~13.8V MOT power supply

[asand1]

I am considering building a power supply in the 50-80A range for running CB gear in my garage. It would be simple two hand-wound secondaries of ~13 turns each, dual rectifier bridges mounted on a CPU cooler, and large cap ~1000uf above 50V.

My question is, should I remove the magnetic shunts or I leave them in?

Here is a simple unregulated supply I just threw together this evening.

[budm]

Re: ~13.8V MOT power supply

50~80A at 13.8vdc? That is over 1000Watts power supply. How much AC ripple can the load tolerate?
More learning here:
http://www.electroschematics.com/704...r-calculation/
http://www.radio-electronics.com/inf...r-circuits.php
http://waynestegall.com/audio/ripple.htm

[mariushm]

Re: ~13.8V MOT power supply

If you want to go the linear power supply route...

50-80A x 13.8v = 700-1100 watts so you'll need at least a 900-1000VA transformer.

A 250VA transformer is about 50$ so you'll probably need 4 or 5 of them: http://www.digikey.com/product-detai...350-ND/2090088

Then you need some beefy rectifiers... not a single one, you'll probably need at least 2 in parallel and heatsinked well, something like this :

http://www.digikey.com/product-detai...BPMS-ND/773615

Then you're gonna need a lot of 25-50v rated capacitors..

If you use a 12v ac transformer, you get 12x1.414 = 17v, then lose about 2v on bridge rectifier and you're left with 15v ... then you need about 14v so you can afford about 1-1.5v ripple..

F = current / 2 x ac mains x Vripple = 80a / 2x50x1.5v = ~ 53000 uF

Then you have to think about the inrush current such large capacitors would cause, you may blow a fuse... so you need some kind of thermistor, or some method of connecting those capacitors sequentially.

and you'd be left with about 14-15v.... maybe throw a bunch of big power transistors and get 13.8v but you'll dissipate about 30-50 watts in the heatsinks...

It would be much cheaper getting a 1000w switching power supply and using that power supply's output as input for regulating further to 13.8v

[kc8adu]

Re: ~13.8V MOT power supply

you are better off making it regulated.
look to the lm723 and a bunch of 2n3771.
basicly you will be building an astron clone.

[ben7]

Re: ~13.8V MOT power supply

P.S. MOT transformers are very inefficient. If you aren't careful, they will burn up on you!

Try going to the local scrapyard and see if you can find some bigger, better transformers. You might even find one with the right output voltage!

Anyways, I say you should use at least 10,000uF for filtering, not 1,000uF!

P.S.2. Install a fuse also xD

[kc8adu]

Re: ~13.8V MOT power supply

yes a mot is meant to be cheap not efficient.
so how big an amplifier are you trying to run?
maybe a smart charger and a deep cycle battery would be better.
and it will work during emergencies where a cb might actually be useful.

[asand1]

Re: ~13.8V MOT power supply

you are better off making it regulated.
look to the lm723 and a bunch of 2n3771.
basicly you will be building an astron clone....

...so how big an amplifier are you trying to run?
maybe a smart charger and a deep cycle battery would be better.
and it will work during emergencies where a cb might actually be useful.

I am currently running my FILs' Astron RS 35A, prior to that I had a 1200mA wall wort and 55aH AGM. I just want to build my own PS and give his back. I also want this to be a DIY CHEAP project. I've also read in various HAM & CB forums that the major base amp MFGs actually use the same microwave oven transformers (MOT)s frames in their products. 1 MOT supposedly produces ~70A in this application.

I have looked online for a circuit diagram with thelm723 and 2n3771 pieces and have come up empty to this point. Could you link me to a diagram or instructions?

I am currently running a Cobra 148gtl with no amp. I would like to run a 100W amp soon and have plenty of PS to spare.

[SeanB]

Re: ~13.8V MOT power supply

Buy a battery ( or go to a wreckers and get some good used ones instead) and go buy a cheap battery charger to attach to it instead. Simpler and more likely to work first time.

With a supply this big any mistake will result in a very loud band with a lot of expensive confetti all over the place.

[asand1]

Re: ~13.8V MOT power supply

Buy a battery ( or go to a wreckers and get some good used ones instead) and go buy a cheap battery charger to attach to it instead. Simpler and more likely to work first time.

With a supply this big any mistake will result in a very loud band with a lot of expensive confetti all over the place.

I already have a very good battery and charger, But I want 13-14V @ 50A+ continuous power out of a supply that I built myself.
Mainly, I want to do this because I like to tinker and want to say to myself "I built this".

[severach]

Re: ~13.8V MOT power supply

1 80A power supply is a lot harder to make than 8 10A power supplies. You'd only go to the trouble to make an 80A supply if you were going to spend it all in one place, maybe a 1000W amplifier.

Thanks to SMPS high current 12v power supplies are dirt cheap on eBay.

[ben7]

Re: ~13.8V MOT power supply

Mainly, I want to do this because I like to tinker and want to say to myself "I built this".

Don't worry, I am the same way

Besides, a linear PSU, unlike an SMPS, will be able to provide large peak currents for short amounts of time, whereas an SMPS will just shutdown from the over-current.

Check out these good pages:
http://ludens.cl/Electron/Ps20/Ps20.html
http://ludens.cl/Electron/trafos/trafos.html

For that linear PSU, I would add a temperature-dependent fan speed controller, and a thermal shutdown feature. Also, for higher current output, you'd need more transistors, more heatsink, beefier transformer, and a better bridge rectifier. Also use a 10A fuse, especially because you want more than 20A out of it. You could use schottky diodes for the bridge rectifier, so that it won't dissipate too much power. Remember, for the diodes and transistors, they will dissipate power as heat, and you NEED to keep them cool or else they will self-destruct!

[kc8adu]

Re: ~13.8V MOT power supply

most of the solid state cb amps are pure garbage anyway.
no big surprise that these cheap n nasty splatterboxes use a cheap transformer too.running 2sc2879's at 18v+ also gaurantees short life.
so dont take any design ideas from those things!

I've also read in various HAM & CB forums that the major base amp MFGs actually use the same microwave oven transformers (MOT)s frames in their products. 1 MOT supposedly produces ~70A in this application.