# Calculating required resistor devider

#### Mardokx

Joined Mar 12, 2020
3
Hi guys, i want to know how to calculate properly resistors devider in half bridge topology. Two resistors parallel to capacitors. Example below.
Thanks for any suggestion or article.

#### Mardokx

Joined Mar 12, 2020
3
I know that, you can put there 100k 200k or even 1M. How I can decide which one is the best? In this scheme you have 2x 330k why not 2x 10M ohm? Current will be smaller so the smps will be more efficient. In all schemes they are using something like 100k - 500k

#### BobaMosfet

Joined Jul 1, 2009
1,063
Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

#### Alec_t

Joined Sep 17, 2013
11,141
In this scheme you have 2x 330k why not 2x 10M ohm?
Either of those pairs would help to share the voltage equally between C3 and C4, but the resistors have a second function which is to discharge those caps at power-down for safety reasons. With 10Meg resistors the discharge time constant is 680uF x 10Meg = 6800 seconds; plenty of time for someone to get a lethal shock if they were expecting the circuit to be 'off'. The shorter time constant, using 330k, would be safer and the 'wasted' energy would be insignificant.

#### Dodgydave

Joined Jun 22, 2012
9,080
Those resistors are to discharge the capacitors when power is removed for safety, usually they are around 330 to 470K.

#### MrAl

Joined Jun 17, 2014
7,491
Those 330k resistors seem kind of large. The time constant is around 4 minutes. That means it could take 15 minutes to get to a safe voltage value.