Opto-isolator advice

[optodunce]

Hello to all you lovely people.

Right this may go on for a while.... so please stay with me.

I need to control the power output from a.. lets say unit, what it does is not important.
So this unit has a power control line, nice and simple.

0VDC = 0% power
10VDC = 100% power

Default is full power 10VDC

Due to the vagueries of what I am trying to do, it is only possible to to short this voltage
in order to drag it down and so control power output.

So far I have a 5K pot on there and it works a dream, there is no problem shorting the line
(voltage comes from an op-amp with a nice big resistor on the output, either way I have left it shorted for hours with no ill effects.

Incidently shorted current is 32mA.

The problem is that really I need to control the power output from an external analogue signal (PLC D-A unit, voltage output scalable to suit but hopefully 0VDC-10VDC).

So Here's where I come unstuck due to ignorance, untill I started to look I assumed (wrongly) that it would be possible to get an opto-isolator with a photo-resistor output that would be biased in either direction. therefore I could just sit it straight on the line and the worst I would have to do would be condition the external signal a bit to get linearity.

As it turns out I'm pretty sure you can't do this (5K photo-resistor output, low input=low resistance does not exist as far as I know), thinking back I might have known.

So can anyone suggest a good way forward;

Is an opto the way to go?
What type?
will I need to use an external PSU to run an op-amp?

To be honest I am totally lost now and really need help.

Thanks in advance for any time you can spare and for the time you have already spent.

 

[Harald Kapp]

You could use an analog (linear) optocoupler, e.g. http://www.avagotech.com/pages/en/o...igh_linearity_analog_optocoupler/gate_driver/ ,this may be a bit expensive, however.
Depending on your requirements with regard to linearity and settling time of the analog control voltage, your can use a PWM signal (http://en.wikipedia.org/wiki/Pulse-width_modulation) and transmit ist via a cheap optocoupler. Add a low pass filter at the output of the optocoupler to recover the mean Dc voltage from the PWM signal. Depending on the duty cycle of the PWM you can set up any voltage between 0% and 100%, as required by your circuit.

Regards,
Harald