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LED light for driver output visualization?

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Moneil

Mechanical
Nov 13, 2008
15
My company has nitrogen dosing units. A PLC sends an output to a driver that sends power to a coil. The driver also pulls current away from the coil after a given time duration to allow the coil to close the valve quickly.

We are able to see the 6 millisecond output signal from the PLC to the driver via an analog output wired to our high speed camera LCD display. We want to see whats is going on with the voltage and current going to the coil from the driver, but the voltage is too much for our signal box (converts the signal to something the camera can see). Is there an LED light that can withstand 170 volts for 4 ms and then 22 volts for the remaining 2 ms? I want to wire it in paralell so we could see it light up on the high speed to see if the coil is getting what it should be getting.

or maybe there is an inexpensive oscilloscope that has an analog output? Any help is appreciated.
 
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Have any resistors?

-handleman, CSWP (The new, easy test)
 
put those inline so an led could handle the driver output?
 
It's been a while since Circuits I, but I'm thinking something along the lines of sizing a resistor to reduce the current to a level that the diode can handle. The voltage portion will take care of itself.

-handleman, CSWP (The new, easy test)
 
I would put a 200K pot in series with an ordinary discrete LED and start at the high (200K) end. Turn it down until you start seeing the signal.

Don
Kansas City
 
What is the reaction time for an LED and what is the reaction time for an LED with a resister in series? I don't know the answer to these questions but it's something to consider. 2ms seems pretty quick.

I think you need to use a different tool, like an oscilliscope.
 
The scope has to be the ideal solution here - the LED is only going to tell you that "something" has appeared across the coil. It won't tell you how much (either in voltage or current terms) because it's such a non-linear device.

Have you calculated how long it will take the current in the coil to rise when you first apply power? Thisis one area where a scope looking at the current draw could be really valuable.

Didn't get what you're planning to do with an analog output from your desired cheap scope.

A.
 
You could use a oneshot as a pulse stretcher to drive the LED, if you only need to distinguish between two pulse widths.
 
Belated thought:

If you try an LED and associated resistors, it would be a sensible precaution to wire another diode in parallel with your LED, but pointing the other way.

In circuits with coils, there's always a risk of getting some reverse voltage. As designed, the chain of resistors reduces the voltage at the LED to a safe level provided there's current being drawn through them. If the voltage is the other way round, the LED won't conduct, so there's there's no current through (and hence no voltage drop across) the resistors and all the available volts appear across the diode junction. Mere ohnoseconds later, the LED becomes a smoke emitting diode.

The safety diode doesn't have to be anything clever. You might choose another LED (different colour might look nice on your high speed movie)

A.
 
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