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I want to know the function of the three 1Mohm resistor between the HVDC+ and HDVC-, they are R4 R14 and R17 in the below circuit, thanks for your attention and time!

I want to know the function of the three 1Mohm resistor between the HVDC+ and HDVC-, they are R4 R14 and R17 in the below circuit, thanks for your attention and time!

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Those are "bleeder" or "discharge" resistors. They drain off the high-voltage charge on C3 when the power is removed. Else C3 could retain a charge that could injure someone working on the circut, even after removing the power.

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  • \$\begingroup\$ My calculation says it would take about an hour to discharge the 330uF cap down to 10V. \$\endgroup\$ – Daniel Mar 29 '16 at 2:43
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    \$\begingroup\$ Anything below ~48V is considered "low voltage" and relatively safe to work around. 3M is relatively high for bleeder resistance. But this may be a very low-power circuit, however we don't know because we can see only part of the picture. \$\endgroup\$ – Richard Crowley Mar 29 '16 at 2:45
  • \$\begingroup\$ Any reason they are 3x 1Mohm rather than a single 3Mohm? \$\endgroup\$ – MichaelK Mar 29 '16 at 2:47
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    \$\begingroup\$ 'Normal' small-ish 1/4W resistors will have a maximum voltage rating which would probably not be as much as 390V. 3 equal value resistors means they each only see 130V - much more manageable. \$\endgroup\$ – brhans Mar 29 '16 at 3:03

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