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I have npn transistor circuit and want to change the input from 48V to 12V. My question is can i used same circuit digram using same npn transistor (S8050) for input 12V by changing the R1 and R2 value only?enter image description here

Kim
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  • This circuit is marginal on 48 V; I wouldn't use an SS8050 (a 40 V rated transistor) for it. Though neither of the transistors should be exposed to 48 V in normal operation, a fault--or an innocent modification that was poorly thought through--could easily exceed the ratings. – Hearth Nov 07 '23 at 04:29
  • Does it mean that I cannnot used S8050 ( 40V rated transistor) for 12V input? – Kim Nov 07 '23 at 04:38
  • @Kim Read here on EESE and here on EESE. Much is covered in one place by that author, and in more detail than I elsewhere find here. And yes, the circuit can be made to work just fine on 12 V. Though that depends upon how many LEDs and of what type you plan to stack up. – periblepsis Nov 07 '23 at 04:58
  • @periblepsis ok. Got it. Thank you for explaintaion. – Kim Nov 07 '23 at 05:38
  • @Kim I'd say that you can have at most 3 of your 3 V LEDs running off of a 12 V supply given that design. – periblepsis Nov 07 '23 at 07:27
  • @Kim No, it's fine for 12 V. I just wouldn't use it at 48 V. – Hearth Nov 07 '23 at 12:32

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The circuit is a current regulator. R2 sets the current, using the B-E voltage of Q1 as a reference. So you wouldn't change R2 unless you want to change the current through the LEDs (at least I assume they are LEDs) — this is independent of the supply voltage.

R1 provides bias current for the two transistors (mainly Q2). Its value is not critical, as long as it provides enough current for Q2 without putting too much current through Q1. It's quite possible that this resistor doesn't need to change either.

There's a third issue: The diagram implies that the two LEDs need 6 V each; that adds up to well over 12 V if you include the drop across Q2 and R2. 12 V may not be enough to operate the circuit at all.

Dave Tweed
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