In Reply to: sharing resistor sizing... posted by Vinnie on May 2, 2007 at 16:59:03:
Vince, hi. Here is a simple calculator. Take the value of two caps you are putting in series as C. Assumption here is both caps are same value same voltage. You can do a calc with dissimilar types (voltage and value) but the lowest voltage value trumps everything. The composite electrolytic cap will always be less than the lowest voltage cap in your compsite. This gets confusing and complicated. So -let's say its 150 uF at 350 WVDC for example. Stacking the caps yields 75uF. That value is C. Anyone building up a higher voltage composite cap in this way should use equal value caps.Here's the calc: 1000 divided by 0.015 times C = R in K Ohms. In this case it computes to 888K Ohms. So, going with a 910K would be fine. I selected a precision 1% metal film at 3W. You don't need a 3W but what the hey. Over engineering is no sin. :)
My personal bias steers me towards precision 1% wire wound and precision carbon film. It is difficult to find WW R at higher values (like 910K) so other types are required. I still would not use a metal oxide type - even in this application.
Happy voltage balancing.
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Follow Ups
- simple formula to calculate voltage balancing R - elektron 12:52:48 05/03/07 (2)
- I don't trust that web site (acts as if all caps are equal quality)(nt) - Russ57 07:44:57 05/04/07 (0)
- Re: simple formula to calculate voltage balancing R - Vinnie 14:20:19 05/03/07 (0)