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 Post subject: Diode rectifier question
PostPosted: Jun Sun 16, 2019 11:26 pm 
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Joined: Jan Thu 24, 2013 11:50 pm
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Location: Bristol, Rhode Island
Hi all,

Trying to put together a little amplifier and had a question about voltage coming out of the rectifier.

The transformer HV secondary is 300-0-300VAC. There is no 5VDC winding for a standard tube rectifier so I'm using diodes.
I figured the rectified voltage (two diodes in series coming out of each leg of the HV secondary, then paired, with HV center tap to ground, would give me about 410 or 420VDC.

But I'm getting much lower than that in practice. With diodes in place, the rectified voltage is only around 265VDC to ground. There are no filter caps in place, just the rectifier.

I have no explanation for this. Does anyone?

Thanks,

Ted


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 Post subject: Re: Diode rectifier question
PostPosted: Jun Sun 16, 2019 11:32 pm 
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Diverted wrote:
The transformer HV secondary is 300-0-300VAC. There is no 5VDC winding for a standard tube rectifier so I'm using diodes.
I figured the rectified voltage (two diodes in series coming out of each leg of the HV secondary, then paired, with HV center tap to ground, would give me about 410 or 420VDC.

But I'm getting much lower than that in practice. With diodes in place, the rectified voltage is only around 265VDC to ground. There are no filter caps in place, just the rectifier.
Hi Ted,

You MUST have a proper filter in place, i.e. two electrolytic capacitors with a choke or resistor between them, to get proper DC from the supply.

- Leigh

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 Post subject: Re: Diode rectifier question
PostPosted: Jun Mon 17, 2019 12:01 am 
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Joined: Jan Thu 24, 2013 11:50 pm
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Location: Bristol, Rhode Island
Thanks. Assumed as much.

I had seen the old guideline that solid state rectified voltage is about 1.4x the AC voltage. I did not know that was also including the filters.

Thanks.


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 Post subject: Re: Diode rectifier question
PostPosted: Jun Mon 17, 2019 12:21 am 
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Diverted wrote:
I had seen the old guideline that solid state rectified voltage is about 1.4x the AC voltage. I did not know that was also including the filters.
You need at least one filter capacitor at the output of the rectifier to get that peak voltage.
You do not need a full filter if the supply is unloaded.

- Leigh

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 Post subject: Re: Diode rectifier question
PostPosted: Jun Mon 17, 2019 1:01 am 
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Joined: May Thu 14, 2015 4:15 pm
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Location: Dallas, TX
Actually the peak voltage is there. It is a half wave waveform, if you looked at it on a scope you can see that. This issue is that the waveform ( since it contains a lot of AC) fools the meter that is expecting DC. A large capacitor smooths out the waveform and charges to the peak voltage (with no load).

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 Post subject: Re: Diode rectifier question
PostPosted: Jun Mon 17, 2019 1:26 am 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 10015
Location: Santa Rosa, CA
The Hammond rectifier guide is very useful. It shows what you might expect for various configurations of rectifiers and transformers.

http://www.hammondmfg.com/pdf/5c007.pdf

Note: Voltages shown are with a load connected.

Rich


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 Post subject: Re: Diode rectifier question
PostPosted: Jun Mon 17, 2019 2:33 am 
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Joined: Nov Tue 14, 2017 5:09 am
Posts: 2025
Location: Austin, Texas
Notimetolooz wrote:
Actually the peak voltage is there. It is a half wave waveform, if you looked at it on a scope you can see that. This issue is that the waveform ( since it contains a lot of AC) fools the meter that is expecting DC. A large capacitor smooths out the waveform and charges to the peak voltage (with no load).

Adding to the above.

The blue waveform below is what you would see on an oscilloscope. The peaks of the waveform are about 407 volts.
The red line is what a DC meter that averages the waveform will read. That is about 258 volts.
With a capacitor added, it will stay charged to near 407 volts and the meter will then read the expected voltage.
Attachment:
Full wave.jpg
Full wave.jpg [ 93.68 KiB | Viewed 362 times ]


Jay


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