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Ballance resistors

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Braga

Member
Joined 2010
Hi there,I`m restoring a Sonic Frontiers SFS 40 tube power amp.Original main caps 2 x 1700uf/300V in series to achieve 600V. Two 75K 3W resistors where used to ballance the caps. One to each cap.Need formula to calculate the value of the resistors, as im going to replace both caps. Probably will be 1500uf/450V Kendeil caps x 2 . What should be the correct value for the resistors ? Thanks and regards, Braga
 
HollowState

HollowState

Member
Joined 2006
Braga said:
Need formula to calculate the value of the resistors, as im going to replace both caps.
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There is no "formula" per se' for choosing these balancing resistors. The main idea is to use values that are well below the internal leakage value of the capacitors used. By doing this, you will assure proper voltage division. The value you already have (75K/3W), while low, is a safe bet and will surely work with most any electrolytics. For high quality capacitors, with low leakage, the value could be much higher and still work properly. I have seen resistor values anywhere between 47K (5W) up to 470K (½W) serving as dividers. So the resistor value is based upon knowledge of the capacitor or any leakage resistance you might measure. The actual value is not critical as long as it's below the leakage value. Personally, I usually use 100K 1W carbons and never have any troubles.
 
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