Re: [CAD_CAM_EDM_DRO] Transformer Soft Start

Tony Jeffree wrote:

>Hate to disagree with both of you but...I saw exactly this problem on a PSU
>that I built a couple of years back - there was absolutely nothing wrong
>with the way it was wired up (single primary, and you have to try pretty
>hard to mis-wire a bridge rectifier & a capacitor), but I had to use a much
>larger mains fuse than I had expected in order to deal with the inrush
>problem. At the time, this was a simpler solution than using an inrush
>limiter (I didn't have one to hand, but I did have a bigger fuse) but I
>will fit one when I can remember to add one to my next components order &
>will then replace the fuse with one of the correct rating.
>
>

Well, this may be a difference due to the 230 V mains supply in the UK, etc.
as opposed to the US. Inrush could be a lot worse at twice the voltage.
It just seems that in the US, it is extremely rare to see this sort of
problem.
I have a number of large power supplies (both 120 and 240 V) that do blink
the lights when they are turned on, but I've never had one that tripped
breakers.
Maybe that is the other difference - we rarely see fuses in the power
distribution
systems in houses and businesses anymore. They were phased out in favor
of magnetic circuit breakers in the late 1950's. We still use fuses inside
equipment, though. A slow-blow type fuse would be a good choice in an
application like this. I have a slow-blow fuse in the power supply for my
milling machine's servo power supply. it has 2 transformers that weigh
about 60 Lbs together, and 200,000 uF of filter capacitors. it has never
blown the fuse, and I DON'T have an inrush limiter in the circuit.

One other thought - it is fairly common practice to put a small series
resistor between one of the wires going from the bridge rectifier to the
capacitor for exactly this reason. Sometimes it will work out to less than
one Ohm. For instance, in a 40 V supply, a 1 Ohm resistor would limit the
secondary surge to about 40 Amps. the primary surge woult be 40 * 40/230
= 6.95 Amps!

Jon