240V detect for PLC input?

[tttonyyy 0]

Hi chaps, sorry if this has been answered before but I've had bit of a search without spotting it... I have an old FX0S which is used to control some pumps and heaters. The heaters are 3kw and switched by SSR. Ideally I'd like to verify that mains is not present when the SSR is signalled off (IE hasn't failed short circuit). Is there a simple way to detect 240V AC with the PLC's input? I could use a relay but there must be some kind of opto-isolator solution that would be more elegant? Regards, Tony

[kaare_t 155]

There's no 240V inputs on the FX-Series, so you'll have to go with a relay or opto-coupler with 240VAC on the switch.

[Inntele 60]

To realize it there are two way and two type of connection inside each. The first way is non-standard and, of course, can not be approved by the manufacturer, but should work with separate module of 24VDC inputs if the last you have in the PLC configuration. It's suitable if the abovementioned module has bi-directional (sink/sourse) type of inputs. Each input terminal connects to the load wire via 20 mA fuse and 36 to 48KOhm resistor with not less than 2W power (but better would be to use two 2W resistors which are connected in series). Between an input terminal and S/S must be connected a plastic capacitor of nominal about 0,33uF. The first type of connection allows you to monitor the voltage on load with respect to the neutral and in this case the S/S terminal connects to the neutral wire. It's suitable both for single phase and three phase connection. In the second type the S/S terminal connects to the phase. And it allows to monitor both state of SSR, and state of load by monitoring the voltage between a phase and load wires. So it's applicable for single phase connection only! Warning!! Do not use base module for this purpose!! Use capacitors instead of resistors, I do not recommend too! Just another way is to use AC version of ALPHA PLC for this task. This way is more legitimate and does not demand an external components. Then the ALPHA outputs may be connected to the FX-series inputs directly. Both way are unusual, so you have to understand that the choice, follow by my proposals or not, belongs to you. P.S. I forgot to say that also is applicable the third way, and it's the best on my vision. Is possible to make home-made optocoupler from neon bulb and optodiod (or optotransistor), enclosed into a black heat-shrink tube. How to connect this adapter is easy to guess. Do not forgot about capacitors on PLC inputs, however their values may be lower, because the glow of inert gas does not disappear instantly with the fall of the breakdown voltage. Edited 6 Nov 2011 by Inntele

[tttonyyy 0]

Thank you for the very comprehensive reply. I do like the homebrew opto-isolator idea a lot - I'd never have thought that one up. :)

[Crossbow 295]

I would opt for simple cheap off-the-shelf relays. No homebrew or custom duct tape and drywall screws approaches when I build panels. Simple low-amp DIN rail mount relays with 230VAC coil. Low cost and easy for the next guy to troubleshoot...

[larry818 0]

Yeah, relays are the best answer. I'm assuming you're doing temperature control with the plc? Adding some mechanical contactors upstream of the SSRs connected to the plc and an overtemperature controller would help in not setting stuff on fire.

[Inntele 60]

Crossbow, I can safely say that the neon bulb in comparision with an electromagnetic relay and even an usual optorelay is the best way to monitor the operation of circuits with a load, controlled by triac, especially if the load is a lamp or a heater, 'cause have an experience in this area. I can prove it, but this discussion will has not the slightest relation to the Mitsubishi controllers.