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New bench power supply

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cowana

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Recently, my bench power supply stopped working properly. It is a digitally controlled supply, and the digital to analogue convertor, the analogue to digital convertor (for the displays) and the current limit have all gone very out of spec.

The error on the current is +100%, the error on the voltage is about +70%. The calibration menu of the supply only allows for 5% compensation - something inside is very wrong. I've examined it in detail, but there are no blown components. In addition, nothing is labelled - no voltages, cables or connections. I've tried adjusting all the pots to try and work out what they do (after marking their original positions), but I've come to realize the supply has had it for precision work, and is now only good for basic tasks - eg powering a battery charger.

So, I'm on the lookout for a new power supply. I like the idea of a double or triple supply, as I often work with high power LEDs, so need 12v for the LEDs and 5v for the control circuitry. Obviously the supply needs current limitting, and voltage and current readouts.

I want to spend below £200 on this supply, and probably get a 0-30v 5A supply.

I've found **broken link removed** power supply which seems to fit the bill - I'd appriciate any ideas on it.

The only downside seems to be a lack of output switches - but I could either fit these (internally or externally), or just disconnect the wire when needed.

Power supply manual
Power supply schematic

I'm impressed the manufacturer published the schematic (see above) - it will make it much easier to repair if anything goes wrong.

Any thoughts? I'd rather get something new than something second hand.

Cheers

Andrew
 
That PSU looks awfully complicated as in the schematic.

You buy that thing and it breaks you are back to square one. You need something well engineered and thus reliable and simple to fix if it (eventually) breaks....

I don't know where to point you though....

Not much good electronic engineering anymore, just a throw away society. And SMD are the reason. I HATE SMD with a passion.

Sorry for the rant,
Cheers
 
That PSU looks awfully complicated as in the schematic.

You buy that thing and it breaks you are back to square one. You need something well engineered and thus reliable and simple to fix if it (eventually) breaks....

I don't know where to point you though....

Not much good electronic engineering anymore, just a throw away society. And SMD are the reason. I HATE SMD with a passion.

Sorry for the rant,
Cheers

hi tvtech
I looked at the schematic and my eyes glazed over,,, Sorry Cowana, it looks a very complex circuit for a bench psu.

I am sure I would hate SMD if I could only see them, let alone solder the pesky things in.!

My shaky hands and dodgy vision, makes handling them close to impossible...:rolleyes:
 
The various models on the schematic:

The display board:
This uses 7106 drivers to drive the LCD - as used in every budget multimeter. Cheap, reliable, no problem at all.

The keyboard:
Just buttons!

The keyboard2:
This uses a 4520 binary counter to cycle through independant, series and parallel (via relays). Nothing too complex, I've used the 4520 before.

The adjust board:
Standard power supply circuitry - relays to select the transformer tap, a rectifier, smoothing caps and a few beefy transistors (15A each) to regulate the voltage. No problem.

The control board:
This is where the fun starts :p. Looking at the various parts:
Top left:
A simple power supply for the parts on this board - giving 5v, 8v and -5v.
Top right:
This isn't too bad either. It looks at the current voltage, then decides what transformer taps need energising, and switches them. Not too bad.
Bottom left:
Getting a little more complex here - this debounces and decodes the switches using schmitt triggers and more 4520s, to control a X9C103WS digipot - simple up and down variation of resistance. 4066 switches are used to enable/disable the output of the digipot.
Bottom right:
The output from the digitpot feeds into a few opamps, which mix the constant current mode settings with the constant voltage controls.

I appriciate that this sin't the simplest power supply out there - but it is much better than my current one, which has a big ATMEGA48 - I have no idea what it's programmed to do. I feed that troubleshooting this power supply would be possible - although there's a lot of it, the schematic is made of simple elements. In addition, there are two power supplies - if one has problems, you can compare voltages with the working side.

As for SMD, I'm fine working with it - nearly all my own projects are SMD (down to 0.5mm pitch).

I feel that I'd have a better shot at fixing a complex power supply with a schematic then a simpler one without. My current broken power supply has chips with unreadable part numbers - I don't have a hope!
 
Last edited:
hi cowana,
I guess my reply did sound a little negative.:)

Looking at that CSI Triple psu, it looks a decent unit for the price.
From a personal usage of psu's, I prefer to have single or dual psu's.
The problem as you have found with your failed unit, if one section fails, the whole unit is down.

For your high power LED projects, do you really need a high specification psu.?

Of course you are paying the bill and its your call, but I would buy 2 or 3 lower cost units, selected to suit your projects.
 
It's a good question about specification.

My LED system doesn't really need a lot of power. The reason I am looking at this high spec unit is that it will become my only power supply for (I hope!) a very long time. Therefore I don't want to get one that I will reach the limitations of. I do quite a lot of mechanical/robotic projects, so having quite a lot of current available is important.

I'm still not 100% sure whether I should go with the 3A version or the 5A version - although my projects/motors sometimes draw around 5A, the 3A unit can be paralleled to make a 6A unit. But then again, the 5A unit can be parallel to make a 10A unit - and some motors I have do draw that much. I think that for the extra £30, the 5A unit will probably suit me better.

It's an interesting point regarding seperate units - in fact, that power supply unit is available as a single power supply (0-30v, 0-5A) for 52% of the price. The way I see it:
Pros of seperate units:
More portable (as one can be moved on it's own)
Totally transformers rather than seperate windings (less noise)
Only one dies if something goes wrong internally
A power switch for each
Takes up 10mm less bench space
Cons of seperate units:
Slightly more expensive, and you don't get a 5v 3A as well
No series/parallel switch - needs to be done manually with wires. I don't know if the LCD readouts on the triple unit say the total voltage current when parallel/series is activated - if so, that's a big advantage of it.
Not quite so neat
 
I've had that exact power supply for a couple of years. I'm reasonably pleased with it, so far it has not failed to meet its specs. The display meters are surprisingly accurate. The parallel/series front-panel switching comes in handy. The fans are noisy, but thermostatically-controlled and only come on if you leave a significant load attached for a while.

My biggest problem is the voltage adjustment multi-turn pots are very low-quality. I ended up replacing the originals with good Bourns units. Also, I would have preferred a plain old pot for current adjustment, the digi-pot and button arrangement is tedious and imprecise. Since the digi-pot is 100-step, the current adjust increment is 50mA. No problem for charging batteries, but not good for precise adjustments.

I agree the schematic may look complicated, but when broken down in sections, it is really a straightforward design. Pass transistor with current source bias, diode-OR'ed op-amps for current and voltage regulation.

Here is a look inside. (NB. large photos)

Left side:
IMG_3598.JPG

Right side:
IMG_3600.JPG

Rectifier, filter cap, pass transistor, tap relay board in rear:
IMG_3603.JPG

Behind front panel. Not exactly a work of art, but this isn't HP or Lambda:
IMG_3605.JPG

One of the control boards (no, I didn't replace the drive transistors, it came that way):
IMG_3613.JPG
 
That's fantastic ModemHead - exactly what I wanted to hear - someone's experience!

The construction looks pretty good - some cheap products come covered in glue or silicone. I'm glad to hear it's still operating after a few years! You make an excellent point about the pots being low quality - replacing them looks to be a good idea.

But generally, I'm reassured that it is a good product - many thanks for the photos. I think I'll splash out for the triple 5A version - the only real reason to go for the 3A version would be if it would need replacing soon after buying it.

Many thanks again!

Andrew
 
Many thanks again!

You're welcome, glad you found it helpful. I've probably doomed myself, the unit will smoke tomorrow! But I'm pretty sure I could fix it, there's nothing too exotic inside, except maybe the LCD units.

About the multi-turns: the originals have 4mm shafts, so chances are you'll need to enlarge the holes and replace the knobs for a 1/4" shaft replacement, as I did.
 
An update for posterity

A new member of the forum with this same power supply PM'ed me about a problem with the 5V fixed output on his unit. It was not capable of producing the 3 amps specified. I rarely, if ever, use this output, so I checked my unit and found the regulation of the 5V output to be worse than poor. In fact, at 1A the output was only 4.76V, and at the rated 3A current it was down to 4.07V.

After a tear-down and examination I discovered that the LM723-based 5V regulator board and the associated transformer windings were all functioning properly, but the wires that lead to the front panel were incompletely crimped to the ring lugs that fasten to the banana jacks. This led to a significant voltage drop at higher currents. The part of the crimp that grips the insulation was the only thing keeping it together. After a re-crimp and wicking in some solder for good measure, the 5V output on my unit is now in good shape. It will deliver up to 4.4A at 5.00V, at which point the current limiting kicks in.

So, as with most gear of this class, one should be prepared to find and correct the manufacturing flaws, or pay the price for better stuff.
 
Great to hear. I prefer something even simpler, and rather than get combined dual, triple, etc., output supplies, I'll get multiple single output supplies, all linear and linear controls. You can get them fairly cheaply, between $50-80 each, very easy to fix or to re-mediate bad workmanship, or even enhance.

**broken link removed**


Fairly simple linear supply by Chinese makers, very little can go wrong even if the QA is shoddy.

**broken link removed**

The digital voltmeter and ammeter boards.

**broken link removed**

1980s style all through hole design.

**broken link removed**

Close up of the op amp and adjustment pots.

**broken link removed**

Front face of the supply. CC and CV, shot circuit protected. No bourne style pots [ they do cost more than the whole PSU above] but you can dial down to zero, but at 1V with stable 10mV precision, 1-10mV above 1V. During testing, easily delivers < 0.01% < 1A, and max at 0.1% AC on DC ripple at 3A. Outputs float if the ground isn't shorted, so you can easily parallel or series a number of these supplies to get more output V or A.

The ammeter is accurate but the voltmeter is off as high as 10%, depending on load. Its ok to dial in what you want, but for real accuracy, best check with a DMM at the DUT input.
 
Hey Andy, i don't know what your electronic capabilities are but if you built your own from scratch, from a schematic in a book or online for instance, you would be able to repair it if it went wrong easier? also you could build it to your own spec, modifying an existing circuit if need be.

Neil.
 
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