Power Supplies - Of The Bench Variety...

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Power Supplies - Of The Bench Variety...

Postby Steve Anderson » Thu Sep 08, 2022 4:51 pm

Many years ago, decades in fact, I bought two cheap bench-style power supplies for use during development of projects. They were cheap yet amazingly have lasted until now. I blew one up a couple of months ago trying to recharge a car battery - yep, I got the polarity wrong! So after 30+ years, replacement time...

"I'll build my own - it can't be that hard can it?" Err...wrong! The pitfalls awaiting the unwary are plentiful and many, this is outlined on Rod Elliot's site in this article...

https://sound-au.com/articles/bench-supply.htm

Rod's site is primarily related to DIY audio gear, but there's a lot more - like power supplies, DIY test equipment and so on.

So reading through that as well as a more recent example of his work....

https://sound-au.com/project223.htm

...I'm still going to DIY it. It's no push-over, but if successful I'll get what I want, and not what a manufacturer thinks I (or you) want or need...though as is pointed out you're unlikely to save money if you buy all the parts required - but if you have a well-stocked 'junk-box', things could be different...

I'm interested in what sort of specification you would draw up given a clean sheet of paper. You need to be realistic as the more voltage/current/power you add the more expensive the exercise becomes. You also need to feel confident and competent enough to handle mains (120/220V) wiring as there's an element of that too.

I'm still in the 'specification phase' so my 'shopping list' isn't yet fully defined. I've thought back over some years on this, what is 'nice to have' and what did/do I really actually need?

I've drawn up an interim specification to suit what I think I need and it's getting close to being 'just right' for what I need, but I'll leave it a week or so before I commit myself. Your requirements may be somewhat different...

If you're considering something similar now or in the future, Rod's articles are invaluable...save/download/bookmark them...

Steve A.

P.S. I have no connection with Rod Elliott nor the site...
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Re: Power Supplies - Of The Bench Variety...

Postby Steve Anderson » Wed Sep 14, 2022 2:30 pm

I've made a start on the variable plus/minus supplies as these are the most challenging parts of a bench power supply.

I settled on a split arrangement with the positive and negative outputs being variable from zero to 15V, positive and negative. The current limiting cuts in at a selectable 60 or 400mA. This may seem too low for some users, but in all the time (30 years) I used the cheap versions this is replacing I rarely, very rarely needed anything more than 100mA. I tend to build in stages, design, breadboard, build, test. OK - that bit is done, now the next.

As your project expands you are going to have to build its dedicated supply at some point, so I do that quite early on, then transfer the completed stages to its eventual permanent supply. It's the lowest risk method I've found in practice.

If you're building 100W audio amplifiers and the like, well that is different. The nearest power requirement in NBTV is generally going to be disc motor-drive and/or LED drive. For 99% of all the other stuff I build outside of NBTV/SSTV this is enough.

In addition to the two variable supplies there's also fixed outputs at +3.3, +5.0, +12, -5.0 and -12V all at 1A. The most common voltages I use and enough current to 'get things going'.

An initial sketch of the variable supplies follows, some of it might seem like, "Why have that bit?" There is a method in my madness...more later...

But it would be quite easy to increase the current...with suitable heatsink(s) for the series-pass transistors and a simple adjustment of the current limiting.

At this stage this has not been built or tested or even simulated. I'm quietly confident so I'm going to skip the simulation stage...here goes!!

Steve A.

I've just noticed the NPN transistor in the negative part is unlabeled, it's TR823 (the BC547B)...the master has been updated...

Variable PSU 1.gif
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Re: Power Supplies - Of The Bench Variety...

Postby Steve Anderson » Wed Sep 14, 2022 6:53 pm

I'm sure that someone is going to say, "Why not use a LM317 for the positive supply and a LM337 for the negative.?" Yes, it is the most obvious and an easy way to meet my requirements. Really, really simple. However there is one caveat. The most usual arrangement and the simplest arrangement is very similar to this, from the datasheet:-

LM317-ADJ.gif
LM317-ADJ.gif (22.82 KiB) Viewed 5948 times


The LM337 is the same with the polarities reversed.

R2 in the formula is a pot. What happens if the wiper goes open-circuit? Remember this is an elctro-mechanical part. The track suffers from wear and tear (the 'crackly volume control' syndrome), moisture and dust ingress too. In fixed voltage use (say you want +6.8V for some reason) two fixed resistors as per formula is fine, I've used them many times in this mode. R2 can also be a good quality enclosed pre-set, once set - forgotten. But even with an expensive conductive plastic. cermet or wire-wound panel-mounted pot, one day it will fail. If you use the usual carbon versions, you're asking for trouble.

Your precious 5V circuit attached to the output gets the full unregulated supply on the input of the 317/337 less a volt or two - say goodbye...

That's why I have not chosen the 317/337 for this application. My version may be more complex, but it is fail-safe...more on this later...

Steve A.

Yes, I know, there are many, many versions using the 317/337 on the 'net, but they all suffer from this potential failure with few exceptions.
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Re: Power Supplies - Of The Bench Variety...

Postby Steve Anderson » Thu Sep 15, 2022 2:53 pm

Follows is the simplest version of the dual variable power supplies. 0-15V, 60 or 400mA each, individual voltage controls.

Note that the fail safe requirement is met by the 100k resistor from the wiper to 0V of each pot. If the wiper does go open-circuit that resistor will pull the non-inverting input of the op-amp to 0V - the output will follow. The op-amps have JFET inputs so the bias current can be forgotten about. It does make the voltage control very slightly non-linear, but I doubt most would notice it.

Two separate op-amps are required (TL081s) as the +/-25V unregulated supply is too high for a dual version.

This version removes the requirement for an additional buffer and inverting stage for the negative supply.

Steve A.

PSU 2.gif
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Re: Power Supplies - Of The Bench Variety...

Postby Steve Anderson » Sun Sep 18, 2022 3:39 pm

Originally I was going to use a 7805 regulator for the +5V/1A supply and a LM317 configured suitably for the +3.3V/1A supply. in most cases that should be sufficient. But depending on the transformer(s) used they may end up dissipating a lot of heat. However I came across this from a local supplier to me, it supplies +5V/3A and a variable/preset output of 1.3V to 30V at 3A depending on the input voltage. I would simply set the second output to 3.3V.

It is a switching regulator however, efficient, yes, but maybe electrically noisy. As I intend to generally only use the 5 and 3.3V outputs for logic, that should be OK. If testing a sensitive analogue circuit, e.g. a microphone preamplifier, simply shut off the DC input to it. The other supplies should all be nice and clean now. It's unusual but not unheard of for sensitive analogue circuits to use such low voltages as 3.3V or 5V

It costs the equivalent of US$10 here which is cheaper the buying the components alone. I would be daft not to use it...

Steve A.

...the output 'catch' diodes are on the underside of the PCB...the board measures 43 x 56mm...


et-mini pwr5-adj_2re.jpg
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Re: Power Supplies - Of The Bench Variety...

Postby Harry Dalek » Thu Sep 22, 2022 8:26 am

Looks good Steve just come across your post with little of late but i will study your posts and schematics .
Nothing more handy than a good bench power supply .
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Power Supplies - Of The Bench Variety...

Postby Steve Anderson » Thu Sep 22, 2022 2:05 pm

Thanks Harry. I'm still in the process of design, something like this needs to be 'belt and braces' in terms of protection with the expected abuse it'll get. I'm a bit stuck at the moment with the temperature sensing and shut-down, but I'll get there eventually - I'm just waiting for one of those 'Eureka' moments!

Steve A.
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Re: Power Supplies - Of The Bench Variety...

Postby Steve Anderson » Thu Sep 22, 2022 6:37 pm

Would you believe it? Now my multi-meter has decided to quit! I bought it new from GW-Instek (Sth Korea) about three/four years ago. To replace it with a similar device from Fluke (my preference) is about three times the price. It wasn't overloaded or abused - it simply decided to become wholly inaccurate one day of its own volition....

The question is...do I need (that word again) something, A) That portable? B) That accurate? When you're doing something basic like fixing/testing something to do with the house electrics - no. Something that involves the car electrics? Again no. The only real justification for accuracy may be on the workbench. So a bench multimeter is the answer. But - the price of such devices makes them simply a no-go.

Time for DIY again...and analysing what you need and the 'nice-to-have' extras...

It never rains, it pours...

Steve A.

So the upshot of all this is the bench multi-meter has priority, then the bench PSU...I'll start a separate thread for the bench multi-meter...

For the more mundane uses I'll just buy a cheap $5 throw-away version...
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Re: Power Supplies - Of The Bench Variety...

Postby Steve Anderson » Sat Sep 24, 2022 4:03 pm

Steve Anderson wrote:..I'll start a separate thread for the bench multi-meter...

Oh no I won't! Having looked at the requirements for such a device, it's a nightmare! It is not as easy as you might think...and nowhere near as easy as I thought! I take my hat off to those that design such things!

So, I'm gonna have to buy a replacement whether I like it or not!

Steve A.

The bench power supply seems easy in comparison, though that's no piece of cake either...
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Re: Power Supplies - Of The Bench Variety...

Postby Steve Anderson » Sat Sep 24, 2022 5:03 pm

A word to the wise...just like newer cars these multimeter manufactures are stitching you up for future income. Looking at current Fluke models, if you don't return it for re-calibration after a certain period of time (unknown) it shuts down and will not let you use it. Your car, if you don't get the dealer to do the routine servicing, it beeps and farts at you until you do. In both cases it needs 're-setting' with the dealer's computer saying it has been maintained/calibrated when due.

This is violating my 'right to repair' or 'right to maintain'. Apple have had/are having an issue over this. An insidious infringement of my rights in the creeping 'after-market profit rip-off'.

It's going to get worse before it gets better...

Steve A.

The concept is reasonable(?) within the guarantee/warranty period. after that, it should be my choice if I want to get my hands dirty...
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Re: Power Supplies - Of The Bench Variety...

Postby Harry Dalek » Sat Sep 24, 2022 8:19 pm

Oh that sounds like crap ! i would never buy from them again what a load of rubbish !
It does sound like all the mess that happens if your Iphone mucks up who could be bothered trying to repair it ,lucky my son has the patience for that i don't .
So its getting worse now built in shut down for devices to make money no wonder i like old electronics !
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Power Supplies - Of The Bench Variety...

Postby Steve Anderson » Sat Sep 24, 2022 9:13 pm

Harry Dalek wrote:Oh that sounds like crap

Agreed...but it seems the way things are going...
"Oh! Your TV is three years old...in needs a colour recalibration, if you don't do a recalibrate (at a cost) it will shut down until you do...here's our payment plan....sorry, we forgot to mention this when you bought it...enjoy a blank screen until then...utter BULLSHIT!

Steve A.

This might not be right now, but I see the thin edge of the wedge coming...

...anyway, getting off-topic again...
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Re: Power Supplies - Of The Bench Variety...

Postby Steve Anderson » Sat Sep 24, 2022 11:26 pm

But there'll always be those clever chaps that can get around it....but again at a price...

Steve A.

Anyway...bench power supplies.....with no expiry date or software updates you really don't need (or even want forced upon you)....
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Re: Power Supplies - Of The Bench Variety...

Postby Steve Anderson » Wed Oct 05, 2022 3:37 pm

This 'simple' exercise of a DIY bench power supply is/has (and probably will) go/gone through a number of alterations/mutations and has almost come back full-circle. Read Rod Elliot's pages referred to above.

It will use the LM317/337 devices, but without those dreaded pots...they'll be replaced with rotary encoders that program the output voltages on the +ve and -ve variable outputs. It may seem this adds complexity but it may confer other advantages and flexibility....and in getting rid of those pots, improve reliability.

The other 'disadvantage' of the LM317/337 duo is the output as usually configured cannot go down to zero volts. The minimum is +1.25V and -1.25V respectively. However there is a way to get around this if you really want or need to. Myself, I'll probably not bother, how often do you need less than 1.25V of either polarity?

I'm still in the early stages of (re)design so no schematic to post as yet.

Updates as and when...

Steve A.
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Re: Power Supplies - Of The Bench Variety...

Postby Harry Dalek » Wed Oct 05, 2022 8:08 pm

Steve Anderson wrote:This 'simple' exercise of a DIY bench power supply is/has (and probably will) go/gone through a number of alterations/mutations and has almost come back full-circle. Read Rod Elliot's pages referred to above.

It will use the LM317/337 devices, but without those dreaded pots...they'll be replaced with rotary encoders that program the output voltages on the +ve and -ve variable outputs. It may seem this adds complexity but it may confer other advantages and flexibility....and in getting rid of those pots, improve reliability.

The other 'disadvantage' of the LM317/337 duo is the output as usually configured cannot go down to zero volts. The minimum is +1.25V and -1.25V respectively. However there is a way to get around this if you really want or need to. Myself, I'll probably not bother, how often do you need less than 1.25V of either polarity?

I'm still in the early stages of (re)design so no schematic to post as yet.

Updates as and when...

Steve A.


Yes the pot part is a bit of problem i think you looked into this before many posts back .
The pot adjustment is handy at times but fiction adjustment device not the best thing to use over and over pity you can't do this with a variable capacitor nothing to wear out thinking of those big old radio tuning caps .
Perhaps with a few LM317s or higher amp version devices and single resistor for each and just switch the outputs and a few volt increments between them ? well bit more pricy for the regulators but have a safe higher or lower voltage on the output switching /
How many volt increments do we need half the time most stuff is run on 5 or 12 volts and some times 9 volts some times dual 15v
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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