ESP soft start circuit

[454Casull]

http://sound.westhost.com/project39.htm

Some questions:

Digi-key doesn't have the BC558 in stock. What's a suitable replacement?
Can I replace the aux transformer and rectifying diodes with a DC wall-wart supply? How does the earthing work?
I don't know anything about relays. What should I use?

[richie00boy]

Yes you could use a wall wart as long as it is switched from the same switch/plug socket as the power amp.

Any small general purpose PNP will do for the transistor, just make sure you are aware of the pinout.

An automotive relay would be a OK to use.

[454Casull]

Do I need to earth the case if I use a wall wart?

If I can use any old PNP transistor, is the 0.65V switching determined by the circuit around it, then?

Do all relays have the same pin-out? What type of current and voltage ratings are important? Coil current, coil voltage, contact rating, control off/on voltage? Ugh.

[454Casull]

When a voltage of 0.65V is sensed across the relay, Q1 turns on, and instantly completes the charging of C2

Will this work for the PNP transistor?

http://search.digikey.com/scripts/Dk...me=BC327GOS-ND

Will this work for the relay, or is it overkill?

http://search.digikey.com/scripts/Dk...me=255-2387-ND

This one's rated higher, but is cheaper for some reason.

http://search.digikey.com/scripts/Dk...l&name=Z215-ND

[pacificblue]

Originally posted by 454Casull
Do I need to earth the case if I use a wall wart?

Yes, because there is mains voltage present in the case at the relay contact.

Originally posted by 454Casull
If I can use any old PNP transistor, is the 0.65V switching determined by the circuit around it, then?

All silicon transistors switch at that voltage level. You can also use new PNP transistors, they need not be old.

Originally posted by 454Casull
Do all relays have the same pin-out?

No.

Originally posted by 454Casull
What type of current and voltage ratings are important? Coil current, coil voltage, contact rating, control off/on voltage? Ugh.

The coil voltage should be the same as the supply voltage. If it is smaller, you will have to use a series resistor. Then you will need the coil current or resistance to determine the resistor value. The lower the coil current, the better, because you want to waste as little energy as possible.
Contact rating is important. Voltage rating should be AC and greater than or equal to the mains voltage. Current rating should be greater than or equal to the current you are switching and should be given for inductive load. The current depends on the transformer you use, but Mr. Elliott's recommendation to use 10 A or higher will be okay for the vast majority of audio amplifiers.
All other values should be of no concern for this application.

Originally posted by 454Casull

Will this work for the PNP transistor?

http://search.digikey.com/scripts/Dk...me=BC327GOS-ND

Yes. Direct replacements for the BC558 are BC556/BC557/BC559/BC560. They are all the same, except that some of them have higher voltage ratings than the BC558.

Originally posted by 454Casull
Will this work for the relay, or is it overkill?

http://search.digikey.com/scripts/Dk...me=255-2387-ND

This one's rated higher, but is cheaper for some reason.

http://search.digikey.com/scripts/Dk...l&name=Z215-ND

Both will work. The Omron relay will hold ~double as long, but both will probably live longer than the entire rest of the amplifier, as long as there is no short at the relay output.

[454Casull]

Thanks, you're the best!

[coloradosound]

Never had any problems with Omrons. Haven't
tried the Panasonics yet.

[454Casull]

Does the polarity/direction of current through the relay coil matter?

Also, I'm still not sure about what the earth symbols mean here: http://sound.westhost.com/p39-fig2.gif. If one takes mains into the chassis, then I could see needing to earth it and then you have your earth, but if I use a wall-wart I won't have that.

[pacificblue]

Originally posted by 454Casull
Does the polarity/direction of current through the relay coil matter?

Usually not. Only, when you have a relay with built-in flyback diodes or LEDs

Originally posted by 454Casull
Also, I'm still not sure about what the earth symbols mean here: http://sound.westhost.com/p39-fig2.gif. If one takes mains into the chassis, then I could see needing to earth it and then you have your earth, but if I use a wall-wart I won't have that.

That will be the common ground then.

[MJL21193]

I built the ESP soft start on an earlier project and it works fine. It is a bit complex and the need (or preference) for a auxiliary supply to run it is a downer.

Below is something simpler - it runs off the main power supply and uses a thermistor as the inrush current limiter (safer than the power resistors). There is a 1 second delay before the relay closes, bypassing the thermistor.