DMX Switch or Relay Pack

[LeeStoddart]

I'm looking for a small unit that I can use to control non-dim type kit - specifically the first use is for a set of egg-strobes, haze machine, fan. So far I've found a 4-channel switch pack (Transcension DSP-405 DMX Switch Pack) in various forms eg here

 

Other (old) threads here discuss the use of relays rather than the "zero crossing circuitry".

 

What are the benefits of relays compared to "zero crossing circuitry"?

 

Other than a bare circuit board such as the one from Milford Instruments on the same page as the link above are there ready built packs available.

[KevinE]

I suppose relays can be more robust if for example a lamp blows or you get a short on the load; whereas a triac would possibly fail, a relay will probably be ok unless you have a silly fuse in which might weld it.

 

The problem with relays of course is that they wouldnt last long in a chase programme, because they'd soon wear out. They will also cause switching noise which might upset other equipment, triacs dont have this problem.

[Matthew Robinson]

If you want to operate a haze machine, you may find that you can't operate it effectively by switching its power on and off.

[Jivemaster]

http://www.milinst.co.uk/ and look for DMX modules, will give you all the modules that they supply. Semiconductor switching tends to distort the waveform at the zero cross point giving occasional bad effects. Relay switching means some random switch points but not one every cycle so the artefacts are different.

[TomHoward]

The Milford Instruments one is available as a boxed unit as well - but it would need casing further to include power connectors, it's a set of relay outs along the back of the case.

[Tomo]

"Zero-crossing circuitry" is tosh. It's marketing-speak for "Dimmer that's permanently in switch mode" (usually also omitting the choke as the rise time doesn't matter)

 

The Botex unit is a Triac (or possibly discrete SCR) based solid-state switch.

It is therefore almost identical to using a dimmer in the 'switch' setting as far as the end device is concerned.

 

You will note that many devices specifically say not to control them with solid-state, Triac or SCR switches.

This is primarily due to the 'wibble' around zero-cross as the triac/SCR goes from conducting to non-conducting, and is then switched on in the other direction.

This happens 100 times per second for a 50Hz supply.

 

Relays do not suffer from this, and are preferable in any situation where you are switching a non-resistive load.

 

Purely resistive loads like tungsten lamps obviously don't care, and that is the kind of load the Botex unit is aimed at - it's a 'disco flasher' for want of a better term.

 

I would guess that your egg strobes and hazer are likely to dislike the triac in the Botex, while the fan will be OK *if* it has an AC motor and represents a sufficiently large load to allow the triac to properly turn off.

 

- Mirror ball rotators are classic 'tiny' AC motors that often won't turn off from this kind of switching.

[David Duffy]

"Zero-crossing circuitry" is tosh. It's marketing-speak for "Dimmer that's permanently in switch mode" (usually also omitting the choke as the rise time doesn't matter)

I remember back in the 80's, some chasers had TRIAC outputs but no zero crossing. The TRIACs were driven by either pulse transformers or the earlier MOC3021 optos and would tun on anywhere in the cycle they liked. The (zero crossing) MOC3041's came out later, but were still fairly expensive at the time.

 

I do agree that relays are the best options for non-resistive loads. Even if the TRIAC output will drive the load, any partial phase angle will see some loads go BANG! Yes, accidentally did that to a small gobo rotator (with switch-mode PSU) once. :** laughs out loud **:

[LeeStoddart]

Thanks all (especially Tomo with the clearest explanation)

 

Apart from the Milford Instruments cased example any other suppliers of true relay packs?

[dbuckley]

I have a Northlight Relay board and will get another sometime soon.