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Im not really interested about this kind of electronics (i work with embedded systems as programmer) but recently i came up with need for adjustable very high power regulated power supply. First i thought to buy one but when i saw how much device like that would cost - i got suddenly needed motivation to make one
Once i made one like this: AC in = transformer - rectifier - capacitors - LM350T + PNP pass transistor - capacitors = DC out
I have suitable transformer, capacitors, rectifier, LM350 and bunch of resistors etc needed. I have also three NPN-power darlington bricks mounted on HUGE heatsink. There is actually 2 darlington circuits in on brick. I know basic principle about linear regulation BUT i have no idea how to use PNP darlingtons as pass devices with LM350. If someone could explain this to me (or better: draw picture with paint) i would be happy. All 3 bricks can be used if possible.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Basically that looks ok except I would
-connect the collectors directly to the positive supply and -put the 0R1 resistors one in series with each emitter
because putting the resistors in series with the collectors will not encourage current sharing.
The voltage regulation will be good but not great because the three base-emitter junctions will have an effective voltage drop that -increases with output current -decreases with junction temperatures.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
If you're using half of 2 bjt bricks there you must really need some serious current > 50A or something?! You'll need to put the resistors on the emitter sides like sulaiman said, but you may also need some base resistors because the current Ic = Beta Ib, and if Ic ~= Ie is unequal between the two bricks, then Ib will be unequal as well. You might want something like 10R feeding the bases of the bricks, but I dunno for sure because its going to require some playing. Also, you will probably need a load on your output of say 1KR to keep some current flowing or the supply might swing a bit when you add a load and remove loads. EVR knows more about this, and I would know for sure but I never really had a chance to get into it seriously... too much college bs to take care of and all ;p
Good luck with the power.
Oh, forgot to mention, if you need > 50A you'll need some huge resistors, 10W ain't gonna cut it, and you don't want to use the 10W resistor to 10W because they tend to get red hot, so if you want serious current the power is going to be IIR, so just double the losses and the resistors will get hot but not die.
Good point you got there. I will make both emitter resistors out of 2 50W resistors in parallel mounted on same heatsink with darlingtons.
And it is true, i need some serious power this time, and i want that powersupply is not the weakest link (meaning that the load burns before this supply) :)
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Need help with linear power supply desing
