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1-10 of 14 messages
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reduce 40 volt transformer output
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by TOMCAT1 on November 5, 2009
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hi fellas. i new here and wanted to find out some info about a transformer i have. i have built a couple of dc power supplies before for a radio i have. some its not something new to me. i've got a hold of (2)40 volt,8 amp transformers. i have been trying to build one for a new power supply, i'm putting together. i have a 25 amp, 100 volt piv rectifier already hooked up to the tranformer, the problem is i can't get the voltage down enought(at least 20 volts) to where i might could finish out hooking up the caps and voltage regulator for a complete 13.8 or variable power supply. alot of my problem is i'm not experienced enough to figure this out. lol. i hope i have made sense to you guys about this and provided enough info. anyways any help would be appreciated as i'm trying to teach my onnself. thanks alot.
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RE: reduce 40 volt transformer output
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by N5LRZ on November 5, 2009
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Please direct yourself to the ARRL Handbook under the section called Power Supplies. You should find several basic power supply circuits in it that you can easily modify if you need to alter the current and or voltage.
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RE: reduce 40 volt transformer output
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by N5LRZ on November 5, 2009
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PS Please make sure that you put voltage regulation in the circuit if you have not yet done so already.
This is an important feature.
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RE: reduce 40 volt transformer output
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by WB6BYU on November 5, 2009
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Do the transformers have a center-tap on the secondary?
If so, ground the center tap and run each side of the
secondary through a SINGLE diode to a common output and
you should have, in effect, a 20 volt winding good for
more than 8 amps.
Without a center tap the rectified DC voltage out of the
transformer should be around 56V. Reducing that down to
13.8V will waste about 3/4 of your power in heat. That's
probably tolerable for a 1 amp supply, but not if you
are trying to draw the full 8 amps out of it. You
really want about half that output voltage - or a bit less -
to regulate down to 13.8.
One way to do that is to use both transformers with the
primary windings in series across the AC line. That puts
half the voltage across each primary, so the output
voltage is half of what it would be otherwise. Then
you can put the secondaries (CAREFULLY!) in parallel
for higher current capacity.
Or you can drive the inputs with a Variac.
Or you can disassemble the transformer and rewind the
secondary with half the number of turns but using
larger wire to increase the current capability.
There are other methods - series dropping resistors (or
lamps) in either the input or the output, or even a
series capacitor (which will reduce the output voltage
without dissipating as much power.) These require some
further calculating based on the allowable input voltage
to the regulator and the maximum current draw of the
supply. But overall I think the best approach is to
reduce the output of the transformer, either by using
the center tap or reducing the voltage across the
primary winding.
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RE: reduce 40 volt transformer output
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by TOMCAT1 on November 5, 2009
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sorry, forgot to mention. the transformer has (4) wires coming out the output (dc) side of it. one set of wires is 40 volt and the other set of wires is 10 volt. thanks for replies so far.
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RE: reduce 40 volt transformer output
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by WB6BYU on November 5, 2009
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In that case you can put the 10 volt winding in series
with the 40 volt winding (presuming it has sufficient
current capacity.) The result will either be 30 or 50
volts AC, depending on whether the windings are in phase
or out of phase. But 30 volts AC (which would give around
42 volts out of the rectifier) is still pretty high for
a linear regulator.
There are some commercial DC-DC switching converters that
are designed to run from a 48V bus and deliver +5V, +12V,
etc. These are often used in equipment with multiple
racks or bays, so each board provides its own required
voltages from the main power bus. (The higher voltage
reduces the wire gauge required to distribute power
through the equipment.) When you've got that much DC
headroom above the desired output voltage, a switching
regulator of some sort should be considerably more
efficient than a linear regulator.
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RE: reduce 40 volt transformer output
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by VE3EFJ on November 5, 2009
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To put it simply.... a 40 volt secondary transformer is impractiacal to use to make a 13.6 volt regulated supply using a linear regulator. There's simply too much heat being dumped into the heat sinks - like around 250 watts of heat to throw away at an 8 amp load. Yes, you can do it, but the energy efficiency is very bad. In engineering terms... energy efficiency sucks.
BUT. You ***did*** say you had 2 of these transformers and they have secondaries of 40 and 10 volts. If you were to use both transformers and wire the 10 volt windings in series, the 20 volts would still be a little high, but perfectly workable. Thats assuming that the 10 volt winding is good for 8 amps or more. Now we're dealing with a no load voltage of 28 volts minus the voltage drop in the rectifiers. Still high, but "workable".
Point? A 40 volt secondary is completely impractical for a 13.5 volt linear supply. Wired in bucking, the 10 volt winding will provide a 30 volt secondary... still impractical. Take 2 transformers with 10 volt windings and series connect and the 20 volt secondary becomes workable. Its still "high", but not too far out of line. A stiff transformer that would provide about 16 VAC would be close to ideal.
Now.... If you have all the parts sitting around and you have the will and you think this would be a fun project, then by all means give it a shot. Keep your eye on "reality" and be aware that an 8 amp 13.6 volt supply can be purchased for "around" $100.00. Just a guess on pricing but I'm pretty close, I would think.
Me? 13 volts at 8 amps seems to be a lot of trouble at this level. It wouldn't be a bad project, but the transformers that you are working with are far from ideal.
You might believe otherwise, and in that case, go for it.
Wayne
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RE: reduce 40 volt transformer output
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by WX7G on November 6, 2009
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If the transformer windings are isolated - the 10 V and the 40 volt are isolated - and both can handle 8 amps you can construct a 17 volt, 8 amp transformer.
Connect the primary of xfmr 1 to 120 VAC. Connect the 40 V output of xfmr 1 in series with the primary of xfmr 2. Connect them so they are out of phase. Connect the modified xfmr 1-2 to the 120 VAC line. It now receives 80 VAC. The 10 V output of xfmr 2 outputs 6.7 volts. The 10 V output of xfmr 1 outputs 10 volts. Connect them in series to obtain 16.7 volts.
Now you have a reasonable voltage for your 13.8 VDC regulated power supply. It will be good for around 4 amps DC continuous and more intermittently. Between transformer loading, diode drop, ripple on the output and headroom for the regulator it might come out a bit low on voltage. You might have to set the regulator for 12 VDC. You won't be able to charge lead acid batteries with it but you will be able to power a transceiver.
All of this assumes that the transformer secondary windings have sufficient insulation to allow for the normal AC line transients to 1500 VAC to GND. Intalling appropriate MOVs line-to-line and line-to-GND will limit transients to 400 volts or so.
Would I do such a thing? Yes and I have.
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RE: reduce 40 volt transformer output
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by WX7G on November 6, 2009
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Or connect the primarys of both xfmrs to 120 VAC. Connect the 10 V secondaries in series. You now have a 20 volt, 8 amp transformer. What if the 10 volt secondaries have a low current rating or you want higher current?
Connect the primary windings in series. Each xfmr sees 60 VAC. The 40 V secondaries output 20 volts. Connect them in parallel for 20 volts at 16 amps.
If the transformers do not have the identical number of windings there will be circulating current in the secondaries. You can measure this with an AC ammeter a couple of amps should not hurt anything.
Or separately rectify each transformer. Connect the outputs of the rectifiers in parallel. No circulating current problem.
Now take your 20 volt, 16 amp transformer and connect it to the series pass regulator circuit. It might have to drop 10 volts. Size the transistors and heatsink appropriately.
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RE: reduce 40 volt transformer output
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by TOMCAT1 on November 6, 2009
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hi, WB6BYU & others. i did like you said and put the 10 volts in series with the 40 volts. i came out like you said, but with 30 volts coming out of the rectifier. i was wondering if i might could use a variable pot to lower the voltage? if so what size do you think, or would this be the practical way to go. i have read the other replys and i can see i will be learning alot here. the stuff they are talking is starting to sink in. thanks gentlemen.
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