Re: System for switching circuits of high-wattage devices

On 30 Jun 2005 10:18:41 -0700, "DJ Craig" <spit@xxxxxxxxxxxxxxx>
wrote:

>Forgive me if I've posted this message in the wrong group. I'm not
>really sure where I should post this.
>
>I'm a mobile DJ. I often DJ outside people's houses, or at hotels,
>camp grounds, appartment blocks, churches, etc. These places usually
>will let you draw about 2000 watts from one of their circuits before
>you blow the circuit breaker. My equipment requires between 4000 and
>5000 watts. The lighting effects, fog machine, etc, requires about
>2000-2500 watts, and the audio equipment requires about 2000-2500
>watts. I have about 1000 watts-worth of equipment that *must* remain
>on througout the event, such as CD players, mixer, amp and speakers.
>It is a nightmare trying to distribute my equipment across multiple
>circuits so that I dont blow fuses, which I often do. It is not too
>bad if the lights and fog machine turn off during an event, but it is
>important that my audio equipment keeps running thoughout an event.
>
>I thought that it may be possible to build a gadget that would hook
>into 2 circuits, where i would plug my audio equipment into the
>"reliable" plug, and the lighting into the "unreliable" plug. If
>either of the two circuits blow, then the box would switch the audio
>equipment over to the remaining circuit, and turn off the lighting.
>Does such a gadget already exist, or would it be possible, and
>practicle to build?
>
>One issue is that if my CD players are turned off, even for an instant,
>the music will stop and I have to re-cue the music. So a box that just
>uses a relay to switch the circuits may not work. I'm not sure, but
>the CD players may not be able to handle the delay caused by the relay.
> Especially if the voltage from the circuit that is blowing fades
>before it goes right out, creating even more delay and causing the CD
>player's condensers to empty. Could it be done with a triac instead?
>Would a triac be able to handle the high wattage?
>
>Can anyone think of another solution to my problem? I always try my
>best to hook into as many circuits as possible, and distribute my
>equipment evenly, but I still have trouble.

---
I like the relay idea to switch the lights and audio stuff, but I'd
get an uninterruptible power supply for the CD players and wire its
input to the audio side of the relay. That way, even if you lose
power and it takes the relay a few tens of milliseconds to switch the
audio stuff to the good line, the CD player would have power supplied
to it all the time and it would just keep playing.

I'd use a DPDT relay with contacts rated for 30 amps and hook it up
like this:



L2>----+----------------------------+
| |
| |
L1>----|----------------+ |
| | |
O-->\ <--O O-->\ <--+
NO \ NC |NO \ NC
O | O--+----[AUDIO STUFF]
| | | |
[LAMPS] [COIL] +--[UPS]---|---[CD PLAYERS]
| | | |
| | | |
NEUT>--------+----------+-------------+-----+

That way, when you plug the thing into line 1 (L1) the relay coil
(120VAC) will be energized and will cause the common contacts to move
over to the normally open (NO) contacts. That will cause the lamps to
be connected to L2 and the critical stuff to L1.

Now, if L2 drops out the lights will go out, but the critical stuff
will keep going. However, if L1 drops out the coil will no longer be
energized, which will cause the common contacts to fall back on the NC
contacts. That will disconnect the lamps from L2 while connecting the
critical stuff to L2, and the UPS on the CD players will keep them
from getting the slightest hint that anything happened, which is what
you want.

There is one small snag though, and that's that there's no guarantee
that neutral and hot won't be reversed on on of the sockets you plug
into, so uless you keep everything separate, nasty *** could happen.
Fortunately, there's an easy way out and that's to use another relay
to switch the neutrals, like this:

L2>----+----------------------------+
| |
| |
L1>----|----------------+ |
| | |
O-->\K1A <--O O-->\K2A <--+
NO \ NC |NO \ NC
O | O----+----[AUDIO STUFF]
| | | |
| +--+----+ | |
| | | | |
[LAMPS] [COIL1] [COIL2] +--[UPS]---|---[CD PLAYERS]
| | | | |
| +-------+--------+-+-----+
| |
O O
NO / NC NO / NC
O-->/K1B <--O O-->/K2B <--O
| | |
N1>----|------------------------+ |
| |
| |
N2>----+------------------------------------+

A good relay would be an OMRON MGN2C AC120, $29.70 each at Digi-Key.

--
John Fields
Professional Circuit Designer
.

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