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Help with ALWSR

IDM

pfm Member
Hi All sorry for basic questions but I really need some help.

I bought a secondhand ALWSR several years ago. I had too many projects on so it went in the spares box. I have now finally got it out and am trying to set it up for +5V.

So the changes I made were:
Change ZD1 to a TL431.
R8 and R9 were changed to 1K each.
R5 and R7 were strange values so I changed them to 249R and 499R.

When I tested the board the output is 7.8V not 5V!

I checked the voltage across ZD1 and it 2.49V which sounds right.
Voltage across R8 is 4.37V
Voltage across R9 3.51V

Can anybody advice me on what to try changing next or is the board previously damaged?

Cheers
Ian
 
D2 needs to be an LED, not the 4.7v zener of higher voltage versions, else the error amp cannot pull the drive to the output transistor down far enough. Otherwise your numbers look fine.
 
Thanks Martin.

If I replace the zener does the LED go in the reverse orientation (I seem to recall that zeners are reverse biased to give the correct breakdown voltage)?

I have a green LED to hand I assume this will be ok

Cheers
Ian
 
Green LED will be fine. It needs to go in with cathode toward T2, so yes, opposite polarity to the Zener. Look at thw flat spot on the LED which identifies teh cathode. Edit: It's marked-up in the ALWSR manual:

alwsr5v.PNG
 
Great. I will make the changes.

Interestingly my version of the ALWSR instructions make no mention of the LED replacing the zener in the instructions, however, the diagram for the 5V vrsion does have an LED in that position!

This is the second 5V version I have built I must have a look at the old one and see what I did.
 
Interestingly, the ALWSR BOM from the manual (version 2.9iss009) has 1N5333B for the 5V version, whereas the actual circuit board shows an LED. I'm running 1N5333Bs in my 5V ALWSRs... weird!
 


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