Non-electrolytic capacitor voltages

[poppydog]

This is a question that has been asked before, however I cannot find a definitive answer.

I need to order axial non-electrolytic replacements for one or two of the hunts replacements for a couple of the Roberts I have, however I can only find capacitors whose voltages are well above the ratings on the faulty ones i.e. the faulty ones are 150/200v dc and the replacements are rated anywhere between 400 and 600v dc. (also within budget.) The question is: Will these be ok?

Regards
Poppydog

[ms660]

Voltage wise yes.

Lawrence.

[Station X]

See here:-

http://www.vintage-radio.net/forum/s...ad.php?t=88767

[poppydog]

Originally Posted by ms660

Voltage wise yes.

Thanks for that Lawrence, I didn't know how much higher I could go than the original rating, also I have to be a bit careful with regards to the capacitors physical size due to the voltage rating.

With regards to the link that Station X sent, I have viewed that recently and saw that there was a very small piece regarding voltage, perhaps that should be updated to tell us how far over we can safely go over - if there is a max.

Regards
poppydog

[Station X]

What the post linked to refers to non-electrolytic capacitor and says is this:-

As well as capacitance, capacitors also have a working voltage. You must make sure that the working voltage of a new capacitor is equal to or greater than that of the capacitor it replaces.

I think that is very clear and unambiguous.

[Radio Wrangler]

You can go as far over the original voltage rating as you like with non-electrolytic capacitors.
At some point you may find them too big or too expensive and those will set your limit.

David

[ms660]

Originally Posted by poppydog

With regards to the link that Station X sent, I have viewed that recently and saw that there was a very small piece regarding voltage, perhaps that should be updated to tell us how far over we can safely go over - if there is a max.

What's said in Graham's link is clear to me, what you have to remember is that the capacitors in question don't have to be subjected to the voltage stated on them in order for them to do their job, it's just the maximum voltage they should be subjected to for reliable operation.

Lawrence.

[GeoffK]

Just how do manufacturers actually determine what is a maximum value, (for any type of component for that matter) do they just keep applying voltages, either AC signal or DC blocking until something gives way? We take it for granted that components will work without giving it to much thought as to how values are arrived at.

[poppydog]

I asked because I understand, from what I have read on here, that you have to be careful about going too high with electrolytics and because to someone like me that didn't know, it wasn't clear. It can be easy to read something that you already know the answer to and think it clear to everyone.

Apologies Station X for asking a question that was so blatantly obvious in the sticky, if you feel the question/thread isn't relevant I won't feel offended if you remove it.

Regards
Poppydog

[Radio Wrangler]

Non-electrolytic capacitors are lengths of foil interwound with lengths of plastic or impregnated paper. They work from the very beginning. Ceramic ones are stacks of deposited metal and ceramic, and get fired like pottery. The trick is making connections to the metal layers of the sandwich. They too just work.

Electrolytics are different.

Two strips of plain aluminium foil are rolled up with two strips of paper. The paper is impregnated this time with a conductive liquid (water-based)

This assembly doesn't work, it leaks badly. The device is fed with current, and the aluminium oxide film on one foil strip is broken down, while that on the other builds up.

The dielectric of this capacitor isn't the paper or the impregnating fluid. It is the aluminium oxide layer. As time goes on and the layer thickens, the capacitance falls and its voltage capability increases pro rata. The surfaces of the foils were textured to increase their areas to make a bigger capacitance per volume. The conductive fluid forms one electrode, and being liquid, it self-adjusts to follow the surface of the oxide to keep the capacitance high.

So the electrolytic capacitor had an electrochemical forming process in its manufacture.

Apply reverse voltage and it undoes quickly, heats up, boils the water to high pressure and Bang!

Apply too low a forwards voltage and it can very slowly undo what the forming operation did. Try to stick to 20% of rated voltage, and try to keep below 60% of rated voltage for long life. Ripple current causes heating and drives out the water, so this sets life if the voltages are OK.

David

[Boater Sam]

You should have no problems finding caps for a transistor radio at low voltages, in the range of 10v to 35v, so I assume your Roberts is a battery or mains valve radio?

If battery, with one exception, there is no need to go much higher than the HT battery voltage. The exception is any capacitor connected to the anode of the output valve and/or the output transformer where high peak voltages develop in the transformer primary.

If mains, stick close to what is fitted if possible but with NON electrolytics you can use anything that is higher.
Stick close to the original with electrolytics.

You can assess the voltage likely to be across a cap from the voltages given in the service sheet. Manufacturers often use the same component voltage in a high or low voltage situation to avoid stocking more different parts, or because they could buy a high voltage part cheaply.
There are situations where they use a 350v cap in a cathode connection that needs at best 10v!

[Sideband]

Originally Posted by Boater Sam

You can assess the voltage likely to be across a cap from the voltages given in the service sheet. Manufacturers often use the same component voltage in a high or low voltage situation to avoid stocking more different parts, or because they could buy a high voltage part cheaply.

There are situations where they use a 350v cap in a cathode connection that needs at best 10v!

That is very true. There are many situations where I have removed a paper capacitor rated at 350V (for example across the cathode resistor of an I.F amplifier) where the maximum voltage is around 3.5V or the AGC rail where it will never go above 4.5V. In those cases I have often used a lower rated component (say 100V) than the original knowing that it will still be operating well below it's rated voltage. This helps where for instance your personal stock of 400V capacitors is low and they can be reserved for the places where they are definitely needed.

Only do this when you have enough confidence and knowledge of circuits.

[GMB]

When choosing a different voltage rating, don't just consider the voltage that it will be subjected to in normal operation.

Consider switch-on and switch-off situations, there can often be transients much larger than the normal state.

Also consider what would happen if a valve is pulled out. That might make a big difference.

Finally, consider what normal transients it will be exposed to and what will happen in that situation. You will see capacitors subjected to the mains and DC rated at only 400V - but these are the type that can self repair when a voltage spike hits them. If you use a different type then you need a very much higher rating (and the life is then a matter of luck and statistics).

[60 oldjohn]

Originally Posted by poppydog

I need to order axial non-electrolytic replacements for one or two of the hunts replacements for a couple of the Roberts I have, however I can only find capacitors whose voltages are well above the ratings on the faulty ones i.e. the faulty ones are 150/200v dc

If it is a battery transistor set any capacitor with a voltage Higher (within reason as already discussed) than the total battery voltage would be OK. Early transistor sets used up stocks of capacitors intended for Valve sets. I expect it may have been quite a while before low voltage caps came readily available in all values. Hence 200v, 350v & 12v caps in a set where one would expect a 12v capacitors.


John.

[G8HQP Dave]

Originally Posted by Hetrodyne

Just how do manufacturers actually determine what is a maximum value, (for any type of component for that matter) do they just keep applying voltages, either AC signal or DC blocking until something gives way? We take it for granted that components will work without giving it to much thought as to how values are arrived at.

A combination of design, testing and production engineering (so they can repeatedly make more or less exactly the same thing millions of times).

For example, the dielectric strength of plastics is known so for a given film thickness the maker knows what voltage it should stand.

[Maarten]

Originally Posted by Hetrodyne

Just how do manufacturers actually determine what is a maximum value, (for any type of component for that matter) do they just keep applying voltages, either AC signal or DC blocking until something gives way? We take it for granted that components will work without giving it to much thought as to how values are arrived at.

The values are probably calculated from theoretical and practical data but also verified regularly. I think Philips designed their foil capacitors to withstand 50% or 100% overvoltage in an actual test.

Also I don't agree that a capacitor in a transistor set need never have a higher WV than the battery voltage. Sometimes charge pumps are used (in output stages for example) where the rating needs to be at least twice the battery voltage. In other cases, a capacitor is used for resonating with or damping of inductions, where higher peak voltages can also be seen.

[60 oldjohn]

Originally Posted by Maarten

Also I don't agree that a capacitor in a transistor set need never have a higher WV than the battery voltage. Sometimes charge pumps are used (in output stages for example) where the rating needs to be at least twice the battery voltage. In other cases, a capacitor is used for resonating with or damping of inductions, where higher peak voltages can also be seen.

Ok I stand corrected.

John.

[1100 man]

Hi Poppydog,
I have just been through the mill of ordering various non electrolytic caps to replace Hunts caps in my Pilot TV project. Trust me, its not as straight forward as it first seems! Finding suitable sized replacements at the right price can be a challenge!
If you would like to post the values/ voltage ratings of the old ones, I can tell you the most suitable ones I sourced as replacements and what the sizes were if that is any help to you. Also where I got them from as most suppliers only do a very limited range.
I have to agree that what is blindingly obvious to one person is often very obscure to another- technical text books are a prime example. They are usually written by someone who is expert in their field and are frequently baffling to someone trying to learn! Like me
So never be afraid to ask and try not to take any negative comments to heart- I guarantee there will be a dozen other people who would like to ask the same question but have not plucked up courage to ask
Cheers Nick

[Biggles]

I'm sure I read somewhere once that it isn't good to use a much higher voltage rated electrolytic than required due to the fact that the capacitor requires a constant small current to remain "good" and higher voltage rated ones don't receive the current required in a low voltage circuit. But that's where my knowledge/theory ends unfortunately! In practice I find that it doesn't seem to do any harm by using higher voltage rated capacitors than the circuit requires. Hope this hasn't added confusion to the thread.
Alan.

[G6Tanuki]

Electrolytics apart, there are some places where I would use a capacitor whose voltage-rating is significantly, nay massively, greater than the voltage it will generally "see".

Example: AGC-line decouplers. Reason being that in such situations though the voltage is low the impedances and resistances involved are very high, so any leakage is to be avoided; a "high voltage" rated capacitor will (generally) have a lower leakage when operated at low voltages than would be the case for a capacitor designed to only survive the low voltages in the circuit.

Another example - the capacitors from the anode and cathode of a classic 'split-load' phase-splitter to the grids of a push-pull output pair. In particular the one from the anode to one P-P grid will "see" the full HT voltage until the valve warms-up and starts frawing current (this voltage will be potentially higher than the 'usual' HT-voltage too since there's initially no load on the HT lineso it will rise to the peak as-fed-to-the-rectifiers voltage).

Another example: in some ham-radio transceivers there is a capacitor between the anode of the driver and the grid(s) of the PA - sort-of the equivalent of "that capacitor" in single-ended audio amplifiers. This capacitor 'sees' the driver-stage HT voltage at all times, but when the radio is switched to 'receive' a significant negative bias - sometimes up to -350V - is applied to the PA-grids to turn the output-bottles fully 'off'. This negative voltage is in series-aiding with the driver HT voltage and it's not unknown for this to 'pop' the capacitor, which goes short-circuit and then you get full driver-HT-voltage on the PA grids, with catastrophic fireworky results!
"I can't have blown the PA stage - I've only ever used it on receive!" being the usual moan.

Last example: again in transmitters/transceivers - the capacitor from the PA anode to a pi-tank ATU. This is usually a silver=mica or ceramic job and *must* be of high voltage rating: on modulation peaks (assuming AM and classic high-level anode-modulation) it sees *four times* the resting HT voltage. If it fails you can have the antenna become 'live' at HT-voltage - woe betide anyone who touches it.