voltage regulator circuit

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DazFM

Joined May 4, 2020
2
Hi guys,

I am kinda stuck with the question. I have managed to work out the zener breakdown voltage when maintaining a regulated 25V output using this equation and re-arranging it slighlty.

Vout = (1+(r1/r2)*vz, Vz = (vout/(1+r1/r2) = 25V/(1+4/1) = 25V. I made r1 = 4k.

I am just not sure about how to to calculate the maximum power in the pass transistor? Would it be the max vin of 50V - 25V * Load, although there is no load resistor, so i am confused. Any guidance would be much appreciated.

Thanks
 

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ci139

Joined Jul 11, 2016
1,898
it may expect trivial ... as how much current the switch (NPN) passes in case of the 25V output while providing max avail power (unspecified) to R.LOAD (not shown)

... the input may be varying from 30 to 50 but lacks it's other specs ? (so it's ideal) ... as also the NPN (can be switched fully on assuming the op amp has different supply) ... so the wiring ... so the output power capability \(\rightarrow∞\)

you can make 2 assumptions the op amp is powered from the same 30 50 supply (i dont know any 50V op amps) . . . but anyway then V.inp should be V.outp+700mV
but since the ideal supply it bares no meaning the power at NPN is \(P_{SWITCH}≈\left({\left({V_{IN}-V_{OUT}}\right)·\frac1{1+\frac1{h_{FE}}}+\frac{0.7}{1+h_{FE}}}\right)·I_{OUT}\) in reality it may give a huge error depending the actual currents and device (NPN) type
 
Last edited:

MrAl

Joined Jun 17, 2014
11,396
Hi guys,

I am kinda stuck with the question. I have managed to work out the zener breakdown voltage when maintaining a regulated 25V output using this equation and re-arranging it slighlty.

Vout = (1+(r1/r2)*vz, Vz = (vout/(1+r1/r2) = 25V/(1+4/1) = 25V. I made r1 = 4k.

I am just not sure about how to to calculate the maximum power in the pass transistor? Would it be the max vin of 50V - 25V * Load, although there is no load resistor, so i am confused. Any guidance would be much appreciated.

Thanks
First, 20/(1+5/1) does not equal 25, but you are on the right track.

Since the output load or current is not given, you may have to express the power in the transistor based on the output current. For example, P=Iout*X where X is some other value you should determine yourself.
 
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