This is a question about substituting one capacitor for another. When you have a bad cap but you don't have exactly the same rating cap as the bad one to replace it with. How far can you deviate from the original spec?

Voltage is ok. You can go from 25V to 35V, for example, because if you only have a 25V feed that's the maximum voltage the cap is going to accrue. So the rule would be greater than is ok but not less than unless you are certain what the supply voltage is. Size is a consideration
as well because size tends to be bigger the higher the voltage.

Temperature and ESR also have to be considered too. The main question is about the capacitance rating. That's the grey area for me.
Are there any rules?

Recently brought a dead Toshiba TV back to life. There was a repair kit for the power board on eBay that consisted of 7 caps. Came with a list of locations. Very convenient because there were 16 caps on the board and they all looked fine.

But I noticed that one of the locations on the board had a 2700uF 16v cap that wasn't original. It's replacement in the kit was 3300 uF 16v. But on the wiring diagram that I found the spec was 4700 uF 6v. I queried this with the kit vendor and they said they were aware of this difference but had difficulty getting the 4700 uF 6v caps so went with the 3300 uF 16v rather than the 2700 uF 16v that "some people use". I found a 4700 uF 6v cap on eBay and used that. But the difference between the 2700 uF cap on the board and the 4700 uF on the wiring diagram seems pretty broad. I should point out that the 2700 uF 16v cap appeared to be fine when I removed it and tested it. Although all of the other caps looked fine 4 of them were well out of range. The TV works fine now.

So I was wondering is there any guidance or best practice as far as capacitance is concerned?

Thanks.

 

I don't think it's too critical for power supplies. I've fitted near to values in the past but 2000uf deviation is probably pushing it.

 

I don't think it's too critical for power supplies. I've fitted near to values in the past but 2000uf deviation is probably pushing it.

Cheers. It does seem intuitively large and inappropriate although it doesn't appear to have been an issue in itself. Having said that it may have had an impact on any caps downstream.

 

Electrolytic capacitors generally have a large tolerance on the value, at least ±20%, so the circuit will be designed so that the value is not critical.

 

It really depends on what the capacitor is used for in the circuit. When used as a filter cap, going up in value doesn't hurt at all, as long as you maintain the same (or higher) voltage rating. But doing this may cause problems in the physical world, such as the new cap being too fat, or too tall, or the pins now being too wide for easily mounting on the original PCB. If you find a cap that is swelling (ie, visibly bad), it usually means that voltage spikes are exceeding the voltage rating, thus gradually degrading the device. It is a good idea to increase the voltage rating of these to increase the life of the circuit board, provided there is enough room to mount a larger cap.

 

It really depends on what the capacitor is used for in the circuit. When used as a filter cap, going up in value doesn't hurt at all, as long as you maintain the same (or higher) voltage rating. But doing this may cause problems in the physical world, such as the new cap being too fat, or too tall, or the pins now being too wide for easily mounting on the original PCB. If you find a cap that is swelling (ie, visibly bad), it usually means that voltage spikes are exceeding the voltage rating, thus gradually degrading the device. It is a good idea to increase the voltage rating of these to increase the life of the circuit board, provided there is enough room to mount a larger cap.

Thanks for the input.

 

Note that in a rectifier circuit, while increasing the reservoir capacitance will reduce the ripple voltage, it will also increase the diode peak current.

 

When you have a bad cap but you don't have exactly the same rating cap as the bad one to replace it with. How far can you deviate from the original spec?

It depends.

Sometimes a higher capacitance will work, sometimes a lower capacitance will work. Similar for voltage ratings. A higher voltage rating in the same package size has more compromise than the one with a lower voltage rating.

 

It depends.

Sometimes a higher capacitance will work, sometimes a lower capacitance will work. Similar for voltage ratings. A higher voltage rating in the same package size has more compromise than the one with a lower voltage rating.

Or is a more modern component.

 

1) Find a replacement that fits in the same physical location.
2) Choose the same voltage rating or higher. If you know the max voltage in circuit, go with a voltage rating 1.5 - 2.0 times the circuit voltage.
3) Choose capacitance 1.0 - 1.5 times the original value for power supply applications. For timing or frequency dependent applications replace with the same value capacitance.