Physical Electrolytic Cap Size Question

Thread Starter

John Czerwinski

Joined Jun 19, 2017
44
My question has to do with through-hole Electrolytic Capacitor physical size and finding the correct specification in ordering a replacement.

Example: I’m working on an old video arcade monitor. The parts list calls for two (2) 6800uF, 50V Electrolytic Capacitors (ECap). Based on that specification, you can go to Mouser, DigiKey, etc and get a through-hole ECap at that spec…and get a far smaller one (physical size-wise) than what was on the board.

From my limited knowledge, I understand that size does matter in this application because of the stress that the circuit puts on the ECap in charging and discharging, so I’m assuming that its something to do with the operational frequency of the ECap.

My question:
Is there something on the old cap I can look at to get the right replacement part? Going just by the parts list, it doesn’t state size or frequency.

Is it better to measure the ECap that’s on the board first, and match-up the physical size and capacitor specs with the suppliers? Wondering how other experience repair shops solve this…especially when building a spare parts inventory.

Any help or direction would be most appreciated.

thumbnail_IMG_1536.jpgthumbnail_IMG_1540.jpg
 
Last edited:

MrChips

Joined Oct 2, 2009
23,520
I would be surprised if the 6800pF capacitor calls for it to be electrolytic.
Usually values lower than 1μF tend to be non-electrolytic.
If it were 6800μF then yes, it would be an electrolytic capacitor.

Post a photo of the capacitor in question.
 

dl324

Joined Mar 30, 2015
12,871
The parts list calls for two (2) 6800pF, 50V Electrolytic Capacitors
It would be unusual for a 6.8nF cap to be electrolytic. I can't recall seeing anything smaller than 1uF.

EDIT: Cornell Dublier gives a range of 0.1uF to 3F for electrolytics. They didn't date the guide or include a revision number; I downloaded from their site around 4 years ago.

Is there something on the old cap I can look at to get the right replacement part? Going just by the parts list, it doesn’t state size or frequency.
Size and function are important considerations.
 
Last edited:

Ian0

Joined Aug 7, 2020
2,196
The parameter you need is the ripple current. I'm assuming that they are the smoothing capacitors for the power supply.
If the ripple current is not specified, then you can make a reasonable guess by dividing the power consumption of the board by the power supply voltage.
 

MrChips

Joined Oct 2, 2009
23,520
Thanks for posting the photo.
Now we know for sure that it is a 6800μF 50V electrolytic capacitor.

There are three fundamental specifications in making a capacitor, capacitance, voltage, physical size.

A non-polarized 6800μF capacitor would be enormous in physical dimensions. Hence they have to create them using electrolytics.
The next parameter is the operating voltage. You must adhere to the max voltage rating of the capacitor plus some more, i.e. give the capacitor some headroom. If the operating conditions are 50VDC, don't use a capacitor rated for 50V. Go higher, such as 63V.

Next come ESR. This will depend on circuit applications. For this we need to see the circuit schematic or at least know what is the function of the capacitor in the circuit.

Then comes size and orientation, radial or axial. What you have shown is a radial capacitor.
 

neonstrobe

Joined May 15, 2009
145
Yes, that's 6800uF not pF.
Most capacitors are made smaller these days. I just replaced a worn out 32 uF 400V terminal ended one in a valve amp with one which had axial leads and was about 1/4 size and had a higher ripple current rating than the circuit needed.
What is the power of the equipment? You can check the ripple current of electros and match it. I'm guessing it will be exceeded from what you need, but there is a range of different ratings out there.
 

ElectricSpidey

Joined Dec 2, 2017
1,653
As long as the ratings are correct I wouldn't be concerned about the physical size except for the pitch (distance between leads) because you want the replacement to seat properly.
 

Tonyr1084

Joined Sep 24, 2015
5,969
The parts list calls for two (2) 6800uF, 50V Electrolytic Capacitors (ECap)
The TS has edited the post to reflect 6800µF instead of 6800pF.

Different manufacturers may have different sizes. A tall skinny cap can have a similar capacitance and voltage rating as a shorter fatter cap. When ordering replacement parts, look at the data sheet for the dimensions and find the right one that fits properly.
 

wayneh

Joined Sep 9, 2010
17,152
See if you can find the specs of the original part, including lead spacing and lead diameter. You can tolerate some deviation but it can be a pain if the leads don't fit the holes, for instance.

Look closely if the cap is billed as "power supply", "low ESR" or anything other than general purpose. You definitely need to keep ESR at or below the previous rating, just as you want the temperature and lifetime ratings to meet or exceed the original.

Matching the capacitance rating and voltage rating is the minimal and easiest part of replacing an EC.
 

Thread Starter

John Czerwinski

Joined Jun 19, 2017
44
Thanks. Had to edit because the parts list is wrong compared to the photos of the real devices. The board is a deflection board for vector monitor. The one in the photos was an example, but I have a few that need replacing for sure.

Sounds like the best approach is to get the dimensions of the bad cap and find a replacement that matches capacitance, voltage, and dimensions.
 

Thread Starter

John Czerwinski

Joined Jun 19, 2017
44
Here's the parts list with the specifications. Doesn't provide much execpt for the capacitance and voltage (note: parts state pF, real part is uF).

1610734293710.png
 

andrewmm

Joined Feb 25, 2011
1,467
Look at the parts you have, in particular the manufacturer on the side,
then you can probably find the specs of the old part.

Note, the length they can work for is very dependent on the size,
I recently found a capacitor rated at 1000 hours at full temperature ( 60 )
that's just over a month !! but it was cheep and small !!

Out of interest,
have I missed it , why do yo think its this part thats gone ?
 

dl324

Joined Mar 30, 2015
12,871
Had to edit because the parts list is wrong compared to the photos of the real devices.
The polite way to make changes after someone has already responded to your post and referenced the text in question is to leave the original text with strike through and to note that it was edited. Not doing that makes the people who responded to the incorrect information appear to be clueless if they didn't quote the original text.

I wouldn't advise using a physically smaller cap. The distance between the plates will be smaller and there will be less water in the electrolyte. It will be more affected by high temperatures and less water means higher resistance.
 

Thread Starter

John Czerwinski

Joined Jun 19, 2017
44
Out of interest,
have I missed it , why do yo think its this part thats gone ?
The capacitors in the photo are just an example and don't need replacement on that board. But I have other boards where the capacitors are bad (bulging, etc). The bigger concern is building a spare parts inventory. Since most of the manual and parts list are equally as vague, seems that getting the original part, measuring, then purchasing the part is the best approach.
 
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