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 Post subject: Replacing seleniums in a Perma Power
PostPosted: Jun Sun 09, 2019 2:09 pm 
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What would be good replacements for the seleniums in a Perma Power model E? I figure a 1N4007 would work for RA1 but what about RA2? I have never seen a symbol like that before.

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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Sun 09, 2019 2:27 pm 
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Hi Jim,

RA2 is a dual diode with the ANODES connected together.

The cathodes go to the transformer winding.

- Leigh

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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Sun 09, 2019 2:30 pm 
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Now that I look at it closer I see the anodes being tied, thanks Leigh. I need to be more observant.

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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Sun 09, 2019 2:38 pm 
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This is a twist on the traditional full-wave rectifier circuit. Generally on these designs the output winding center tap is grounded, with each output winding feeding the respective anodes of two diodes that have their cathodes connected together; the output voltage is taken off at the cathode junction.

RA2 appears to be two selenium rectifiers connected anode to anode. Since the junction is grounded, the center tap will be at a positive voltage with respect to the ground point. This accomplishes the same thing as "traditional" full-wave rectification, it just reverses two of the elements. Two reversals= no net change.

I have no idea as to why this was done. It may have something to do with the unusual low-voltage/high current output (2V/750mA).

If you change to silicon, you will presumably need to replicate the relatively high voltage drop of the selenium part. One way to do this is to series-connect several silicon diodes until you get the needed voltage drop (10 silicon diodes would have a Vf of around seven volts). You could also use a series resistor, but that would make the output voltage load-dependent, which may not be desirable.


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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Sun 09, 2019 3:35 pm 
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It will take some experimenting to get the proper output voltage. I first need to replace the capacitors.

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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Wed 12, 2019 9:38 pm 
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lorenz200w wrote:
This is a twist on the traditional full-wave rectifier circuit. Generally on these designs the output winding center tap is grounded, with each output winding feeding the respective anodes of two diodes that have their cathodes connected together; the output voltage is taken off at the cathode junction.

RA2 appears to be two selenium rectifiers connected anode to anode. Since the junction is grounded, the center tap will be at a positive voltage with respect to the ground point. This accomplishes the same thing as "traditional" full-wave rectification, it just reverses two of the elements. Two reversals= no net change.

I have no idea as to why this was done. It may have something to do with the unusual low-voltage/high current output (2V/750mA).

If you change to silicon, you will presumably need to replicate the relatively high voltage drop of the selenium part. One way to do this is to series-connect several silicon diodes until you get the needed voltage drop (10 silicon diodes would have a Vf of around seven volts). You could also use a series resistor, but that would make the output voltage load-dependent, which may not be desirable.


The low voltage/high current is due to the fact that the supply is built for 2 volt battery radios using 4 to 8 tubes. The tube filaments are in parallel. For the radio I have the current draw would be 560ma.

Now, using silicon diodes in place of the seleniums in this eliminator, what would a good wattage be to minimize heat? 5 watt too much overkill?

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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Wed 12, 2019 10:36 pm 
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Location: Santa Rosa, CA
Quote:
Now, using silicon diodes in place of the seleniums in this eliminator, what would a good wattage be to minimize heat? 5 watt too much overkill?


Maybe I misunderstand? Diodes don't have wattage ratings (except Zeners). I would use 1N540x series (3 amp) diodes. I only keep 1N5408 in my bins (1000 V). Two 1N540x in a CT arrangement theoretically gives 6 amps output, but I would keep the load under 4 amps as a more conservative approach.

Rich


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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Wed 12, 2019 11:17 pm 
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Location: Lexington, KY USA
You might use a packaged bridge rectifier, just for the convenience of having a way to mount it and terminals for wires.

A full wave bridge with four diodes can be a good choice. Just don't use the + terminal.

You may need to add a bit of series resistance to bring the loaded output voltage down to what you need.

BTW, it would be a nice touch to add a bleeder resistor to the A supply on this thing.

Ted


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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Wed 12, 2019 11:43 pm 
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Sorry, I meant for the dropping resistors to use for making up the lack of voltage drop from selenium to silicon.

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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Fri 14, 2019 8:13 pm 
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Replacing the seleniums in this, I started with the 2 volt section. Without any kind of dropping resistor I am getting the 2 volts at an input of 75 VAC. Now, if I put a 1K resistor in series with the A+ my 2 volts occurs at an input of 44 VAC. WHAT :?

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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Fri 14, 2019 10:54 pm 
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Do you have a load connected across the power supply when you take these readings? No-load voltage is likely to be higher. Also, bear in mind that the original capacitors probably had considerable leakage even when they were new. Modern capacitors don't have as much leakage so voltages will tend to be higher for that reason too.

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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Fri 14, 2019 11:04 pm 
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Yes, I have connected to the radio it was used with. However, I did not connect the B+ to the radio if that would make any difference.

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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Sat 15, 2019 4:25 pm 
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OK, I am a little confused or maybe I am missing something simple. Using a 3.3 ohm 10 watt resistor to sub for the filaments I am getting a 2 volt output with an input voltage of 68 VAC. Placing a 7500 ohm resistor in series with the + output I get the same 2 volt output with only a 43 VAC input. Why is it taking a smaller input to achieve the same output? Shouldn't it be the other way around?

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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Sat 15, 2019 5:06 pm 
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Joined: Jan Tue 16, 2007 7:02 am
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Location: Lexington, KY USA
We are confused, too!

What you seem to be saying can't possibly be correct. (Perhaps what we don't understand is that what you said is not what you meant!)

One or another of your resistors is not actually the value posted, or something is not connected where we think it is. Or one of the actual meter readings is not exactly what was posted. Could there be an auto-ranging meter involved?

You can reality check the resistors with an ohmmeter.

A 7500 ohm resistor in series with a 3.3 ohm load should leave almost no output voltage at all. Under 10mV.

Placed between the output of the A supply and the load, the required resistor value should be less than 10 ohms.

The "best" place for a series resistor is before the first filter capacitor, but predicting the needed resistance value is difficult. You might start with 1 ohm there and see what you get.

I expect that when you look at this the next day, you will see the problem right away.

Ted


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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Sat 15, 2019 5:12 pm 
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I'll take a picture of the setup later. Yes, I too am totally confused on this and no, no auto-ranging meter is involved.

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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Sat 15, 2019 7:20 pm 
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First photo is with just the 3.3 ohm 10 watt resistor connected as a load. The second picture is with a 7500 ohm resistor in series with the 3.3 ohm load. You can see that with the 7.5k connected I get the 2 volts across the load at a much lower input. This is with 2-1N4007"s connected anode to anode and new capacitors installed.

Image

Image

OK, never mind. I just realized my mistake. I did not have the probe of the DC meter on the correct point. It was on the input side of the 7.5K resistor instead of the load. Sometimes I just jump to conclusions which gets me all befuddled. Sorry to waste every ones time.

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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Sat 15, 2019 8:25 pm 
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Figured out the dropping resistor needs to be 9 ohms to give me 2 volts on the tube filaments with a 122 volt input. Next will be the B+ supply and I will be extra careful not to make the same stupid mistake when reading the voltages.

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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Sat 15, 2019 9:25 pm 
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Location: Lexington, KY USA
We have all done that. Some days, much more than others...

With such a large drop across your resistor, you might try a choke input filter, by disconnecting C3. This will bring down your output voltage, and also reduce the strain on the transformer, rectifier, and input filter capacitor.

Is the voltage across the filter capacitors exceeding their ratings? Looks as if 6V might not be high enough.

Does your radio actually draw 750mA? This A supply is very poorly regulated; the output voltage will rise with a lighter load. A choke input filter improves regulation, too.

And don't forget to add a bleeder resistor.

Ted


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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Mon 17, 2019 5:04 pm 
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Location: Powell River BC Canada
The power supply is made by a transformer company. They have control of
what flux level, core gapping and winding resistance they choose, to make
the device perform as desired, i.e., self regulating Just as constant voltage
transformer does.



Back in TV days there were tiny selenium rectifiers, three leads,full wave,
common anode, common cathode, and series. They were used as horizontal
phase discriminators.

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 Post subject: Re: Replacing seleniums in a Perma Power
PostPosted: Jun Tue 18, 2019 2:55 am 
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Location: Lexington, KY USA
Unless the transformer is actually a CVT, it won't regulate the output anything like the way a CVT does.

If there is no high voltage AC resonating capacitor connected to its own winding on the transformer, and magnetic shunt in the core, it is not a CVT.

I'm pretty sure no Perma Powers were made using anything but an ordinary transformer.

Ted


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