Re: [CAD_CAM_EDM_DRO] Re: What size capacitor should I use? resistors....

cnc_4_me wrote:

>
>
>>I understand the desire to limit the starting inrush current as
>>
>>
>well
>
>
>>as the need for the smallest time constant.
>>
>>would it not be prudent to charge the cap slowly up to some value,
>>and at that point, switch over to the raw voltage directly ?
>>
>>It would seem a simple electromechanical relay would be a
>>
>>
>possibliity
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>>to eleminate the hum of initial power on.
>>
>>Dave
>>
>>
>
>You are exactly right dave...And for a example Jon Elson used to have
>on his site a schematic of what you just described (not sure if it is
>still there). He used a time delay relay to slowly charge the caps...
>
>This method also helps save the bridge rectifier from the huge
>inrush...I usally spec the bridge rectifier current rating 50 to 100%
>larger than desired current because of this huge inrush...
>
>

Actually, that circuit is to reduce the inrush when a big DC supply is
connected to
the smaller caps in a bank of servo amps. Connecting two caps at
different voltages
with a big relay is one way to define a spot welder! But, the circuit
is about
the same.

The transformer limits the inrush a bit due to leakage inductance. If
you just rectify
the line directly, the inrush can reach 4000 A! Power Switches don't
handle too
many surges like that. With a big transformer, the surges may only
reach 400 A
or so, but that can still kill the rectifier and power switch after a while.

Jon