Power Supply Soft Start Board

[nrg2009]

Originally Posted by TANDBERGEREN

But otherwise I totally agree with You. Although I always bypass them with a relay after a suitable inrush-time. Again to secure extended service-time.

So effectively what you are doing is replacing R15-R18 with an NTC (CL30)?

- this has the inrush running through the CL30
- when the relay connects it shorts out the CL30

Is that safe? Is it effective and are there any benefits of replacing R15-R18 with a CL30?

I am trying to decide if I should build this SS-board for an F5Tv2 with 1000VA and 200k uF caps.

Thanks for all your contributions!

[RCruz]

CL30 seems to have only 2.5ohm at 25º

Maybe you should use CL80 or two CL70 in parallel.

[AJT]

Originally Posted by TANDBERGEREN

Just pointing at a tiny little case here Tony.

It does matter wether the thermistor is to be mounted on the primary or the secondary.
One should always mount them where the nominal current is the lowest.
This to secure the best (longest) service time for the cirquit.

But otherwise I totally agree with You. Although I always bypass them with a relay after a suitable inrush-time. Again to secure extended service-time.

the ntc resistors i have seen are on the primary(mains side) of ATX psu's and connects to the mains rectifier....no relays are used to short them out...

though it can be done shorting the ntc out with a relay, i still say using fixed resistors such as that shown in the schematic is the way to go.....

[AJT]

Originally Posted by nrg2009

So effectively what you are doing is replacing R15-R18 with an NTC (CL30)?

- this has the inrush running through the CL30
- when the relay connects it shorts out the CL30

Is that safe? Is it effective and are there any benefits of replacing R15-R18 with a CL30?

I am trying to decide if I should build this SS-board for an F5Tv2 with 1000VA and 200k uF caps.

Thanks for all your contributions!

resistor shorted out with a relay is strongly recommended here.....

[RCruz]

Just built the softstart by the book, using:
C9 1uF
C10 1000uF
R15 - 18 180r 11w
G5L 24V relay
D13 24V 1w zener

I am testing the build without load and the relay actuates very fast... half a second.... How can I increase the delay ? (Higher C10 ?)

Or should I expect the circuit to work differently when connected to the power amp psu ?

[AJT]

Originally Posted by RCruz

Just built the softstart by the book, using:
C9 1uF
C10 1000uF
R15 - 18 180r 11w
G5L 24V relay
D13 24V 1w zener

I am testing the build without load and the relay actuates very fast... half a second.... How can I increase the delay ? (Higher C10 ?)

Or should I expect the circuit to work differently when connected to the power amp psu ?

or change C9 to 0.5ufd...

[RCruz]

How low can I go with C9 (I do not have any 0.5uF AC caps).

I simulated the circuit in LT spice and it seems to not be able to reach 24v on the relay if this cap is 100nF...

[AJT]

Originally Posted by RCruz

How low can I go with C9 (I do not have any 0.5uF AC caps).

I simulated the circuit in LT spice and it seems to not be able to reach 24v on the relay if this cap is 100nF...

you can series connect 2 x 1ufd caps...

remember that C9 is your current limiter, the impedance it present to the line that is....

so that the zener current and the relay coil current added together is your required current after the rectifier....

Ohm's law is pretty much at work here....

TBH, i'd rather put a small housekeeping traffo in there, small enough to supply a few mA at 24volts...but it's just me....

[RCruz]

Correction... simulation with C9 100n and C10 1000u seems to work fine giving 4.5s to reach 24v. Th relay should work before that so I estimate it will close at 3.5s.... is it too much time ? Will it heat up too much the resistors ?

[AJT]

so try 200nF...