Building this psu but not fully understanding diode use

[multime]

This is a psu i am constructing, pcb has components soldered but i am not fully understanding the use of some of the diodes. These are d4 to d7 and d8  which in parallel r4. I was planning to use r4 value changed to 0.01r as a current sense resistor and measuring the voltage dropped across it to drive a digital current meter but it doesn't look like it will work with d8 in place. Can anyone advise on this.

 

[ledtester]

Those diodes look like the protection diode D1 in this LM317 circuit:



From section 9.2.1 of the TI datasheet:

https://www.ti.com/lit/ds/symlink/lm317.pdf

Protection diode D1 is recommended if CO is used. The diode provides a low-impedance discharge path to prevent the capacitor from discharging into the output of the regulator

This can happen, for instance, when the input supply voltage drops (like when it is powered off).

[Kim Christensen]

d4 to d7 are protection for Q4 to Q7 incase a DC voltage is applied to the output of the power supply.

D8 protects the MC1466 if the pass transistors (Q4 to Q7) fail when there is short circuit on the output.

 

I was planning to use r4 value changed to 0.01r as a current sense resistor and measuring the voltage dropped across it to drive a digital current meter but it doesn't look like it will work with d8 in place.

That will change the current limiting setpoint and adjustment range. Changing R4 from 0.05R to 0.01R means the max limit setting goes from 4A to 20A. Setting it to 20A will result in smoke. D8 wouldn't be a problem for using R4 as a measuring shunt since the voltage across R4 should always be much less than 0.7V
Instead of changing R4, you could put an appropriate trimmer pot across it and scale the voltage that way.

[multime]

Kim Christensen / ledtester

Good info  I can now see the why the diodes are there.  Here is some info on the psu spec

300 VA toroidal transformer twin 15v secondary's  these are to used in parallel at up t 12.5 -13v at max 16amp above this voltage they are relay switched from parallel to series to give 30 to 40v at max 8amp so the psu is 0 to 30 to 40v output.

so i will need to change r4 to accommodate for the 16amp output.

Series pass transistors are 6 x Tip35C mounted on a 0.3 c/w fan cooled heatsink.