Opions on components in series sharing the load??

[ElectronTubesRule]

Ok first I fully understand that identical parts in a series will divide and share the load so long as we are talking about stead state operation no inrush currents or anything like that. So that is the given that I am working from right or wrong.

So my question if you have to wire two cap's in series to come up with the needed values how does load sharing work if they are not identical. Second do they need balancing resistors?

So I need to replace a 500V cap in main power supply area of a tube linear. I have 450V caps and 50V caps. I also have on hand some 1 watt 10ohm carbon resistors. So the two caps I am using to replace the old Cornell Dublier 500V 10uf paper caps will be in series to make 500V total 450-50=500. I will install these in both lines coming off the HV transformer feeds that go directly to the rectifier board.

As usual I have read two different takes on what to do from two different online home brew amateurs both well respected authors for ARRL related material over the years. One guys thinks balancing resistors are needed another say no they are a waste of time and are old school bad technology. I can see them as being a waste if the components are identical and in series but with differing values for the caps I am using I have to think it makes a difference. If it matters I plan on putting the larger 450V cap first or closest to the transformer then the smaller 50V cap down stream of that.


Thanks for the input!

 

[ElectronTubesRule]

Third question: Should each cap have it's own resistor or should I bridge a single resistor across the entire series from + on the one cap to the - on the other cap so they are tied together as one cap?

 

[vu2nan]

Using capacitors in series is not about load sharing or getting needed values.

It's about not exceeding the voltage rating of each capacitor.

Consider a 500VDC power supply requiring a 16μF filter capacitor. Assuming only 32μF 350V capacitors are available, 2 of them in series would give 16μF. To ensure that the voltage rating of each is not exceeded under all conditions, a resistor of equal value is to be connected across each of them. They would also serve as bleeders.

The resistor values are calculated considering 100Ω/V as optimum. In this case the value of each would be 25kΩ10W (the calculated dissipated power being 5W).

The 10Ω resistors are too small for this application.

Using capacitors of the same value ensures that the voltage is equally distributed.

Connecting 450V & 50V capacitors in series unnecessarly complicates the issue and is not done.

73,

Nandu.

 

[wavrider]

. So the two caps I am using to replace the old Cornell Dublier 500V 10uf paper caps will be in series to make 500V total 450-50=500. I will install these in both lines coming off the HV transformer feeds that go directly to the rectifier board.
Thanks for the input!

Are you sure that is how it works?

Do you have a face shield? welders mask and leather gloves when you apply power to this circuit?

 

[Captain Kilowatt]

You do NOT want to connect the capacitors in series if the voltage ratings are very different. The smaller value capacitor will always have the greater voltage drop across it so unless the 50 volt cap is many, many times the capacitance of the 450 volt cap you will need that face mask waverider was talking about. Do not be cconcerned about making a capacitor rated for 500 volts just because that is what was in something originally. Nothing wrong at all with far exceeding that rating and in fact you may not need to install multiple caps. You have a 450 volt cap. What is the voltage on it? If the voltage is less than 400 volts you can use the 450 vollt cap as it is close enough. Just don't push the limit or exceed actual voltage ratings. If you need to install two caps it is far preferable to install two 450 volt caps of twice the value needed so they will then provide the value needed at twice the voltage rating. It is a BAD idea to mix cap values and voltage ratings in a series string.

 

[vu2nan]

wavrider's 'armour' is a must if ElectronTubesRule actually follows through with his statement on connecting the capacitors across the AC output of the HV transformer!

73,

Nandu.

 

[Captain Kilowatt]

I will install these in both lines coming off the HV transformer feeds that go directly to the rectifier board.

wavrider's 'armour' is a must if ElectronTubesRule actually follows through with his statement on connecting the capacitors across the AC output of the HV transformer!

73,

Nandu.

YIKES!! I missed that! Probably because I was thinking about after the rectifiers which is the norm not after the transformer.

 

[vu2nan]

Sure, Captain, even I missed it the first time!

Regards,

Nandu.