slow turn on

audiofan · 2nd November 2006, 01:35 PM

I expect to build a power amp and unfortunatly the power tranfo is quite small so to protect the transformer I will use a slow turn on device.
I would like to use a relay in the bleeder circuit. Relay has a nominal voltage of 24 volts and pick-up at 10 volts ( 42% of nominal voltage ) with 3 K resistor will pick-up around 24 volts and nominal voltage will be set OK.
A TRIAC in primary of transfo will short current limiting resistor.
Did anybody try a similar circuit? Should I go ahead ?

richie00boy · 2nd November 2006, 01:54 PM

Firstly, you have misunderstood what soft-start is. It's not going to protect your transformer for being too small, in fact if your transformer is small (300VA or less) then you don't even need it. What it does is prevent your lights dimming and wear on your power switch and a 'thunk' sound at power on.

Now on to the circuit. The triac is pointless, you might as well short the resistor with the relay. I personally don't like the relay drive on the secondary like you have, it's prone to causing a flaming the primary resistor if the secondary voltage doesn't quite get up to full voltage. Better to drive it all off the mains, it doesn't take a complicated circuit.

Gold_xyz · 2nd November 2006, 02:09 PM

The slow turn on is very suitable for toroidal transfo
and with a large capacitor supply.

in many application this device is used.
Professional amplifier, microwave owen, etc. etc.

I have seen this schematisc above all without triac,
but with a little cap for eliminate the switch on transient.

audiofan · 2nd November 2006, 03:35 PM

Slow turn on will reduce stress on all componets: main, switch,fuse, transfo & rectifier.

I will use a lot of capacitor to set time constant of power supply below time constant of feedback & input filter, but this lot of capacitor is very demanding on transfo & rectifier, this is why I expect to use slow turn on. I woul like to usea small relay to keep bleeder current to a minimum value. Using a relay is simple and will control time constant of sytem but a low power relay with small switching capacity cannot switch full power of transfo, this is why I use a TRIAC.

My question are will the triac need a snubber ? since there is a low value resitor across

Will the TRIAC switch close to zero enough ? or will I get a lot of harmonics in tranformer voltage ?

Gold_xyz · 2nd November 2006, 03:50 PM

normally the transformer is an inductive load

and I think that an AC snubber is suitable.

edl · 2nd November 2006, 04:23 PM

What about this?

2nd November 2006, 01:54 PM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	Firstly, you have misunderstood what soft-start is. It's not going to protect your transformer for being too small, in fact if your transformer is small (300VA or less) then you don't even need it. What it does is prevent your lights dimming and wear on your power switch and a 'thunk' sound at power on. Now on to the circuit. The triac is pointless, you might as well short the resistor with the relay. I personally don't like the relay drive on the secondary like you have, it's prone to causing a flaming the primary resistor if the secondary voltage doesn't quite get up to full voltage. Better to drive it all off the mains, it doesn't take a complicated circuit. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

2nd November 2006, 02:09 PM	#3
Gold_xyz diyAudio Member Join Date: Dec 2005 Location: Italy	The slow turn on is very suitable for toroidal transfo and with a large capacitor supply. in many application this device is used. Professional amplifier, microwave owen, etc. etc. I have seen this schematisc above all without triac, but with a little cap for eliminate the switch on transient. __________________ My English originates from here

2nd November 2006, 03:50 PM	#5
Gold_xyz diyAudio Member Join Date: Dec 2005 Location: Italy	normally the transformer is an inductive load and I think that an AC snubber is suitable. __________________ My English originates from here

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2nd November 2006, 03:35 PM	#4
audiofan diyAudio Member Join Date: May 2003 Location: quebec	Slow turn on will reduce stress on all componets: main, switch,fuse, transfo & rectifier. I will use a lot of capacitor to set time constant of power supply below time constant of feedback & input filter, but this lot of capacitor is very demanding on transfo & rectifier, this is why I expect to use slow turn on. I woul like to usea small relay to keep bleeder current to a minimum value. Using a relay is simple and will control time constant of sytem but a low power relay with small switching capacity cannot switch full power of transfo, this is why I use a TRIAC. My question are will the triac need a snubber ? since there is a low value resitor across Will the TRIAC switch close to zero enough ? or will I get a lot of harmonics in tranformer voltage ?

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