After this discussion I realised that the trimmer actually needs to be rated at at least 1KV since that is the intended max voltage for the meter, and I don't trust the "gimmick" idea at that voltage. The highest voltage ones (that don't cost a small fortune) I can find are rated 500VDC, so I was wondering, if I make this a series arrangement with a fixed HV capacitor of smaller value (3.3pF for now) than what the trimmer will be set to, this will protect the trimmer I know you are not **supposed** to put capacitors in series to increase the working voltage like this, but it should be ok since the smaller, fixed capacitor will bear the brunt of it.
I also thought I could make a capacitor using a little bit of d/s pcb and adjust the value by trimming some copper off - is this a better solution?
Hi there,
If i understand you correctly you want to put two caps in series in order to raise the voltage rating of the original cap. Well, this works to some degree but there is a catch. The catch is that in order to double the voltage rating (and that is usually the goal) the two capacitor values have to be the same. So if you have a 1uf 100v cap in series with a 1uf 100v cap you might get by with using them up to 200v in theory, but limiting that to 150v would be a better idea. But for this discussion lets go by pure theory for now for and remember to derate by some amount like 25 percent of the total.
So with 1uf and 1uf we roughly double the voltage rating. But this 'works' to some degree because we had both caps the same value and that means the voltage divides equally between the two caps. But if we had different values then the voltage would not divided equally between the two caps so the rating would not roughly double like it did before. In fact, the new rating would highly depend on the two values as well as their individual voltage ratings.
With 1uf 100v and 2uf 100v for example, two thirds of the voltage appears across the 1uf cap so that means we end up with only one third across the 2uf cap. This means we'd only gain about 33 percent of the original rating, which in this case is only 33 volts. So the rating with the two together would only amount to about 133 volts, and that's before any safety derating. With two 1uf caps or two 2uf caps we doubled the rating which is a 100 percent increase, but with one with a value twice as high we only went up by 33 percent. So you see that it quite different.
For two caps one with a lower value CL and one with a higher value CH the increase factor 'A' is:
A=CL/(CL+CH)
so the total increase is 1+A.
The example was 1uf and 2uf, so there CL=1 and CH=2 (same units) and so A comes out to:
A=1/(1+2)=1/3
so the total increase is:
1+1/3=4/3
and that works out to about 133 percent. Derating that by about 25 percent we get:
133*0.75=99.75 percent, which is still around 100 volts.
But it's always best to get the right rating to begin with. Spend a little more and it should last a long time.