Adjustable power supply for your bench

[antonis]

I should call it "internaly trimmed" rather than "adjustable"..

P.S.
I know you like to get yourself better Kevin so I strongly suggest you to study Chapter 25 in below:
https://nvhrbiblio.nl/biblio/boek/Self%20-%20Small%20signal%20audio%20design-2e.pdf
(especially about mutual shutdown circuitry for bipolar supplies..)

[Kevin Mitchell]

Woah where did that come from?
I could have used this a long time ago thank you so much! I'll keep a copy with my early 1970s electronics handbook. Though this would prove to be far more useful 

I have seen that sort of implementation of that circuit for "mutual shutdown" before, perhaps in the datasheets. Personally haven't seen it used in the real world. It really should be included in a production design as one should expect anything and everything to go wrong - within reason.

Thanks again!

[Rob Strand]

(especially about mutual shutdown circuitry for bipolar supplies..)

What should be done is an interesting topic.

A lot of professional Lab quality power supplies have a tracking mode which typically is used when you want a bipolar +/-V supply.  If you haven't used one of these before the voltage setting on the master supply controls the outputs of both slave and master DC outputs.  The pot on the slave channel is disabled.   In non-tracking mode both channels behave independently and identically and the master/slave labeling means nothing.

In one configuration the second channel (slave) tracks the first (master) by literally tracking the voltage on the output terminals of the master.   When you overload or short the master the voltage drops and the slave will follow.     However when you overload or short the slave, the slave voltage drops but the master does not follow.     The behaviour is asymmetrical.

In a second configuration the second channel (slave) tracks the first (master) by following the voltage *pot* setting.   When you short or overload either channel only the voltage on that channel is affected.

I reviewed a number of professional power supplies and both behaviours are out there.   As far as I can rememeber there's none which pull both rails down if *either* one of the rails drops, like a mutually tracking idea.   No doubt that's because of the messy analog electronics required to handle this off the radar case.    (There was one Tektronix power supply which had a lot of funky stuff going on but I can't rememeber if it did mutual tracking at the end of the day.)

All being said you might not want or care if the supplies to mutually track!

Anyway the take home message is there's no standard for this behaviour so there's no point enforcing such requirements on a simple supply because not even the big-boys are consistent.

Having a tracking mode can be convenient.

[akc1973]

I should call it "internaly trimmed" rather than "adjustable"..

P.S.
I know you like to get yourself better Kevin so I strongly suggest you to study Chapter 25 in below:
https://nvhrbiblio.nl/biblio/boek/Self%20-%20Small%20signal%20audio%20design-2e.pdf
(especially about mutual shutdown circuitry for bipolar supplies..)

Not meaning to butt in but thanks for that link. What a great book!

[anotherjim]

I think there were some of those synth IC's (can't recall if it was Curtis or SSM?) that were prone to self-destruction if the bi-polar supplies were not always equally present. This was fixed in the new replacement chips.

[ElectricDruid]

I think there were some of those synth IC's (can't recall if it was Curtis or SSM?) that were prone to self-destruction if the bi-polar supplies were not always equally present. This was fixed in the new replacement chips.

I haven't heard about it with the CEMs (although it wouldn't surprise me) but the SSM2164 quad VCA is famous for this. Removing the negative supply while the positive is powered up fries the chip permanently, detailed here:

https://www.njohnson.co.uk/index.php?menu=2&submenu=2&subsubmenu=3

[Rob Strand]

I haven't heard about it with the CEMs (although it wouldn't surprise me) but the SSM2164 quad VCA is famous for this. Removing the negative supply while the positive is powered up fries the chip permanently, detailed here:

https://www.njohnson.co.uk/index.php?menu=2&submenu=2&subsubmenu=3

I guess the main point is if you want to avoid problems the protection needs to be on the device or on the board.

As far as a bench/lab power supply goes there's no point making it a mutually tracking for these special cases.   You could just as easily pull one of the power supply leads out of the power supply which will instantly remove one of the supply rails.

There's worse mistakes than that:   connect the supply leads around the wrong way.   accidentally dial up 30V on a 3V3 rail.
(I noticed the Rigol lab supplies have a programmable upper voltage limit setting which overrides the normal voltage adjustment setting to stop silly mistakes.)    Pulling out the ground when you have 3V3 and -30V will cause problems I'm sure!


If you use special chips (or you only have one part left  ) it's always wise to put protection on the breadboard.   It still doesn't stop a rogue wire flinging off and falling on the board.