[radiorich]

Hello Everyone,
I have one of my Heavy-duty bench powersuplies made by Samlex it's 13.8 volt dc 20-25 amp supply.
What I would like to do is replace it's main filter electrolytic capacitors which it uses six 4700ufd capacitors to add up to 28,000ufd .
they used a no-named brand and I was wondering about getting some higher value capacitors to replace those six .
or should just stick with the original design ?


Here are a couple photos of this monster        


Sincerely Richard
P.S. the other thing I thought would be handy is volt and amp meter to add to it !
this unit really used seven 4700ufd capacitors

[DaleHCook]

I suggest sticking with the original values. Depending upon the design if you increase the cap values too much the increase in inrush current may damage components ahead of the caps.

[radiorich]

Hello Dale,
What I plan to do is to use a couple  of  14,000 ufd capacitors and put them in parallel and take that circuit board with the 6 4700ufd capacitors are mounted on them to the chassis .

Sincerely Richard

[morzh]

Rich


Any real reason you want to do this, or it is one of those itches "I can make it better"?
BTW, looking inside, I do not see (well, of course it is not like looking at a sch) what would regulate to 25A.
I mean, I see something bolted down to the bottom on some sort of heatsink, which does not impress me as a real oomphy one.
Of course it could be that the input voltage before the LDO (which might be driving an output transistor; I do not know of LDO that is a 25A one, usually it is a transistor with LDO or a zener, or an OpAmp circuit that drives it), and the dropout is small, but even a half-volt one at 25A will dissipate 13W.
Relying on the case as a heatsink is not a good thing.
And, judging by the transformer, it is a linear supply, not a switcher.
Do you have a sch of it?

These (I've seen a few on Amazon), especially considering 13.8V output, are usually made to float Lead-acid batteries.
Also, keep in mind that 28,000uF is a lot, and I mean A LOT of capacitance. Sometimes it is really more of a number in our head, but I once made a small demo to a seasoned contractor EE who designed our power supply for a multy-dwelling modem, run from a 12V 200Ah lead-acid battery as a backup.
His board would reset the unit, and he did not understand why a measly 200uF would do that to a battery. And he did not believe me when I said that that was what did it.
To demonstrate I took a 10uF small 3mm diameter 10mm long aluminum cap, put the scope at the end of 9AWG wires that connected the unit to the battery, set the trigger at 3V (the battery was 12V) and put the 10uF cap across the scope probe. The scope triggered down to 2V.
The guy was picking his jaw off the floor and then stayed with his mouthh agape for about 15 seconds or so.

In your case, the ripple will obviously depend on the load. Do you really intend to exercise that thing to the fullest? If not, also keep in mind that the regulated supply brings the ripple down by the very act of regulation, even with a smaller capacitance. The voltage ripple transfers to the input of that regulator.
That ripple will be given the frequency of the rectifier is 120Hz, the current of 25A and the capacitance , about 7.5V. But that is at 25A. At 2.5A it will be 0.75V. No biggie for the regulated voltage input.

Doubling the capacitance will half the ripple.
And, all large caps (yours are 4700 uF) have their ripple I of a few amps, which in parallel makes it huge.

I would stick with the original, save for a situation where you clearly know you need more.

[radiorich]

Hello morzh ,
Well first all the filter capacitors have gone bad and this unit uses 5 T03 type transistors mounted to a large heatsink mounted to the back of the unit .

I used this unit in my shop to run high power car audio and ham gear when I am repairing it on he work bench and no I don't have Schematic for this unit i might look on the companies website since they still build a lot powersupplies and yes this is a older linear supply and they still build them plus newer switch mode type.

Sincerely Richard

[DaleHCook]

What voltage are the 4,700 microfarad caps? Using 350 volt replacements may be risky.

[morzh]

Dale:

WHy would there be 350V caps in a 14V output unit? I think, Rich just showed this as a representative type of the caps he was intending to use.



Rich:

A-ha...you should've started with that . I thought you were simply trying to improve the existing unit.
Yeah, if you have space, sure.
What was the original caps WV?
50V caps is probably what you want.

[David]

Have 2 Aston power supplies replaced the Ecaps in both several years ago. Have had them for over 20+years used 25 volt rated  caps. Ordered all parts from Mouser.  David

[morzh]

25V is a bit low. That is, it is higher than the applied voltage, but you want some decent margin.
In this case, you have almost 14V out. This being the regulator (an LDO and a bipolar transistor), you need a volt or so of a dropout.
Make it 15V.
15V is the DC average. The Vp (peak) is 3.14/2 times that, or roughly 23V.

And the unit mfr probably chose to have some margin, so adf another 2-3V.

Of course the cap smoothes the pulsation p-p, but there is still some.

Your 25V is very close. If not under.
This is why I prefer 50V, as life of the cap is inversely proportional to the ratio of WV/Vapp.

[radiorich]

hello Morzh ,
First of all the 4700ufd caps where rated at 25 volts .
Yes , I plan to put some higher voltages ones in and I agree that does not give much headroom .
Far as the output transistors there is a big heatsink mount on the back of the case the only thing that is heatsinked to the case is the bridge rectifier .

Sincerely Richard

[morzh]

Rich

I do not know how long the caps have lasted, but if the power supply was not used much at large currents, the higher voltage caps could've lasted longer.

BTW, have you measured the voltage straight from the rectifier, before it gets to the regulator?

[radiorich]

Hello Morzh,
No I have not tested the output voltage from the regulator yet but since the capacitors are reading below their value I thought I would be a good time to change them .
The only time this unit was used when I was repairing high wattage car audio .
back when my shop was running full time I did a lot car audio and some of the brands I did factory warranty work too.

Sincerely Richard