Bench Power Supplies.
Posted: Thu Sep 02, 2021 4:24 pm
Power supplies, more specifically the 'bench' versions. Incredibly useful, abused and taken for granted...until they fail. I bought two 0-15V/3A versions in Hong Kong some 30 years ago, cheap, certainly not precision, but they did the job. I blew one up recently by trying to recharge a car battery. Wrong polarity. Goodbye...
Not worth trying to fix, and one on its own is pretty useless, so it's replacement time. Commercial versions are ridiculously expensive for what they are, so time to make my own. Power supplies are boring, but a necessary part of the whole.
Just like buying anything, what do I need? Those two supplies easily did 95%+ of all I needed for semiconductor work, I rarely (perhaps never) needed 3A, 1A would have been perfectly adequate. So 0 to +15V and 0 to -15V both at 1A would be a good start. Easy...LM317 for the positive, LM337 for the negative, though they start at minimum of +1.2V or -1.2V, that's acceptable.
I also thought it would be useful to add a few fixed voltage outputs too...
+3.3V/1A, Micros, memory, logic etc...
+5.0V/1A, as above...
+12V/1A, mainly analogue stuff...
-12V/1A, as above...
...those in addition to the variables. Not all currents delivered at maximum simultaneously though! All easy with 78xx's/79xx's, except the +3.3V, use another LM317 with two fixed resistors. To limit dissipation for the 3.3V & 5V regulators some power resistors in series with the unregulated input. Though a large heatsink may still be required.
Protection? The regulators have current and thermal limiting but no reverse polarity protection if trying to recharge batteries! But the regulators are cheap and easy to replace if you fry one. Fuses and diodes may help on the outputs...
However, there's one major issue I have with the variables...
A typical arrangement from the datasheet...
What happens if the pot(s) fail? And they will fail, they're electromechanical. The regulators will output the unregulated input supply, less a volt or two. What's the most common failure mode of pots? Open circuit wiper, maybe only over a small part of the track, but the damage is done. Your 5V load circuit is toast!
So maybe not LM317/337's, more research needed. What is required is an arrangement that when a pot fails the supply shuts down.
Steve A.
Not worth trying to fix, and one on its own is pretty useless, so it's replacement time. Commercial versions are ridiculously expensive for what they are, so time to make my own. Power supplies are boring, but a necessary part of the whole.
Just like buying anything, what do I need? Those two supplies easily did 95%+ of all I needed for semiconductor work, I rarely (perhaps never) needed 3A, 1A would have been perfectly adequate. So 0 to +15V and 0 to -15V both at 1A would be a good start. Easy...LM317 for the positive, LM337 for the negative, though they start at minimum of +1.2V or -1.2V, that's acceptable.
I also thought it would be useful to add a few fixed voltage outputs too...
+3.3V/1A, Micros, memory, logic etc...
+5.0V/1A, as above...
+12V/1A, mainly analogue stuff...
-12V/1A, as above...
...those in addition to the variables. Not all currents delivered at maximum simultaneously though! All easy with 78xx's/79xx's, except the +3.3V, use another LM317 with two fixed resistors. To limit dissipation for the 3.3V & 5V regulators some power resistors in series with the unregulated input. Though a large heatsink may still be required.
Protection? The regulators have current and thermal limiting but no reverse polarity protection if trying to recharge batteries! But the regulators are cheap and easy to replace if you fry one. Fuses and diodes may help on the outputs...
However, there's one major issue I have with the variables...
A typical arrangement from the datasheet...
What happens if the pot(s) fail? And they will fail, they're electromechanical. The regulators will output the unregulated input supply, less a volt or two. What's the most common failure mode of pots? Open circuit wiper, maybe only over a small part of the track, but the damage is done. Your 5V load circuit is toast!
So maybe not LM317/337's, more research needed. What is required is an arrangement that when a pot fails the supply shuts down.
Steve A.