Re: Power supply softstarts

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Graham Stabler" <eexgs@n...>
wrote:

>
>
> I asked a lecturer in electrical enginnering about this, despite
> being a lecturer he is very practical ;)
>
> "I KVA is about 4 amps. A big supplly current. Toroidal

transformers

> have an excellent efficiency due to having no small air gaps in

their

> magnetic circuit. Unfortunately this means that they can retain

flux

> on swiths off of the supply. If they are switched on at voltage

zero

> such that the rate of change of flux required causes the

transformer

> flux to grow in the same direction as the residual flux then
> saturation quickly occurs as the flux is offset by the residual
> value. The only current limit is then the resistance of the primary
> winding and the very low leakage reactance. Hence bang. 1KVA should
> not be enough to blow the fuse though normally.
>
> The very big capacitor with no current limit on the input will
> compound the problem of current surge as again there is little to
> limit the current.
>
> The simplest solution would be additional primary resistance on a
> timer as Ron (enother chap I asked) says."
>
> So I will go for some form of power resiter on the primary, shorted
> by some means after some time. No need for ebay parts, I have a
> budget :P
>
> Cheers,
>
> Graham

Sounds like a good plan.

If you read up on the thermistos, they are similar to resistors but
vary with temperature, and are much less expensive than power
resistors.

Regardless, once you bypass it, who cares.

My questions is why the gigantic capacitor ?

Most of the calculations I have done on the power supplies yields
capacitors in the 10k to 20k uF range, Never the 100k Uf range.

Dave