 10 lbs or more. Greater weight is a good analog for power handling ability.

Have fun
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KC2MMI
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« Reply #5 on: September 01, 2004, 04:17:57 PM »

Thanks, all, interesting points. On the sag...I don't remember and don't want to look right now, but the twin filter caps are each the size of a small forearm. Total of either one or two farads between them. (Parlez-vous overkill?<G>) Well, that's how surplus goes sometimes.<G>

Sag? Na, the voltmeter lamps and pilot (nice old green jewel type) that I use as a bleed take a couple of minutes to die, after the switch is off.<G>
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WB6MMV
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« Reply #6 on: September 01, 2004, 05:52:55 PM »

When I refer to sag, the way to test it is start pulling a 25-30 amp continuous load on the supply and check the output AC voltage at the transformer secondaries then compare that to the no load voltage.  That will illustrate the sag.  You can also measure the output voltage under load to determine the regulation.  A well designed linear supply should be able to pull less than 1% load regulation at the rated load as measured at the supply output terminals.

On the filter caps for  a 20-30 amp DC supply you can usually get away with 75-100K uf to provide adequate filtering. Good luck with your project
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N8FVJ
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« Reply #7 on: September 03, 2004, 08:50:29 AM »

A transformer of approximately 17-18 ac volts with a bridge rectifier is required for the pass transistors. The rectifiers will drop 1 volt and at least a 3 volt 'window' is best for clean DC output.

The regulator side should have a few volts higher DC voltage for good dynamic response when driving the pass transistors.

Look at an Astron power supply schematic & one will find the design mentioned above. Homebrew is fun, however if parts are required to be purchased, it is less expensive to buy a used Astron RS-20.
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KC2MMI
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« Reply #8 on: September 03, 2004, 01:19:12 PM »

Thanks, James. The parts are all here, the box was built 25 years ago, I just got diverted, stopped by CW and never pursued the need for tx power back then.

That box taught me how critical the resistance of wiring is when you're bootstrapping eight transistors in parallel. Make one wire ust a titch longer...and that transistor plays the part of burnt offering. (And that was following the mfr's recommendations!)

So, no big deal, just a transformer swap planned. Otherwise I'd probably look for an Astron30 switcher or something similar, way smaller and more elegant but they don't have the nice jeweled surplus power lamp.<G>
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WA1RNE
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« Reply #9 on: September 03, 2004, 03:39:26 PM »

 
 There's a lot more that goes into Linear power supply design than meets the eye:

 Transformer:  Depending on the rectifier configuration, the transformer's RMS current rating must be selected accordingly. A simplified guide for specifying the required RMS rating is:  

 Full wave bridge: Full load DC current x 1.8

 Full wave center tapped:  Full load current  x 1.2

 Others have touched upon regulation. If you live in an area where the input line voltage runs in the 110 vrms range instead of 117-120, I would also take line regulation into account when specifying the transformer's secondary voltage - go up a bit to compensate for this. But remember, if you go too high, any extra watts must be dissipated in the pass transistors which could stress them thermally.

 Rectifiers:  Need to have the required peak surge current rating during the short time interval the filter capacitor is being charged at full load current. This can be in the HUNDREDS of peak amperes and is normally designated as Ifsm.

 Capacitors:....can be a big deal which type you specify. Often times the Ripple Current rating is ignored and can result in disaster. If you plan to run this supply continuously, this needs to be considered. Bottom line; sometimes it's better to parallel 2 or 3 big electrolytics to spread out the ripple current requirements instead of using 1 large capacitor.

 Here's a great guide that takes you through the design with a basic straightforward approach without all the complex analysis: refer to pages 35-38:

 http://www.belfuse.com/Data/DBObject/signalcatalog.pdf

  Good luck, 73....
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