Replacing a 1500 uf 65VDC Cap with a1500uf 63 volt cap

[benmmaddog]

I know I have seen this question many times , but I have the schematic  to help
I have an old Zenith stereo (c. 1974) .  Sounds like i need to replace the caps on the power supply
but the supply has this Axial 1500 uf cap @ 65 VDC

here is the schematic , I have attached the original to this post also


The Cap in question is C504
I see that the rail its on is only 47 volt DC  and was thinking that a 63 volt cap should work
its listed as a 1500 uf 65 V cap
also the part numbers they have are:
Cornell-Dubilier   WBR1500-100
Sprague              TVA-1319.8

If you are wondering why not get a 100 V and call it done is cause the price of the cap are crazy and mostly where I have looked its request a quote , Min. of 40/80 . at least the 63 I can get at a decent price and not have to buy a boat load

[tpowell1830]

No worries...

Uh, BTW, welcome to the forum.

[tautech]

A 63V cap there will be fine just be sure not to select a low ESR type for this linear PSU usage.
It's likely if that unit was built today it would be specified with only a 50V cap such is the cost cutting these days. 
63V is the next voltage range up and appropriate for this application.

[Bashstreet]

I would think it is totally fine.

[coromonadalix]

normally low esr are used in smps  power supply, computer supplies/motherboard  etc ..  simply put : everywhere thats not at 50 or 60 hz  linear based power supply 

[NiHaoMike]

Too low an ESR can make some linear regulators unstable. But where it is, a low ESR probably wouldn't hurt but wouldn't offer that much benefit either.

[tautech]

... just be sure not to select a low ESR type for this linear PSU usage.

Why?

Old design that most likely won't specify the need for low ESR caps.
Power on current inrush is higher into low ESR.....therefore risk of exceeding rectifier diode Recurrent Peak Forward Current ratings.

We can't see all of the schematic but presume the transformer is fed from AC mains (low frequency) whereas if from a SMPS primary and higher frequencies the current peaks would be lower but then of course you'd have fast recovery diodes there too.
Part replacement with modern parts need always take the 'type' of circuit into consideration rather than 'shotgunning' components with typically used parts of today.

[Ian.M]

OTOH, finding caps with a high enough ripple current rating that aren't low ESR, and/or 105 deg C rated can be difficult.   I created a quick & dirty sim of the O.P's cap at the specified 740mA load current, and it looks like it will need a minimum ripple current rating of 1.3A RMS. That's easy to find in a low ESR 105 deg C radial format, but much harder and more expensive to find in axial format with an ordinary ESR.   The sim gave the peak surge current as about 18.5A for the first half cycle at 10% high mains voltage, and 50% excess capacitance, assuming the only limiting factor was the transformer secondary resistance (all other components ideal).

The O.P's schematic doesn't identify the rectifier diodes, so lets assume they are generic 1A silicon similar to a 1N400x series diode.  A 1N400x diode's  peak non-repetitive surge rating is 30A for a single cycle, and by the next cycle the peak's dropped to 6A worst case and its within its 1A average continuous rating.  Therefore I'd have no concerns with replacing that cap with a low ESR one unless the diodes looked really wimpy or already showed signs of overheating, which would encourage me to replace them as well.

If its for your own use, bodging in a radial cap in place of an axial is fairly simple if you crop whichever lead will go to ground and splice on a length of solid core insulated wire, bent over the rim and secured with a small cable tie near the top of the can.   If you bend the lead over, you *MUST* support it at the bend so you don't stress its seal in the bung as any damage to the seal typically results in premature failure.  For liability reasons don't bodge it with a radial if you are being paid for the repair.