Building a 12 volt Power supply

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Building a 12 volt Power supply

Post by tuberadiogeek on Mon Jun 08, 2015 6:18 pm

I have a nice somewhat large old doorbell transformer that puts out 27 volts with no load on it. I was thinking about using it to make a power supply but I just can't find a good simple schematic online that uses a transformer that large. I know I will need some large electrolytic caps for filter caps, some diodes for a rectifier and some resistors. However lots of schematics call for a voltage regulator, which I don't ever recall one of my small regulated supplies ever having one. I was also thinking why couldn't I use a 5Y3 or 5U4 in place of diodes? I have both tubes on hand and wouldn't mind using one. I would like to have maybe a 12 or 15 amp supply that would actually do 13-13.5 volts. Is something like that possible? I have an old computer PSU that does that much, but I still want to build my own.
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Re: Building a 12 volt Power supply

Post by Bill Cahill on Mon Jun 08, 2015 9:48 pm

Door bell transformers can't handle that much amperage. Tubes will work, but, will work better at line voltage, than that low.
You'd need fairly high amperage diodes for that much current. Even a 5U4 can't handle more than four amps. Two would get you eight amps.
5Y3 is about half that.
Most battery radios don't need more than 300ma.
Anything heavier you'd need stronger parts.

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Re: Building a 12 volt Power supply

Post by tuberadiogeek on Tue Jun 09, 2015 12:02 am

Well the Power supply would be used for a wide range of things. However that transformer may not be the best if I want a large power supply. I tried researching the make and model of that transformer and couldn't find but similar models listed. I believe that transformer originally only powered a small single gong bell, so it likely isn't more than a 20 VA transformer, which would only supply around 1 amp of power at 27 VAC. That would maybe get me a 2 or 3 amp power supply right? I know I could transformerless, but I know that is not wise and presents a possible shock hazard. I was just trying to come up for a use of parts I have collecting dust.
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Re: Building a 12 volt Power supply

Post by willy3486 on Tue Jun 09, 2015 8:31 am

If you want a really nice power supply that will run most 12 volt stuff find one of the old Radio shack 12 volt supplies in a metal case. I bought one back in the late 70s and still have it. I have replaced the transformer and trasistors at least once. But it is a simple and easy to build one if you can't find one. Its a clean an simple circuit. I used it for years on car radios and CBs to power them. You can see a diagram at the link below. I have made about 4 or 5 power supplies based on the diagram of it.


Power supply diagram


If you want a high amp power supply look for a good used one as well. I picked up a 10 to 15 amp 12 volt supply used by someone into ham radios. I think I gave 40 for it. I don't think I could built one for that even if I had a bunch of parts.

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Re: Building a 12 volt Power supply

Post by Bill Cahill on Tue Jun 09, 2015 9:10 am

In the fifties, Mallory made one heck of a good 12-24 volt power supply, in fact, It would vary down to four volts because it was also designed for six volt car radios. It would handle up to twenty amps.
They almost always need rebuilding, including new electrolytics, and, isn't regulated, but, works well. I still have the one we used to use at the tv shop. I rebuilt ittwenty five years ago, and, used it to restore two car radios for customers, including a six volt one...
Great little supply.... And, yes, it's fused.

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Re: Building a 12 volt Power supply

Post by Guest on Tue Jun 09, 2015 1:57 pm

This is the way to go;

http://makezine.com/projects/computer-power-supply-to-bench-power-supply-adapter/


Leave the other transformer to door bells!

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Re: Building a 12 volt Power supply

Post by 75X11 on Tue Jun 09, 2015 2:55 pm

You'd make the goodwill store mighty happy taking a computer chassis off their hands!
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Re: Building a 12 volt Power supply

Post by tuberadiogeek on Tue Jun 09, 2015 3:20 pm

I actually have 2 computer power supplies. One currently powers my car radio down the basement and the other was collecting dust because I didn't know how to make it come on. The one on my radio is on came on automatically as soon as there was something drawing power. Hopefully the other one will work once I hook it like the instructions say. The one I am going to use can supply up to 18 amps at 12 volts.
Goodwill here only sells complete computers, and they are quite expensive. However I can get complete in tact computer towers in unknown condition for 5.00 all the time from a guy that has a small scrap metal business one block from me. They are all machines people scrapped. I'm sure most have good power supplies in them.
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Re: Building a 12 volt Power supply

Post by tuberadiogeek on Tue Jun 09, 2015 10:27 pm

Power supply turned right on as soon as I connected the correct wires. I never knew how to make a PC power supply turn on outside of the case. If I would of known how to do that in the past, I would of had quite a few power supplies that could be used. This one is currently powering a 2 channel 85 watt amp for my computer.
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Re: Building a 12 volt Power supply

Post by Guest on Wed Jun 10, 2015 9:30 am

There are several articles on the internet about converting PC power supplies. I've been doing this for a few years.  I believe these are the most plentiful cheap/free power supplies around.

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