Strand -10v signal current requirement

[mutley]

Situation in brief:

I have 2x Strand Tempus dimmers which operate on NEGATIVE 10v analogue signals, not +10v like most others!

I want to use these dimmers on a demux which only does 0 to +10v.

I intend to make a simple op-amp "voltage inverter" circuit to convert positive voltages into negative, but need to make sure that the op-amp can supply enough current. If the dimmer draws say 60mA per channel, and the op-amp can only provide 30mA, I have a problem :unsure: !

Also, I would like to power the op-amp circuit from the dimmer or the desk's aux supply feed (on pin 7), but that is fused at 250mA, so I can't exceed that B-) .

Anyone know that dimmer's current draw on it's analogue input lines?

[And before somebody suggests metering it, I can't meter it if I don't have a negative signal! And no, you can't just reverse the wires to invert the polarity, because it will short out via the earth wire on the dimmer/demux. :huh: ]

[patrick_keys]

We used to use a homemade op-amp circuit with our old Mini 2 dimmers which had the same 0 to -10V control system. I can't remember exactly what op-amps we used, but it's a fairly safe bet that the control inputs of the dimmer draw practically no current at all. Remember that all you're doing is providing a voltage to a comparitor...

 

The 250mA supply is designed to power the desk itself but could be used for powering the circuit that you've designed. Probably worth remembering though that op-amps usually need a +/- 15V supply, although you can reduce this to usually 0.25V above/below the maximum voltage you'll be using (check the datasheet). You may be lucky and find a suitable DC-DC converter that will make this for you from the desk supply... otherwise it'll be an external power supply.

 

HTH

 

Patrick

[Ian Ferguson]

If you put a cheap bridge rectifier in each of the six control lines you will be able to use the dimmer on both +ve and -ve voltages at a minimal cost.

[matt c]

Situation in brief:

I have 2x Strand Tempus dimmers which operate on NEGATIVE 10v analogue signals, not +10v like most others!

I want to use these dimmers on a demux which only does 0 to +10v.

I intend to make a simple op-amp "voltage inverter" circuit to convert positive voltages into negative, but need to make sure that the op-amp can supply enough current. If the dimmer draws say 60mA per channel, and the op-amp can only provide 30mA, I have a problem B-) !

Also, I would like to power the op-amp circuit from the dimmer or the desk's aux supply feed (on pin 7), but that is fused at 250mA, so I can't exceed that

 

It sounds like it'll work to me.

 

Depending on what voltage is coming out of the desk, get a pair +/-12 or +/-15 volt regulators. You can get 2 Amp devices fairly cheaply. Using a 2A device means you're less likely to need a heatsink on the regs, so construction is a bit simpler.

 

Then get some LM324 op-amps, and lay it all out on stripboard (or make your own PCB, if you're adventurous).

 

Make sure you properly decouple the inputs, though, otherwise you might pick up interference, which would mess with the voltage level, and therefore the brightness of your lamps.

 

Matt

[dirkenstein]

If you put a cheap bridge rectifier in each of the six control lines you will be able to use the dimmer on both +ve and -ve voltages at a minimal cost.

Does this wonderfully simple work for Act 3/Act 6 dimmers as well?

Does the forward voltage on the rectifier have any significant impact on the dimming curve?

I always thought that that wouldn't work due to attempting to connect the +ve control voltage output to 0V which was likely to be tied to equipment earth on both systems, resulting in a short.

Also, the 0V circuit is shared across all input op-amps; if each channel inpout had its own 0v referecne you could have just flipped control voltage +ve and -ve for each input.

 

BTW I can vouch that the input current for Strand Act3/Act 6 dimmers must be in the uA range- a 1uF capacitor across an inout and 0v will hold the output level stable for minutes in the absence of an actual input voltage. I can't imagine the tempus will be any different.

 

Dirk

[mutley]

Thanks guys.

To save having to use an external dual-rail power supply, or building one into the circuit, one of the reasons I asked about the current draw is because I intend to use a simple 555 astable voltage inverter. The problem with this is, as the current draw increases, the inverted voltage drops considerably. At 20mA on a 12V supply, the inverted voltage drops by over 25%, but if the current draw is so small that a 1uF capacitor can hold it for a few minutes, then I should be fine.

Once I've designed the circuit I'll upload the PCB layout for anyone else to use if they wish :up: . (I design photo-etch PCBs on the computer B-) - far tidier than stripboard!)

Some people read to relax - I design PCBs! :unsure:

At 26p each for the LM324s and 25p for the 555, plus the other resistors, etc, I expect it to cost less than a tenner for a 12 channel voltage inverter. :huh:

[Floydey]

I built this circuit to run a pair of Strand Tempus dimmers from a Zero88 Alcora desk running on an analogue output. It works a treat.

 

http://sound.westhost.com/project90.htm

[mutley]

OK, I've priced it all up to get all the bits from CPC (CPC is only 10 miles away from me, so I don't pay postage), and total components cost is £15 + VAT, including two 7 pin DIN plugs, 2 chassis mount sockets, aluminium box, etc.

If anybody wants the kit (including computer designed PCB, with tinned copper tracks, pre-drilled), PM me. I'll do the PCB for Blue-Room members for £6.50, or fully assembled and tested for £35. I might as well make them all at once...

(Prices include P+P)

[Paul TC]

including two 7 pin DIN plugs,

 

Thought it was an 8 way connector on a Tempus, or have you a 7 pin output on the Demux ?

[Kazeja]

Inputs on Permus and Tempus are fed directly to one end of a 47k pot, the wiper of which adjusts the top set. The bottom of the pot is connected to Tech Earth.

 

The control system would normally have a 10k resistor and series diode (1N4002).

 

What you effectivly approximatly have is a 60K load with topset adjustment control between 0 and 50K.

 

HTH.

[Don Allen]

I use 7555 or cmos 55 timers as they draw a lot less power. Compare prices as they are more expensive, but don't have that current surge the 555 has when triggering.

[Blaize110]

I have just got my hands on some Strand Mini 2+ dimmers and need to make some voltage inverters for them. I was wondering if the dimmer supply voltage is -ve or +ve? and, if it is positive, can I just invert it rather than every channel and use a dimmer pack-powered desk?

 

However I may end up making an inverter for every channel so I can use powered desks anyway.

[boatman]

The technical handbook for the Mini 2 can be found on the Strand archive. Oh h*ll I might as well do the work for you:

 

http://www.strandarchive.co.uk/control/tec...dbookmini2.html

[Blaize110]

*Sheepish look*. Oops, forgot about the archive.

 

Would this this mean I could use any non-powered desk to control them? (Just want to make sure before I potentially ruin anything)

 

Thanks again

Rory

[boatman]

No, you need to use a desk that expects -15V as its supply. Most of the older Strand desk are this way inclined (ACT, AMC, Mini 2 or Tempus), but Pulsar and other manual desks mostly expect a +15V supply.