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Old 8th January 2008, 11:49 PM   #1
jn2630 is offline jn2630  United States
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Default Transformer hookup question.

I am building a power supply (click here for schematic ), but I'm not sure if I have the transformers I need. The two I have do not have a center post, just two wires in, 2 wires out. Can this work? What would I do differently?
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Old 9th January 2008, 01:41 PM   #2
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The circuit will work without the transformer center tap. A bridge rectifier is being used, which does not require a transformer center tap, and there is no law that says the center of the heater must be at dc ground potential. In fact for minimum hum you may want potentials other than 0 on the the heater.

What I would do is have the keep the capacitors connected to ground as shown, but break the connection between heater centers and ground. Replace with a 100 ohm pot, ends of the pot to ends of the heaters, and center of pot to ground.
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Old 9th January 2008, 02:11 PM   #3
billr is offline billr  New Zealand
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hello

what you have there is two filament transformers wired back to back. So presuming that you are in the USA, what you will get out is roughly 115V from the second transformer.

That arrangement of caps and diodes is a voltage doubler.

The whole idea being that 115V when rectified given a fair wind, would only give you about 150V DC at the far end of the PSU filters. Not much cop really, hence the voltage doubler.

The first transformer has to be rather more substantial than the second as it has to feed the heaters as well.

Also remember that the VA ratings of either transformer should not be exceeded.

hope that this helps.

Kind regards

Bill
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Old 9th January 2008, 02:13 PM   #4
billr is offline billr  New Zealand
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In fact now that I look at it again, normally a voltage doubler uses two diodes and associated caps. THis has three, so I suspect that it is a voltage tripler!
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Old 10th January 2008, 12:18 AM   #5
jn2630 is offline jn2630  United States
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If I connect the centers to ground instead of a 100k pot, what will be different?

Also, the parts list says the .01 uf capacitors paired with the diodes are optional. Is this true, because my .01 uf capacitors aren't right.

also x2, the 1n4007 and 1n4002 diodes I have are rated at 100v, is this a problem?
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Old 10th January 2008, 01:48 AM   #6
billr is offline billr  New Zealand
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Quote:
Originally posted by jn2630
If I connect the centers to ground instead of a 100k pot, what will be different?

Also, the parts list says the .01 uf capacitors paired with the diodes are optional. Is this true, because my .01 uf capacitors aren't right.

also x2, the 1n4007 and 1n4002 diodes I have are rated at 100v, is this a problem?
these little caps, 100nf, really are there to suppress switching noise in the diodes. I've never seen them used in this way, so I wouldn't bother about them. They usually come in 100nf at 630V those would be ok.

You need to use the IN4007s as these are rated at 1000PIV.

kind regards
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Old 10th January 2008, 02:29 AM   #7
jn2630 is offline jn2630  United States
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The parts list (click for list) says to use 1n002 or better for d4-d7. Shouldn't they be alright?
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Old 10th January 2008, 02:45 AM   #8
billr is offline billr  New Zealand
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Quote:
Originally posted by jn2630
The parts list (click for list) says to use 1n002 or better for d4-d7. Shouldn't they be alright?

I wouldn't, just looked at the specs and the 1002 only has a PIV of 100V, not high enough, mind you, what's the output of the transformer?

they don't cost very much, just use the 1007s

bill
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Old 10th January 2008, 01:12 PM   #9
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Quote:
If I connect the centers to ground instead of a 100k pot, what will be different?
Short answer : no difference to BASIC circuit operation.

Long answer :
It is a 100 ohm pot, not 100K ohm, that I was refering to to reduce hum. To be simpler, then yes, you can forget about the pot and just connect the heater centers to ground. This will work fine with or without a transformer center tap.

Or, you can decide not to connect the heater center taps at all, and instead connect one or the other (but not both) of the other heater connections to ground.

Or you can connect the heater to a dc voltage etc etc etc .....

It makes no difference to the basic operation of the circuit, it just changes the heater to cathode voltage. And you may not need to worry about it, and just ground whichever part of the heater you like.


But, there may be a certain amount of power supply hum coupled into the circuit from the heater to the cathode. And to minimize it you can connect a so called hum pot, the ends of the pot connected across the heaters, and the center tap of the pot to ground. Do not connect any part of the heaters directly to ground. This way you can adjust the pot to the point producing minimum hum.
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