[AMPS] Bleeder Resistor

[Dave Kirkby]

Carsten Esch wrote:

> Hi,
>
> I am wondering about 'the' bleeder resistor in a 4kV DC - 1A power
> supply (230V DC input), must of the designs I found are using a 50uF
> capacitor and something like 100k/250W bleeder.
> Is there any disadvantage to use a 400k instead? I remember something
> like up to 100 Ohm per volt is ok? Would be nice not to dissipate too
> much power in the bleeder... Any comments?
>
> Carsten, DL6LAU
>

I would be tempted to put two resistors in parrallel. If one goes, you
have a backup.

Obviously the larger the resistor(s), the longer it takes to discharge
to a safe value. If you start with 4 kV, and consider 50 V safe, then it
needs to fall to -log_base_e(50/4000)=4.4 time constants before its
safe.

Using 50uF and 400k, has a time constant of T=C.R=50e-6*400e3=20 sec.
Hence it would be safe  after 20*4.4=88 seconds.

If you have lots of electrolytics in series, then you need the resistors
to equalise voltage drops too, but I suspect from that you are using one
big cap, as you talk of using one high value, high wattage resistor.

G8WRB


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