[Æ]

I'm seeking more information for design or implementation of a simple soft start circuit for a DIY amp project I'm working on. Something to delay inrush current at turn on. I do know the VA of my torroid transformers and should be able to measure current draw.
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.

I kind of like the idea of a simplified resistor/relay start, but no idea of what relay to use. I've only seen schematics for specific amps and not the parts list. I understand the concept, but have no absolutes. I know there are some soft start with mute kits available, but that's not what I'm looking for.
Just a relay and a resistor. Hopefully I'll be using a relay socket too.
Thanks

[sonicboom]

An inrush current limiter connected in series right after the power inlet/switch should do the trick.

http://search.digikey.com/scripts/DkSearch/dksus.dll?KeywordSearch&FV=fff4000a,fff80391,fffc002d&chp=0

[Æ]

An inrush current limiter connected in series right after the power inlet/switch should do the trick.

http://search.digikey.com/scripts/DkSearch/dksus.dll?KeywordSearch&FV=fff4000a,fff80391,fffc002d&chp=0

Yes, thank you for your reply, but I'm pretty sure those are just Thermistors. Not what I'm looking for.

[el34]

A resistor and relay is not enough, you need circuitry to implement the delay.

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

[Æ]

A resistor and relay is not enough, you need circuitry to implement the delay.

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

Yes, thank you for the reply, that article you linked has a drawing that depicts exactly what I'm looking for and it shows no additional delay circuitry. See schematic below.
I'm going to read and study the article.

We had Nelson Pass at one of our NorCal DIY get togethers and he recommended to me essentially the same circuit, but no specifics as to ohms, wattage, amps or voltage, etc.

[el34]

Sure, no problem.  What's not shown in that diagram, is the relay coil and its driving circuitry.

[cheap-Jack]

Hi.

I'm seeking more information for design or implementation of a simple soft start circuit for a DIY amp project I'm working on. Something to delay inrush current at turn on.

What DIY project is it? A tube amp or a SS amp?

c-J

[Æ]

Hi.
What DIY project is it? A tube amp or a SS amp?

c-J

Solid State, dual mono with two 300 VA transformers (120 volts).

I pulled the below recommendations from the AVEL Lindberg website.

"It is recommended that slow-blow (type T) fuses be used in the primary circuits of all transformers over 100 VA. For larger toroids, either NTC thermistors or circuit breakers designed for motors and transformers (with type D delay characteristics, for example) should be incorporated. Simple relay-switched resistor soft start circuits can also be used effectively, and a delay of between about 30 to 300 mS will usually work effectively (some relays themselves have pull-in delays of approximately this time). Soft start circuits of this kind should be implemented with, and never replace, the proper circuit protection provided by fuses or circuit breakers."

[Æ]

Sure, no problem.  What's not shown in that diagram, is the relay coil and its driving circuitry.

As I understood it, what Nelson Pass described to me, the proper relay coil could be energized by the AC without any additional control circuitry. He had someone else there along with him who actually drew a simple schematic on a napkin. Current would initially flow through the resistors to the power supply/transformers, those same resistors would also supply the relay coil with just enough voltage/current to delay the relay turn on, slow things down for half a second. Once the relay was engaged the transformer would receive full current through the relay. The relay would stay engaged as long as power was supplied.

[Steve]

A simple circuit is to use a power resistor (value determined by you, say 50 ohms 20 watts to start) in series with the power switch and in series with the primary winding of the power transformer.

The relay coil rated 120vac, is across the power transformer primary. The relay contacts are across the 20 watt resistor. (Triple pole, 10 amp contacts, 11 pin design can be plugged in and easily replaced.) When the power switch is turned on, the resistor will limit the initial AC current through the primary of the power transformer and therefore the charging of the B+ filter capacitors. 

As the B+ filter caps charge, less current is drawn, and the AC voltage will rise across the primary. At "X" vac, about 80 vac, the relay initiates contact closure, shorting across the 20 watt resistor, and applies the full 120vac across the power transformer primary.

The primary voltage will jump from approximately 80 vac to 120 vac when the relay engages, so there will be a small surge, but way less than simply turning on the power switch.

Hope this helps.

Steve
 

[HT cOz]

Christi from Connex electronics has one that would work well.

[Steve]

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

[Æ]

Christi from Connex electronics has one that would work well.

Hello,
Do they have a website I can look at?

[poseidonsvoice]

You can also look here on Greg Ball's site. He has a relay based soft start. You might need one for each toroid depending on how large (VA) your toroids are.

Good luck with the project and thanks for sharing.

Best,
Anand.

[HT cOz]

Hello,
Do they have a website I can look at?

Here you go http://connexelectronic.com/product_info.php/cPath/43/products_id/49

[Æ]

You can also look here on Greg Ball's site. He has a relay based soft start. You might need one for each toroid depending on how large (VA) your toroids are.

Good luck with the project and thanks for sharing.

Best,
Anand.

Yes, I plan on two of them.

Wonderful, just what I like, small and simple. But it would have to be shipped all the way from Oz, bummer. I just might go for two of them though.
It would be interesting to see a schematic for it.

[Æ]

Here you go http://connexelectronic.com/product_info.php/cPath/43/products_id/49

Nice and affordable, but not what I want, too complicated. I want something smaller and something simpler. Simple enough that I can diagnose a repair and readily source the components.
Thank you though.

[el34]

As I understood it, what Nelson Pass described to me, the proper relay coil could be energized by the AC without any additional control circuitry. He had someone else there along with him who actually drew a simple schematic on a napkin. Current would initially flow through the resistors to the power supply/transformers, those same resistors would also supply the relay coil with just enough voltage/current to delay the relay turn on, slow things down for half a second. Once the relay was engaged the transformer would receive full current through the relay. The relay would stay engaged as long as power was supplied.

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?

[el34]

Nice and affordable, but not what I want, too complicated. I want something smaller and something simpler. Simple enough that I can diagnose a repair and readily source the components.
Thank you though.

I expect the one transistor on there is a standard part, should be no problem sourcing parts for that, or understanding it.

[Æ]

I expect the one transistor on there is a standard part, should be no problem sourcing parts for that, or understanding it.

Looks very nice, but it has about a dozen more components than I would prefer. It also has a remote switching option which I really don't need either. But thanks anyway.