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Solid State Solution for Fried Autotransformer

alfa88

Dec 1, 2010
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A tale of woe. It started when I bought a tube tester years ago. It was in pretty poor shape but it powered up. Somehow when I pushed a few switches(I shoulda read the instructions 1st)and my new toy was dead. So I tossed it into the corner and forgot about it. Fast forward. I unwrapped the transformer and to my delight I found an open, repaired, tested, found another open repaired and Ohmed out. Problem is when I realized I was dealing with an autotransformer and that my static test was in error. So I'm thinking that if I could make a multi-tap power supply that will fit I might be back in business. But would running DC through the tube test be legit? BTW I'm talking about a simple emissions tester. Nowhere to be found, though, are what exactly the taps are since the manufacturer(Allied/Knight/Lafayette) saw fit not to include this info. I'm guessing 1, 2, 5, 6, 12, 35, 50 since there are 7 filament settings and the current should be at least 500mA. Opinions? Advice?
 

alfa88

Dec 1, 2010
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I got some advice from my tube buddy and have 1.2, 3, 5, 6.3, 12.6, 25 & 50 as my targets. I want to keep things as small as possible but it looks like I have to use an external supply with a regulator array in place of the transformer. My understanding of emission testers is that grids and cathodes are tied together and the tube is treated like a diode and in my case ~5mA shows up on the plate. With that in mind I have to make the Voltage floating so I can use LM-338s. Now if I go with 338s, they have a 32V limit so I have to cascade the 50V - 25V stage. Therein lays a possible conflict with the 1st and 2nd stage. I read where a pullup resistor might be needed
 

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bertus

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Hello,

Take a look at your schematic.
What would happen when the 25 Volts output gets shorted?
As the LM338 allows a maximum differential of 40 Volts, it will likely blow.
LM338_max rating.png
Bertus
 

alfa88

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The max rating is for Input to Output differential. In my case ~25V. Max is 40. Am I missing something?
 

bertus

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Hello,

I said, what would happen when the 25 Volts gets shorted.
Then there is a differential of 50 Volts.

Bertus
 

alfa88

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An excerpt from the datasheet:
Since the regulator is "floating" and sees only
the input-to-output differential voltage, supplies of several
hundred volts can be regulated as long as the
maximum input to input differential is not exceeded.
 

bertus

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Hello,

Yes, as long as you do not exceed the maximum input to output differential, there is no problem.
The problem will arise when the 25 Volts gets shorted.

Bertus
 

alfa88

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It's kind of funny. I was scanning the manual and it seems shorts were always a concern with this tester.
A fuse at the input of the regulator chain would alleviate any worries.
 

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bertus

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Hello,

You could use a tracking voltage regulator to have about 15 Volts on the second regulator and 10 Volts on the preregulator.
In case of a short, the preregulator will see 35 Volts, wich is safe.
LM338_tracking preregulator 15 volt.png
Bertus
 

alfa88

Dec 1, 2010
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Since space so tight I'm forced to use an external 48V PS. To cut down on size I used fixed resistors instead of pots. I'm going to cram an inline fuse in somewhere. I completed the board and have 48.5V, 24V, 12V, 6V, 5.2V, 3V & 1.5V. Pretty much within 10% of filament requirements. Thanks for taking the time to consult.
 
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