PLC out put won't turn off

[pokyj]

To tell you the truth I'm not sure if calling it a PLC is correct. It was made by L & N and it is called Max I . It takes up about 6 cabinets in two different control rooms and is not programed using rungs.

 

[pokyj]

parallel

Thanks for all the input I thought it was somthing to do with the load but I didn't know how to control it. This makes since, but I havn't heard anyone consider the resistance of the relay? So if I have this striaght what we are realy doing is lowering the resistance of the overall circuit to increase the current. So that we meet the minimum current for the triac.

 

[Contr_Conn]

Thanks for all the input I thought it was somthing to do with the load but I didn't know how to control it. This makes since, but I havn't heard anyone consider the resistance of the relay? So if I have this striaght what we are realy doing is lowering the resistance of the overall circuit to increase the current. So that we meet the minimum current for the triac.

Resistance of the relay will be in parallel to the load and total output current will be sum of current to the relay and resisitor.

By selecting resistor based on the minimum current we guarantee, that output will work with ANY relay, including disconnecting relay completely, making circuit load independent.

From my experience, small relay coil resistance is 20KOhm or higher, sometimes 100KOhm that making it insignificant compare to the resistor value.

Also another way - just replace relay with higher current value and you will not need a resistor

 

[CaseyK]

Since I am only in a facility for a few weeks to maybe a couple of years, I gave up on trying to train the hired help on leakage, and went to relay output cards for almost everything. Saved a lot of time.

As previously mentioned on similat threads, with 120vac, test for leakage with a "wiggy", with 24v, a fair sized lghtbulb or "larger" relay.

Small vom's and some dvm's just don't cut it.

I still contend, a hefty relay output is best. Another reason I like Entertron PLC's, the relay versions have nice hefty outputs. Actually, all the outputs are heavier then the majority.

regards all.....casey

 

[UncleBuck]

Forces

You prob checked that is not being Forced on.
I have forgotton to clear forces when debugging before, but it is likely not it.
AB has a green force light on the PLC if there are any forces left active.

 

[rta53]

Another related resistor sizing question

It matters if you want the resistor to survive in the application. The 10ma is a rule of thumb that comes from working around the triac type outputs or by reading the spec sheets that come with them from the manufacturer. As for the power rating or size, once you have settled on the 10ma or other current requirement, the output will supply 120V when on and that defines how larger the resistor must be. P = E x I = 120v x .01A = 1.2watts but trying to find a 1.2watt resistor may be very difficult. So, a 2 watt, 5 watt or even larger is fine. There is the QED of the subject.

I have another issue related to resistors. I have an instrument that has open-collector outputs for alarming and status indicators. I have to wire some of these outputs to 120VAC inputs of a PLC. I am using AB Solid State Relays in conjuction with the O-C outputs powered with 24VDC. The problem is that there is not enough load on the 120VAC input and the SSR will not drop the load when it is off. The AB SSR manual recommends using a 5k 3Watt resistor across the load. This does causes the input to turn off when the SSR is off. However the resistor gets extremely hot, over 200 degrees F, when the SSR is on. Is this normal? Should I use a higher wattage resistor?

 

[yam785]

I'm having a hard time realizing the depth of discussion on this topic. Maybe I'm looking at it to simply. In my experience it all boiled down to what I was using to test the output. I know on A-B output cards, if I use my fluke on an output which is off, I read a 120V. If I use my wiggy, which is my standard practice, I read 0V.
If I used a cheap Walmart meter, I would probably read about 70V. The point is, the lower the internal resistance of the testing device(wiggy), the more load, the less likely you will get fooled by leakage. The higher the internal resistance of the testing device(fluke,which is megohms), the more likely you will get fooled by leakage. But the higher the resistance of the meter, the more accurate it is. Just don't use them on output cards. As far as what resistance you need, use a wiggy, it's low enough.

 

[Contr_Conn]

The AB SSR manual recommends using a 5k 3Watt resistor across the load. This does causes the input to turn off when the SSR is off. However the resistor gets extremely hot, over 200 degrees F, when the SSR is on. Is this normal? Should I use a higher wattage resistor?

This sounds correct 120V/5K = 24mA, 24ma*120V= 2.88W
3W will give you a lot of heat!
Try to go down to 10mA with 12K 2W resistor
or maybe 7.5mA: 16K 1W resistor.
It will reduce heat.

Here is a way to calculate it:
- take a min current of your SS relay,
- add 25%,
- substract PLC input current.
- 120V divide by this current = resistor in OHMs
- 120V times by current = wattage.

 

[Gerry]

I have another issue related to resistors. I have an instrument that has open-collector outputs for alarming and status indicators. I have to wire some of these outputs to 120VAC inputs of a PLC. I am using AB Solid State Relays in conjuction with the O-C outputs powered with 24VDC. The problem is that there is not enough load on the 120VAC input and the SSR will not drop the load when it is off. The AB SSR manual recommends using a 5k 3Watt resistor across the load. This does causes the input to turn off when the SSR is off. However the resistor gets extremely hot, over 200 degrees F, when the SSR is on. Is this normal? Should I use a higher wattage resistor?

You brought up what I was about to say...

If you run a resistor at or near its power rating it will get very hot! The resistor may not burn up, but anything close to it may melt (like wire insulation). Better to use a resistor rated for 2-3 times what it has to dissipate.

There is a kind of Ohm's Law for heat dissipation - temperature is analogous to volts, dissipation watts is analogous to current, and thermal resistance...you get the picture. The thermal resistance has two parts - from the resistance element to the resistor case and from the case surface to surrounding air. Surface to air resistance decreases as surface area increases.

RE: minimum loads and leakage

All triacs leak. They never turn off completely. Triac output modules generally have RC snubber circuits in parallel with the triac to aid in switching inductive loads. These circuits also contribute to leakage current. In the 'off' state, you can regard the output as a current source and the voltage seen by the load will be leakage current x load impedance. With small relays, it may not be sufficient to pull them in, but, once in, will hold them.

A triac needs a minimum load current to stay 'on'. That is why they turn themselves off when the AC line crosses zero. It's also why they don't turn off for large inductive loads where current lags voltage significantly.