Preamp remotely turning on Monoblock PSUs

[elaar]

Hi Chaps,

I partially built a pair of gainclone monoblocks a while back and they have sat on the bench for a good year as at the time I could not work out the best way to "turn them on" through a seperate preamp.
The monoblocks have no power switch and each contains a 225VA (230v 2x 25v/4.5A ) toroidal transformer and 10000uF caps.

I was hoping I could run this by you guys and see what you thought, I am looking for the most simplistic (but safe) method as possible to start with.

The preamp will contain a PIC MCU (that controls an LCD among other things). I plan to use 6v 0.5VA transformers (I have these spare), regulated to 5vDC and feed this voltage to each monoblock using a 3.5mm-3.5mm cable, that will in turn power on a relay inside the monoblock with a protection diode. The preamp's PIC will turn on/off this 5v using appropriate transistor(s).
The idea of this being that space inside the monoblocks is extremely limited and I don't really have space for another transformer. So with this idea I could possibly just get away with a relay and a diode inside the amps, and the preamp can ensure the amps are last to turn on and first to turn off.

The relays I had in mind are 6A rated (15A peak), 5v control 125ohms (40mA) 250v (400V), which I assume are more than capable of handling the inrush current for this transformer.
Is it frowned upon to switch just the LIVE and should DPST be used? Lastly, everywhere I have read, people have stated that under 300VA a softstart isn't necessary, would you agree with that or would you use something relatively simple for 225VA such as NTC?

Any help would be gratefully received and might mean I actually end up finishing these one day!!

Cheers,
Andy

 

[ticknpop]

A softstart circuit for the size of unit you isn't required, but a thermistor (Keystone CL series are good) on the AC primary would be a good idea as toroids draw high curents at turn on which is hard on your power switch. Size the thermistor for above your maximum AC draw at full load - also the thermistor will be warm when it's operating so keep some space and air around it. Also make sure your mains switch has a mains rated (specialy rated and labeled ceramic or film 0.1 uf ) cap across it's contacts to suppress arcing as well. Snubbing the relay contact(s) with the same 0.1 uf cap per pole would also be good. It will save pitted relay contacts over time and avoid early replacement.
Also do not turn the amp off and then on immediately - it takes time for the thermistor to re-set ( maybe a minute) to allow another start without stressing the power switch. A real soft start would reset instantly , but is more costly to build and takes up more space.

 

[krokkenoster]

A softstart circuit for the size of unit you isn't required, but a thermistor (Keystone CL series are good) on the AC primary would be a good idea as toroids draw high curents at turn on which is hard on your power switch. Size the thermistor for above your maximum AC draw at full load - also the thermistor will be warm when it's operating so keep some space and air around it. Also make sure your mains switch has a mains rated (specialy rated and labeled ceramic or film 0.1 uf ) cap across it's contacts to suppress arcing as well. Snubbing the relay contact(s) with the same 0.1 uf cap per pole would also be good. It will save pitted relay contacts over time and avoid early replacement.
Also do not turn the amp off and then on immediately - it takes time for the thermistor to re-set ( maybe a minute) to allow another start without stressing the power switch. A real soft start would reset instantly , but is more costly to build and takes up more space.

I used caps before and it did not help what I did then is to fit a transorb of 240 volts over the contacts and it still works fine after ten years on a freezer thermostat that failed which I replaced with one based on a 555 i.c.