Remote power switch

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
i recently installed a PA system in a church

the mixing desk is at the back while the amps are up front - which means every time i turn the system on i have to turn the mixer on at the back then turn the amps on in front - and vice verca when switching off.

although this isn't really too much of a problem for me - i am worried that the other people who use the sound system will either forget to turn the amps on (and go into panic mode) or forget to turn them off.

so i was wondering if there is an easy way to make a "remote controlled" switch - kind of like a relay to turn the amps on and off.
I was thinking of using a triac - but i am not sure how much leakage current flows through them when they are off - plus i think they need power to stay off (or is it the other way around??)

my plan is to use a pic for control of switching both the mixer and the amps on - so i can ensure they are turned on in the right sequence (simple delay program)

please let me know if triacs are the best idea for the job. thanks
 
Triacs don't need any power to stay off, just a proper snubber and a gate pull down resistor depending on type (there are snubberless ones, insensitive gate ones, etc...) Power is only required for triggering and they stay on until current crosses zero (they they turn off provided that the current and voltage transients associated with the zero cross are not too fast, see datasheets...) Leakage current when off is too low to cause any problem, but fast line transients can cause parasitistic triggering of the triacs during half a cycle (some filtering solves that). Be aware that continuous triggering is required when feeding transformers because current changes direction in the middle of the mains cycle.

Of course you can use a PIC to turn them on in sequence. You may consider turning on each amplifier in a different instant to avoid tripping circuit breakers due to simultaneous inrush currents.

I would use plain relays, though, as the key feature of triacs is cycle by cycle mains switching but you don't need that at all. Furthermore, the disadvantage of the triacs is the 0.7V to 1.5V drop across them that results in some unwanted heat. Remember that relays also benefit from snubbers because turn-off arcing may be prevented.
 
Administrator
Joined 2007
Paid Member
As Eva says relays are best, particularly at low currents, also no R.F.I. I use a remote mains switching arrangement on audio system at home, the amplifier turning on all the source components. The signal to the switch unit is fed via a fibre optic cable from the amp. The switch unit draws zero power when off and has a 3.6v nimh battery backup.
Regards Karl
 
Go to a HVAC parts store and find some large power relays normally used to control compressors. Those relays are rated for high continuous current (25A and above), high inrush current (100A or more for a fraction of a second), inductive loads, and are available with common coil voltages of 24v, 120v, and 208/240v. They're very cheap (about $8 for a single pole) for what they are rated for.
 
Thanks for all the advice - i will try to get hold of a relay.

Actually, i just found 2 relays in my "box of acquired electronics stuff". it is rated 250v, 10A. It also says type 60.12 on it (if that helps)

considering the wall sockets can only deliver 16A (before it trips) - do you think i will be able to use them???

i don't run the amps hard - in fact they barely get warm and i have the gains set VERY low. the only thing is when turning these things on the lights do dim a bit.

what about connecting both relays in parallel???

it uses 24v DC to turn on and off, any ideas on the current needed to run them??
 
230V x 10A = 2300 Watts. Enough for your amplifiers? I sure hope for the sake of your neighbours ;-) Paralleling relays is almost never a good idea BTW - contacts won't close or open at exactly the same time.

A modern power relay of this spec will have a coil rating of about a Watt or less. At 24V, that's less than 50 mA. Just measure the coil resistance and do the math.

BTW, if you drive this coil with a transistor, be sure to include a freewheeling diode across the coil to absorb the inductive kickback. 24V coils can generate spikes of hundreds of volts at turn-off.
 

Attachments

  • flyback.gif
    flyback.gif
    2 KB · Views: 117
it has a diode hooked up to the coil like that already - i thought it was just to make sure the DC is connected with the right polarity, but now i just tested it (without 230V load) and it works no matter what polarity the DC is.

At least now i know what the diode is for - thanks a ton!!

The amps might be able to push out 2300 W together but they will never get used like that - i am just worried about the turn on current.
But then 10A is the continuous current, not the peak current, correct?
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.