[Æ]

Well, if it's true you can obtain a relay with sufficient inherent delay to give you enough of a soft-start, the circuit is fully described.  You just need to find an appropriate relay to run off the line voltage, and maybe add a cap across the relay contacts?

Yes, something like that. A cap (snubber) across any contacts that can arc is always a good idea.

[el34]

Yes, something like that. A cap (snubber) across any contacts that can arc is always a good idea.

Might not need the cap though, if the relay switches after inrush, there shouldn't be much voltage across the resistor.  I built something similar with a time delay relay tube, which then switches in a DPDT relay to jumper out a resistor, and its own heater.  Didn't see any arcing.  Tested, but haven't installed it yet, though.  It's for a subwoofer amp.

[Æ]

Might not need the cap though, if the relay switches after inrush, there shouldn't be much voltage across the resistor.  I built something similar with a time delay relay tube, which then switches in a DPDT relay to jumper out a resistor, and its own heater.  Didn't see any arcing.  Tested, but haven't installed it yet, though.  It's for a subwoofer amp.

Adding a cap is still a good idea and an inexpensive proceedure.
If you remember back when cars had mechanical points and a 'condensor' (capacitor) the capacitor was there to minimize arcing and the arc noise associated with it. I always wondered why too, until I learned about snubbers.

I had a variac that pulled plenty of current at turn on, it caused quite a pop and some serious contact wear. Adding some caps across the On/Off switch took care of that. I had a Rega turntable switch that 'popped' annoyingly through the system, a small cap across the switch took care of that too.

I recently did a little reading on contact material Silver Cadmium Oxide, AgCdO.

Take pics of your project and post them, I'd be very interested in seeing what it looks like.

[Æ]

Forgot to mention that there are dpdt octal relays which use typical octal based tube sockets. 10 amp contacts easy to come by.

Relay delay can be slowed from near instantaneous to a couple of seconds, depending upon the resistor value.

Upon turn off, the switch is responsible for cutting off power/voltage and unlatching the relay.

If there is a short or very near short in a secondary winding of the power transformer or DC power supply, the circuit described will not engage the relay. This offers protection but will destroy the resistor in a few seconds unless its wattage rating is very high. Used this simple circuit decades ago and worked like a charm.

Of course this circuit does not replace a fuse in any way.

Cheers

Steve

Yes, thank you, this is the type of information I've been seeking. Both your replies to my query are very helpful. Too bad they didn't show up right away, I almost missed them.

[el34]

Some of us take some time to reply.  What is this, twitter?      Here's the delay I built up with a TDR tube, but haven't installed yet.

[cheap-Jack]

I'm semi familiar with the concept of using NTC Thermistors and I've also seen some circuits that use resistor/relay to delay inrush.

For most most, if not all, brandname SS amps, there is always a time-delay for switching ON the loudspeakers to avoid start-up 'pop' which can damage any loudspeakers.

So my question is: why you want to install a slow-start for yr DIY SS amp?
If it is for loudspeaker protection per above, you need to install a loudspeaker
delay-switch-on circuit. NOt simply a thermistor or whatever simple relay cirucit at the primary winding of the power iron.

For tube amp with tube rectifier, no slow-start installation is needed at all as the tube rectifier takes many seconds to heat up, enough time for the tubes the tube heater to warm up beforehand. So no danger to the amp tubes.

c-J

[el34]

In my case, I've got a vintage subwoofer (80s) which the manufacturer recommends leaving on all the time.  They imply damage could result from excessive on/off cycles.  I'd like to limit inrush for that reason (protect the capacitor banks).  I'm not comfortable leaving this on when I'm not home, the caps nearing their lifespan limit.  Really, I should recap it also, but that's a major disassembly project.

[cheap-Jack]

Hi.

Some of us take some time to reply.  What is this, twitter?      Here's the delay I built up with a TDR tube, but haven't installed yet.

Good job, but way way not "simple" enough.

If accurate timing is not needed, say for power-on delay, a very simple delay circuit, using a 12V mini relay, a NPN bipolar transistor, a sand diode to protect the DC relay, a timing cap & a timing resitor will do the job. A few bucks cost!

But if accurate timing is needed, like the wireless home alarm system I design/built, I used a timer chip: LM555 ($2), a 12V DC mini relay ($2), 3 diodes to protect the timer chip & relay, 1 timer cap (1,000uF25V) & a timer resistor (745K1/W) to give me up to 2 minutes delay time.

It can be built on a 3"x3" PCB for about $10.

c-J

[el34]

Hi.
Good job, but way way not "simple" enough.

If accurate timing is not needed, say for power-on delay, a very simple delay circuit, using a 12V mini relay, a NPN bipolar transistor, a sand diode to protect the DC relay, a timing cap & a timing resitor will do the job. A few bucks cost!

But if accurate timing is needed, like the wireless home alarm system I design/built, I used a timer chip: LM555 ($2), a 12V DC mini relay ($2), 3 diodes to protect the timer chip & relay, 1 timer cap (1,000uF25V) & a timer resistor (745K1/W) to give me up to 2 minutes delay time.

It can be built on a 3"x3" PCB for about $10.

c-J

Actually this was a simple choice.  Given that I'm mounting it in an outboard box also containing a transformer configured for voltage bucking, and don't have low voltage available, I can run it off the line voltage.  It came in at $10 or under, but I lucked into the delay tube for cheap.

I have played with the 555 for a preamp delay, but I never finished it, because it would sometimes trigger immediately rather than delay, and I haven't spent the effort to debug.  Somewhere I have a datasheet for another delay chip which is designed for one-shot delay only, and looks to be much simpler to use.

[Æ]

For most most, if not all, brandname SS amps, there is always a time-delay for switching ON the loudspeakers to avoid start-up 'pop' which can damage any loudspeakers.

So my question is: why you want to install a slow-start for yr DIY SS amp?
If it is for loudspeaker protection per above, you need to install a loudspeaker
delay-switch-on circuit.

My solid state DIY amp is being built from kit parts and already includes loudspeaker delay turn on (pop mute).

I want inrush protection, soft start for the two linear power supplies.

My pop is bigger than your pop. Ha!

[cheap-Jack]

HI.

My solid state DIY amp is being built from kit parts and already includes loudspeaker delay turn on (pop mute).

I want inrush protection, soft start for the two linear power supplies.

My pop is bigger than your pop. Ha!

So why not simply add a thermistor, cheapie yet effective since the loudpspeakers are already muted?

If you are using toroidal power irons, the first thing I would install would be a powerline DC blocker to stop powerline DC offset from entering the toroid. Mind you. no torroids can handle any DC offset.

Powerline DC offset affects toroids much worse than any switch-on inrush.


c-J

[Æ]

Hi.
So why not simply add a thermistor, cheapie yet effective since the loudpspeakers are already muted?

If you are using toroidal power irons, the first thing I would install would be a powerline DC blocker to stop powerline DC offset from entering the toroid. Mind you. no torroids can handle any DC offset.

Powerline DC offset affects toroids much worse than any switch-on inrush.


c-J

Well, after some research and discussions I've decided against NTC thermistors.

I already know how to block DC, I can handle that.