PLC 5 Output Module

[kevinprior 0]

Hi, Just a quick one to which I know someone will know the reason. I was adding some solenoids to a rack of a PLC 5 system, the output card was a 16point 110V ac output card (I'm unsure of the number as it is plugged in) When checking through I found that there was voltage there (110V) on the output even though the point was not switched in the PLC, when the point was turned on in the program the voltage was still there, and still there when switched off. The only time this dissapered was when the solenoid coil was wired to it, then the voltage would go on / off as normal. So there had to be a load connected to the card in order to get the reaction I would expect. I'm sure there will be a good reason for this, solid state components reaction etc but I would just like the true reason why??

[rpraveenkum 5]

refer to the output card manual http://literature.rockwellautomation.com/i...in024_-en-p.pdf

[paulengr 44]

RTFM. I've heard that one before. It is *not* spelled out in the manuals. Before reading the rest of this, go find yourself one of those "diode I/V curves". A diode is not a perfect switch. It does have some small amount of current leakage when the voltage is reversed across it. Triac's are basically a pair of SCR's connected back-to-back so that it can be fired in either direction. An SCR is a diode except that it has an extra layer in the middle that causes the SCR to build up a field resisting flow of electrons. When you power up the gate, you deplete (drain) this field. It remains in this state until you either drop power on the gate or put a huge reverse voltage across the device to re-establish the field (don't worry about this possibility). This means you can do two things with a triac. You can simply apply power to the gate connection which makes it a solid state switch with no relay chatter and no contacts to wear out over time (triac's have a lifetime measured in years of operation, not the number of times they cycle). The second thing you can do is that if you have a timer (such as a 555) that triggers when the AC wave crosses zero, you can "chop" the AC wave and create a reduced current/voltage output. The circuit is very simple. This is the circuit in most lamp dimmers. OK. Now back to the point. There are two sources of "leakage" that you will see in practice. One is simply that diodes will leak a very tiny amount of current. In practice, you can think of it like having a really big resistor in series across the "switch". The amount of current you will be able to pull is a trickle, but it is still there. That's why the output card manual for triac cards gives you all kinds of warnings about using LED's on the outputs...since an LED requires only a trickle of power to light compared to an incandescent bulb, it will usually light up even when a triac is off. Same thing with your multimeter which usually has an internal resistance of at least 1 Megaohms. By the time you manage to connect any kind of load, think of it like a voltage divider. Now that you have a relatively "small" resistor compared to your multimeter and if you get anything at all, you will measure a voltage of a few volts at best. I haven't tried it but even with NO power applied at the input side of the triac's, you may still measure an output when you trigger the output on. This is simply the power applied to the gate to open the device up. The effect is harmless but it will throw you off when you go to troubleshoot triac circuits if you are not aware that it is going on. It is key that your electricians understand this. Another warning: Notice the "remove power" problem. If you ever end up in a situation where you never get to "zero" volts such as if you have a faulty ground situation or DC applied in a DC/AC system, or somebody that is used to relay output cards attempts to use it to drive a DC output, you can have a much bigger problem. Once energized, a triac can only shut down when the AC wave crosses zero. If it never crosses zero, the triac output is held open until you cut power to it. If you have everything properly grounded and fused and you keep your DC and AC circuits separate, this is never a problem.

[newpageboba 7]

If you measure the voltage with a digital meter you will read about 110V in the off state and about 120 volt in the on state. Connect a load and it will read 0 volts in the off state. Now use an old Simpson analog meter. You will read about 60 Volts in the off state and 120 volt in the on state. The triac output has leakage current in the off state. When you turn on a triac it allows current to flow. When off it restricts current flow (but some gets through), and there is not enough current to power the load off a coil ( but there is enough to power a meter)

[BobLfoot 302]

Fluke's magazine on their meters listed a similiar problem this month.

[Guest]

since no one has mentioned one of the most serious “safety aspects” of this topic yet, here is a “reprint” of something that I wrote a few years ago in this thread on the same subject ... I ALWAYS demonstrate this “safety aspect” of leakage current in my “maintenance technician” classes using the same method that I outlined above ... it turns out that more than a few of my students have been “bit-by-that-dog-before” ... I remember one student in particular who was watching the demonstration very intently a few months ago ... as soon as I disconnected the “feed” wire from the coil, he mumbled: “you’d better put a wire-nut on that thing” ... I could tell he was dead serious by the tone of his voice ... “that’s bitten you before hasn’t it?” I asked ... “yeah ... it almost knocked me off of a twelve-foot ladder” ... Edited 30 Nov 2006 by Ron Beaufort

[newpageboba 7]

We use isolated inputs & outputs for starters/contactors and the 120volt is supplied by the control transformer in the starter compartment. Therefore when the 480 is disconnected, so is the control power. For non isolated outputs (like solenoids and lights) we pull the fuse powering the output and put a lockout tag on it. (same procedure for inputs Forcing an output off is NOT an acceptable means of isolation and we consider that a violation of safety protocol. If we get a panel with multiple starters and one control power transformer, we rewire it or send it back.