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Zero 88 Fat Frog - 'setting a permanent level on a DMX channel'

Andrew H

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On 6/20/2022 at 10:43 AM, Andrew H said:

The power for the Martin Mac's comes from the 'dimmable sockets' and this has been done by not assigning the six sockets to channel 512 (the opposite end of the desk's working range) and then manually setting the outputs on the dimmer packs to maximum.

This is a really, really bad idea. You're certain to accidentally dim the Macs sooner or later, which will destroy them pretty quickly.

As to short-term solutions:

Best is just to run a suitable extension lead (with adapters) from nearby 13A hard power.

If that's not possible, many dimmer packs have the option to change the "dimmer curve" or "mode" to "Switched" or "Always On".

This is much better than setting the level via DMX or manual levels.

  • These modes will disable any voltage regulation or similar features of the dimmer, (usually) it'll "hard-fire" the SCRs to minimise the disturbance around zerocross.
  • It protects against accidentally dimming the load, eg by accidentally using the wrong encoder on the console, loading the wrong showfile, or while manually bringing the level up after a power outage.

That said, your measurements probably did not tell you what you think they did.
Even Switched or Always-On mode is quite likely to cause damage to the moving lights, as the SCR + choke remains in series and these will always cause current disturbance.
Electronic ballasts and many switch-mode PSUs really don't like this.

You won't see this disturbance using an oscilloscope to measure open-circuit voltage, as the disturbance is current, not voltage.
In fact you'll probably see a pretty clean sinewave on a high-impedance oscilloscope with the dimmer entirely "off"!

As other mentioned, the striking of arc lamps is also quite likely to damage the dimmers. Admittedly, new SCRs are probably much cheaper than new ballasts.

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Some electronic power supplies will suffer from the delay between zero crossing and triac switch on which may be small but it's often finite. There used to be "relay packs" as well as dimmers. which were a simple contactor per channel which switched on at some DMX value and switched off at some lower value.

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  • 4 weeks later...

Hmmm. I wonder why ETC, Strand, Zero88, etc all went to the expense of developing dimmer/relay racks when automated and then LED fixtures became prevalent? To sell more racks or to provide the most sensible solution for not blowing up their customers lights? I would echo other posts here and suggest that the relative cost of switched hard power would outweigh the cost of a blown Martin mover (with potentially more devastating H&S consequences).

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