ML1400 FET outputs

[waterboy 7]

I have an ML1400 L32BXBA with some FET outputs. I realize these intended for high speed which I have no need for, but I used them to drive 24vdc LED's indicators on the panel and the LED's will not shut off. I see in the spec a maximum leakage of 1mA so a 20mA LED should be more then enough load to allow the FET to turn off and 1mA leakage shouldn't drive the LED to a visible level. There was a relay coil on these for a while but that was nowhere near the 1.5A max load and shouldn't have generated enough flyback voltage to damage the FET. Anyone seen this, did I get a bad PLC?

[Mickey 71]

I don't know the actual specification for a LED, but I have seen exactly what you have. There is nothing wrong with your PLC outputs. Once a LED is turned on by a solid state device is will stay on even though the solid state device turns off. You need to use a relay or an incandescence light bulb. Edited 8 Oct 2010 by Mickey

[Steve G 1]

I had the same problem on a project I was doing LED's that were turned on would stay on. I tried to use a pull down resister and that did not work. In the end I replaced the output card with relay out and everything is now good.

[waterboy 7]

I find that it does this with those small A/B relays too. I would imagine solid state relays would behave the same way. This cant be right.

[Mickey 71]

Check the documentation on your A/B relays ( what is the catalog number of these modules), some have build-in surge protection. If it is just a relay ( no surge protection) it will work.

[paulengr 44]

Nothing wrong with the output. LED's are not a brick-wall device in terms of "turnon" or output requirements. They have an exponential current vs. voltage output. Any leakage at all will cause an output. As a result pretty much any leakage at all will give some small amount of output. As to the post about a "turn on/turn off" effect, not true. This IS true of TRIAC's and SCR's because they are a 4-layer device...once you break down the voltage barrier internally, only full current reversal shuts them off...hence the reason that an SCR can conduct even with a negative votlage with an inductive load. Free wheeling diodes (and an LED is included in this category) stop conducting at zero or negative voltage, period. Realize that the output device in the PLC is a FET...field effect transistor. This has an insulated "gate" and the two power leads. It's not an on-off device like a relay. It's more like a pinch valve...changing the voltage on the field input (gate) changes the relative amount of "pinching" going on. Kind of like squeezing a hose...you never truly "stop" the flow unless you put several in series or some such to cut off the flow more effectively than just pinching the hose.

[waterboy 7]

I get what you are saying, but there is a specification of maximum leakage in the off state. I should only have to ensure that the "holding" current of whatever device I attach to this output is below that maximum leakage. The manual states that the max leakage is 1mA. With 1mA of current flow, I can't imagine a Panel LED being very bright and I certainly wouldn't expect a mechanical relay to hold. But they do.

[paulengr 44]

http://www.electronicspoint.com/led-question-t139693.html Expect some output even as low as 0.1-0.2 mA. There is a current limiting resistor in the system of course but I don't know how much of an impact it will have especially if you are using green or blue LED's which have roughly double the bandgap of an ordinary red or amber-colored LED (and hence a smaller resistor).

[waterboy 7]

Tested this unit and found that 2 channels are off after power up but are indeed latched once set a single time in logic. Replacement unit doesnt behave like this, so the problem is solved. Thanks for the help.