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 Post subject: Power supply
PostPosted: Aug Tue 04, 2020 3:11 am 
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Location: Warner Robins, GA
I'm redoing a power supply for a device so that I can regulate it to 300 volts using three 100 volt 5 watt zeners. The transformer winding is 212 Vrms and I will need to double the voltage.

I plan on using a full wave doubler.

Image

I know that I will need a resistor to drop the voltage some, but that will dissipate heat.

Would it be possible to use a film cap between the winding and diodes instead of a resistor between the output and zeners?

Or can I use a resistor between the winding and diodes which will reduce the voltage while not needing to be such a large value?

I thought about a 1/2 wave doubler as I could alter the value of the first cap to reduce the voltage

Image

But I would have an unloaded B+ of 600Vdc and I cannot find a 100uF 600 V cap easily.

B+ current I believe is less than 100mA.


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 Post subject: Re: Power supply
PostPosted: Aug Tue 04, 2020 1:59 pm 
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Location: Minneapolis, MN USA
You can place two electrolytic caps in series to cut the required voltage rating in half.
I use resistors across them to discharge them when turned off...others call them equalizing resistors :-)


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 Post subject: Re: Power supply
PostPosted: Aug Tue 04, 2020 2:08 pm 
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Now if I go with the full wave doubler I will be fine with 100uF 350V caps.

If I go with the 1/2 wave doubler (possibly easier as I can adjust the first cap's value to set the final voltage) I could use two 100uF 350 volt caps in series for a capacitance of 50uF or I could use four in series/parallel to still have the 100uF.

That said with the zeners regulating the voltage I won't need as much filter capacitance, right?

What value of resistor would I need across each series cap?


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 Post subject: Re: Power supply
PostPosted: Aug Tue 04, 2020 4:37 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Minneapolis, MN USA
I have used a few doublers and once a trippeler. Never followed by Zeners.
But yes I would expect some filtering from the zeners.
For dissipating resistors I use Ohmic value that will draw 5-10 percent of the expected load current.


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 Post subject: Re: Power supply
PostPosted: Aug Tue 04, 2020 4:39 pm 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
http://www.calculatoredge.com/electronics/zener.htm

Rich


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 Post subject: Re: Power supply
PostPosted: Aug Tue 04, 2020 5:44 pm 
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Location: Warner Robins, GA
I like that calculator, but I don't know the exact load current.

Most likely I will use a 100K resistor across each series cap which at the full 350 volt rating of the cap will only draw 3.5mA unless a 200K resistor will work just fine.

I'll never see 700 Vdc from the supply so the current drawn by the 100K resistors will be a bit less.


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 Post subject: Re: Power supply
PostPosted: Aug Tue 04, 2020 8:23 pm 
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Location: Saskatoon
If you go with the halfwave doubler, be aware that if the first cap is smaller than the second one, it will see a reverse polarity every half cycle until the second capacitor reaches its steady state value. The degree of the reverse polarization will be proportional to the difference in capacitance values. This won't be good for the life expectancy of the the first cap.


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 Post subject: Re: Power supply
PostPosted: Aug Tue 04, 2020 8:32 pm 
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Oh ok. Did not know that.

May use the full wave design then.

Would be easy to add a resistor before the rectifier diodes or between the main filter caps and a smaller value second filter cap.


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 Post subject: Re: Power supply
PostPosted: Aug Tue 04, 2020 10:57 pm 
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Location: Saskatoon
I haven't tried this, but at first glance, in the halfwave circuit, you may be able to avoid the reverse polarity across the cap by adding a third diode. The diode would be connected in parallel with the first cap (cathode to the positive terminal of the cap). That would keep the first cap out of the circuit until the second cap has charged up to the peak AC voltage, and afterwards, it would only conduct whenever the polarity starts to reverse across the first cap.


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 Post subject: Re: Power supply
PostPosted: Aug Wed 05, 2020 2:43 am 
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Interesting.


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