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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 2nd Dec 2023, 1:10 pm   #1
DonaldStott
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Default Hacker 12V Power Supply

The Hacker radio I am restoring has a DC socket for the 12V power supply where the centre pin is NEGATIVE!

To avoid future mishaps if someone tries to connect a centre pin POSITIVE power supply I'm looking to fit a Schottky diode as some form of basic protection.

Too many Schottky diodes to choose from depending on where in the circuit it is to be located, so recommendations please for a suitable candidate for this DC socket : 12V, 14mA at zero volume. Thanks.
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Old 2nd Dec 2023, 2:51 pm   #2
paulsherwin
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Default Re: Hacker 12V Power Supply

It doesn't really matter. Any scrap switching PSU will have at least one suitable power Schottky.

As an alternative, you could fit a diode across the socket which would crowbar if the wrong polarity was connected. It would be a good idea to fit a protective fuse.
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Old 2nd Dec 2023, 8:50 pm   #3
DonaldStott
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Default Re: Hacker 12V Power Supply

Thanks Paul - not sure what ‘crowbar’ means in this context?

What type and value of fuse would you recommend?
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Old 2nd Dec 2023, 9:46 pm   #4
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Default Re: Hacker 12V Power Supply

not sure what ‘crowbar’ means in this context?
I think he refers to a Diode in parallel with the supply feed as opposed to a series diode.
It wouldn't conduct when the polarity is correct - but would blow a suitable fuse if reverse polarity is connected.
I would use a simple 1N4000 if 1 amp or so is enough in series, 0.6 volt drop surely won't affect the operation of the radio.
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Old 2nd Dec 2023, 10:48 pm   #5
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Default Re: Hacker 12V Power Supply

Exactly that. The advantage is there's no voltage drop in normal operation.

The fuse value isn't critical, but you don't want it too small or you'll get nuisance blowing when the decoupling electrolytics charge up. 500mA should be OK.
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Old 3rd Dec 2023, 2:17 am   #6
DonaldStott
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Default Re: Hacker 12V Power Supply

My initial thoughts were just to put a Schottky in series as the voltage drop would be less than, for example, a 1N4000?

A crowbar diode with an in-line fuse now seems like the more attractive option?
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Old 3rd Dec 2023, 2:06 pm   #7
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Default Re: Hacker 12V Power Supply

The inline Schottky solution is simpler and will probably work perfectly well. The crowbar is a bit more elegant. Which I would choose would depend on what parts I had to hand.
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Old 3rd Dec 2023, 2:23 pm   #8
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Default Re: Hacker 12V Power Supply

Yes, I would go for the inline Schottky; it will only drop 0.1-0.2V, and you don't have to worry about shorting the power-supply and killing it, as would be the risk with a crowbar-type diode.

If you want to keep it 'period' then you could use a 50s/60s Germanium power-diode like the GEX541 instead of a Schottky; that will also give a low forward-voltage drop.
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Old 3rd Dec 2023, 2:36 pm   #9
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Default Re: Hacker 12V Power Supply

If you have the usual Strepsils tin of scavenged power diodes, just go through them with the diode test function of your DMM measuring the voltage drop until you find a nice low drop one. This isn't a demanding application so they'll all have adequate voltage and current ratings.
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Old 4th Dec 2023, 1:08 pm   #10
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Default Re: Hacker 12V Power Supply

In all honesty I would not worry at all about using a conventional silicon diode inline even with its 0.7V forward voltage drop. The set will work just fine.
An inexpensive and cheerful 1N4000 series will be just the job - sorted

Steve.
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Old 4th Dec 2023, 10:47 pm   #11
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Default Re: Hacker 12V Power Supply

Thanks everyone - in my 'usual Strepsils tin' I have a number of 1N4000 series diodes so I will see which one has the lowest forward voltage drop.

Another case of over-thinking a problem when the solution was there all along!
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