The simplistic Salas low voltage shunt regulator

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Question on pin designation

In the diagram simplistic-salas-low-voltage-shunt-regulator there is a pin designation that confuses me. It is the second diagram:
  • Q6 (IRF9540, a P-FET) has the D connected to the output of +5 volts.
  • Should that rather be the source?
  • The Vgs is the voltage across the 2SK170.
  • In a P-FET theire is often a reverse-diode; this would short the output to 0,6V if connected like this in the diagram.
  • I looked up also in other diagrams and PCBs and this seems to confirm me.
I thought rather ask than go ahead and have another burned device . . :mad:
I must be stupid as I have not seen corrections :scratch:

352752d1370378004-simplistic-salas-low-voltage-shunt-regulator-low-sshv-ad-3-25v.jpg
 
Sorry for the beginners questions but i need another troubleshoot. I built one shunt reg successfully following the 2008 circuit. Then i build another one using same pcb and components but this time both +/- sides first 3 red leds dont come on. Green leds do, and im able to regulate the v but output mosfets overheat while input ones are cold to the touch..

Why? ;s
 
Hi there!

I am working on a small project and I would like to know if I may expect any oscilations with selected components?
Also I have searched thread a bit for 2sk170 smd replacement and found for BF862. Would this be suitable replacement?
Maximum current that I will be using with this circuit is 400mA~500mA and 5V, so I am guessing that CCS resistor should be >1.5ohm....

BR,
Ales
 

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BF862 is OK for SMD replacement. Select no more than 0.6V pinch off sample for Q7. Avoid C5.
Use IRF9610 as Q1 for much better Ciss. Moderate Q5 for up to 5mA IDSS with some source resistor. R7=10R maybe not enough gate stopper in practice.
IRF9530 has better Crss for Q2 when gfs will not be significantly different from the 9540 at this low Id (comparatively low as 50W MOSFETs can go).
 
BF862 is OK for SMD replacement. Select no more than 0.6V pinch off sample for Q7. Avoid C5.
Use IRF9610 as Q1 for much better Ciss. Moderate Q5 for up to 5mA IDSS with some source resistor. R7=10R maybe not enough gate stopper in practice.
IRF9530 has better Crss for Q2 when gfs will not be significantly different from the 9540 at this low Id (comparatively low as 50W MOSFETs can go).

Thank you for your reply Salas. I have made changes according to your advice, but I don't know exactly how to moderate Q5, in current attached picture Q6.

Thank you again.

Best Regards,
Ales
 

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P.S. The practical problem with those JFETs is they don't come graded and are tiny to handle for selecting IDSS. You would not want more than 10mA anywhere for current noise, self dissipation, and Vref values scaling issues. Especially in Q5 for keeping its Vp up to a 0.6V limit also. So make some kind of jig arrangement to can select them and provide copper area for their G to dissipate on the PCB.
 
Rs between Q6 source pin and R7,Q3 node. What output voltage range is destined for? If its for low Vo you may avoid it.

I plan to use it for 5V(~500mA) and also for 3.3V(100~200mA).

P.S. The practical problem with those JFETs is they don't come graded and are tiny to handle for selecting IDSS. You would not want more than 10mA anywhere for current noise, self dissipation, and Vref values scaling issues. Especially in Q5 for keeping its Vp up to a 0.6V limit also. So make some kind of jig arrangement to can select them and provide copper area for their G to dissipate on the PCB.

I already made PCB in such way that they all lay on ground plane.

BR,
Ales
 
sorry for keep messing it up (both your circuit and the thread) but i caused a short somewhere that overheated the transformer momentarily and after that the shunt produces hiss. intensity of the hiss seem dependent on the v adjust resistor pot.

what components should i troubleshoot first? could the hum come from the trafo itself if it has shorted a wire somewhere?

edit: never mind, the transformer was faulty. how does this introduce hiss into the circuit?
 
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