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Registered Member #2563
Joined: Mon Dec 21 2009, 10:17AM
Location: Australia
Posts: 280
Hey i have a MOT with the secondary windings and filament windings removed, lots of wire to wind a new secondary, and a bridge rectifier capable of 35 amps and 800v. And i wish to make a DC supply with say 10-20 amps and anywhere between 24-50v if possible. but asides from the thing mentioned above i have no idea as to how i can regulate it, limit the current, and give it short circuit protection or whatever it need to protect whatever I'm powering with it from catastrophic failure. so if anyone can point me in the right direction it would be greatly appreciated.
Registered Member #2915
Joined: Fri Jun 11 2010, 10:41AM
Location: Malaysia!
Posts: 101
The 1st thing is: What is it for? If you are pulling 15++ Amps how smooth does it need to be? Using the formula for 10% ripple: C = 5 × Io ..................................................
.....Vs × f Using the inputs: 20A (output), 50V at 50Hz we need:44,000uF at >50V cap as a smoother for a ripple of 10% which is rather large... can you get the cap?
Limiting the current is easy; just use ohms law to calculate a resistance to put in series with the secondary (output) Ohms law: R=V/I
so if you want to limit the current to say... 20 A (at 50V) you need a 2.5 Ohm resistor. However, it will be dissipating up to (V*I) 1,000W of energy... a bit of a problem... But, if you can find a 1KW 2.5 Ohm power resistor Itll work... (I think) BTW, the resistor will limit the current in the event of a short.
Thats all I really know... Please correct me If I have made any mistakes! :P Good luck!
Registered Member #2838
Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333
You could set up a current regulator using multiple LM338's. I have seen this done for up to 0-30v 20A. Keep in mind that the closer that you keep the input/output voltages, the less heat dissipation you will have from the chips.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
As for a overcurrent protector; The old way was a bimetalic Clixon device, 2 terminal-if you shorted the secondary, it just clicked off but allowed 2-3 X current then tripped. After 30 seconds it cooled and reset itself. They were used in low voltage benchtop power supplies for ever and cost a few dollars each. Unaffected by transients too.
Registered Member #2563
Joined: Mon Dec 21 2009, 10:17AM
Location: Australia
Posts: 280
Muttyfutty! wrote ...
The 1st thing is: What is it for? If you are pulling 15++ Amps how smooth does it need to be? Using the formula for 10% ripple: C = 5 × Io ..................................................
.....Vs × f Using the inputs: 20A (output), 50V at 50Hz we need:44,000uF at >50V cap as a smoother for a ripple of 10% which is rather large... can you get the cap?
Limiting the current is easy; just use ohms law to calculate a resistance to put in series with the secondary (output) Ohms law: R=V/I
so if you want to limit the current to say... 20 A (at 50V) you need a 2.5 Ohm resistor. However, it will be dissipating up to (V*I) 1,000W of energy... a bit of a problem... But, if you can find a 1KW 2.5 Ohm power resistor Itll work... (I think) BTW, the resistor will limit the current in the event of a short.
Thats all I really know... Please correct me If I have made any mistakes! :P Good luck!
It is for powering my Mazzilli driver, then I just need a supply in general. Would this capacitor be good for the smoothing? Also I don't think i can get a 1Kw resistor. not cheaply anyways. perhaps ill try for regulating it with LM338's Edit:
Registered Member #2838
Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333
Dude, I used a rewound mot for my driver. Just throw some 18-16awg wire in there, you get approximately 1.2v per turn i believe... As long as you place a fuse in the mix you are ok.
Registered Member #2563
Joined: Mon Dec 21 2009, 10:17AM
Location: Australia
Posts: 280
Yeah, I may go for just fusing it for now, Though I would like to have a more permanent solution.
Edit: Anyways I went and wound a secondary using 6 meters of fairly thick plastic insulated wire. No idea how many turns, and rectified with my bridge rectifier, according to two analog meters i have the amps are over 20, and the volts are around 40-50. When i dead shorted it the wire got hot almost instantly. Anyways. by just limiting the current and fusing it, should I be okay? Because i really don't want to kill my Mazzilli Driver. Also I would have to use power transistors to regulate current wouldn't I? unless i had an epic 1kW resistor.
Registered Member #2838
Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333
Well, to obtain what you want I still recommend looking into a voltage regulator circuit. If you do not mind putting in the extra time. There is a good adjustable voltage/adjustable current circuit on the LM317 datasheet that could be modified to work well with several LM338's. Figure 22 Or you could use one of the simpler designs on the LM338 datasheet to regulate a specific voltage/current output. Each LM338 is rated for 5A so you would need between 2-4 to handle the 10-20 amps that you were looking or. You could also look into a shunt or crowbar circuit. However, I am not sure how well these would work in your application.
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Rewinding a MOT for high current low voltage DC supply
