The simplistic Salas low voltage shunt regulator
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 20th October 2009, 06:47 PM #1401 Salas   diyAudio Chief Moderator     Join Date: Oct 2002 Location: Athens-Greece Resistors yes. MPSA42 no. Its NPN too low hfe.
 20th October 2009, 09:13 PM #1402 massimo   diyAudio Member   Join Date: Dec 2002 Location: Milan, Italy Κατάλαβα! Ευχαριστώ πολύ. (I undestood! Thank you very much)
 20th October 2009, 09:50 PM #1403 HiFiNutNut   diyAudio Member   Join Date: May 2004 Location: Sydney Is there any formula to calculate the Vref based on the Idss of the njfet and the reference resistor in v1.1? For the same Vref, higher Idss requires higher resistor value, right? Anyone has the figure of the input impedance of the Q3 BC550C? Has anyone thought of using a JFET instead of bipolar in place of Q3? lower noise, higher impedance? What is the problem with a JFET? Last edited by HiFiNutNut; 20th October 2009 at 09:54 PM.
iko
diyAudio Moderator

Join Date: May 2008
Location: Toronto
Quote:
 Originally Posted by HiFiNutNut Is there any formula to calculate the Vref based on the Idss of the njfet and the reference resistor in v1.1? For the same Vref, higher Idss requires higher resistor value, right? Anyone has the figure of the input impedance of the Q3 BC550C? Has anyone thought of using a JFET instead of bipolar in place of Q3? lower noise, higher impedance? What is the problem with a JFET?
V_out should be about 0.6 + Idss_Q6 * (R7 + Rtrim)

Yes, for same Vref, higher Idss requires higher Rtrim

Q3 vs jfet: I certainly thought of it, but found other areas of improvement so I addressed those first.

 20th October 2009, 11:41 PM #1405 HiFiNutNut   diyAudio Member   Join Date: May 2004 Location: Sydney Ikoflexer, Yes I derived the same formula but the result did not match, hence the question. In my working example, Idss=7.3mA, so 0.6 + 0.0073 * (2200 + 200) = 18.12 (V). I only got 13V. By the way, if that is the formula, for a given voltage, the higher the Idss would require lower Rtrim. Edit: The above figures are approximate. I have not measured the actual current in this leg. The Idss was measured independantly, i.e. outside the working circuit. Perhaps that is where things can go wrong. Regards, Bill Last edited by HiFiNutNut; 20th October 2009 at 11:45 PM.
 20th October 2009, 11:47 PM #1406 iko   diyAudio Moderator     Join Date: May 2008 Location: Toronto What's the voltage across R5? What's the current through Q4? Insert a small resistor (1R) between drain of Q4 and GND and measure the voltage across it.
 20th October 2009, 11:52 PM #1407 HiFiNutNut   diyAudio Member   Join Date: May 2004 Location: Sydney Yes, I will do those. A bit difficult to do the Q4 one. I think that while there is no load the current going through Q4 has to be very close to that of Q1, just minus the one going through R5.
Salas
diyAudio Chief Moderator

Join Date: Oct 2002
Location: Athens-Greece
Quote:
 Originally Posted by HiFiNutNut Ikoflexer, Yes I derived the same formula but the result did not match, hence the question. In my working example, Idss=7.3mA, so 0.6 + 0.0073 * (2200 + 200) = 18.12 (V). I only got 13V. By the way, if that is the formula, for a given voltage, the higher the Idss would require lower Rtrim. Edit: The above figures are approximate. I have not measured the actual current in this leg. The Idss was measured independantly, i.e. outside the working circuit. Perhaps that is where things can go wrong. Regards, Bill
Under just Vbe derate your 9-10V measured 2sk170 idss by 30-35% as I have wrote here some times before.

 20th October 2009, 11:56 PM #1409 HiFiNutNut   diyAudio Member   Join Date: May 2004 Location: Sydney So to get the same output voltage, a lower Idss_Q6 would require a higher value R7+Rtrim, right?
 20th October 2009, 11:59 PM #1410 Salas   diyAudio Chief Moderator     Join Date: Oct 2002 Location: Athens-Greece Naturally.

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