On Thu, 02 Feb 2006 10:15:23 -0000, Jasen Betts
wrote:
On 2006-02-01, John Fields wrote:
Hi all,
I need to switch high voltage (1kV at 2kHz AC), low current to
sequence some electro luminescent wires. I'm working on the trigger
sequencer and I have the high voltage/ high frequency PSU but I'm not
sure what kind of device/technique to use for the actual switching
between 5 channels. Any suggestions greatfully received.
Thanks in advance,
UK Pete
.. Ooops Make that 200V AC @ 2kHz...
MOC3021 (upto 100mA) If you're in the UK that part (or similar) should be readily
available, it's an optocoupler used to drive triacs but also seems suitable to
switching 200VAC at low currents.
I don't think so.
His 200V drive is almost for sure going to be coming from an
inverter with a rectangular output waveform.
and that'll be a problem ?
could it be solved using some sort of low-pass filter?
---
Not likely, since the color and, to a lesser extent, the intensity
of the light coming from the EL wires will change if the waveshape
of the signal driving them changes.
But, the point is, why bother?
Using a proper SSR will preserve the shape of the driving signal and
eliminate the possibility of any commutation problems which might
come up using a thyristor switch.
---
Take a look at pages 4 and 5 of:
I'm not sure what to look for
I see a curve for on voltage that puts it at around 1.5 for 100mA
150mW is well within the reasonable range.
I see figure 5, and the circuit top of page 5
200vX200Khz 80000V/s = 0.08V/us if a triangle wave
more if a sine wave, yet more if a square wave.
---
You're reading it wrong. Notice that at 20C the maximum rate of
change is 10V/µs. That means that if you try to switch a signal
which goes from zero volts to ten volts in one microsecond you'll be
OK, but if you try to switch a signal which goes from zero volts to
eleven volts in a microsecond, you won't.
That means that for the OP's signal, which is making 200 volt
transitions, those transitions need to take longer than 20µs for
commutation not to occur.
---
but with a square wave would switching on a little late be a problem?
---
No, commutation would.
---
Is a solid state relay likely to perform better?
---
Yes.
With a solid state relay there will be no commutation problems.
---
http://www.fairchildsemi.com/ds/MO%2FMOC3021-M.pdf
thanks for searching that out, I'd misplaced my copy and was looking at a
summary in a paper catalogue.
I see the bit that says "This optocoupler should not be used to drive a load
directly, it is intended to be a trigger device only."
But these electroluminescent wires only draw a few milliamps it's not like
trying to switch a lightbulb where the cold current can be a few amps.
What am I missing?
---
Even with a resistive load, the rate of change of the signal out of
the inverter, when it switches, is likely to turn on the bilateral
switch whether the LED is on or not. With a capacitive load (EL
wires look like lossy capacitors) the problem will be aggravated
aggravated.
Try this link:
http://www.onsemi.com/pub/Collateral/HBD855-D.PDF
--
John Fields
Professional Circuit Designer