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Can you run 0-20mA signals through dry contacts? 2

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bdn2004

bdn2004

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Jan 27, 2007
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We are running 0-20mA signal from two different places into one PLC analog input. The signal source will be determined by the position of two mechanically operated switches equipped with contacts that change state with the position of the switch. Only one switch will be open or closed at any given time. The easiest way to do it seems to be to go through a set of contacts with the 0-20mA signal on each switch.

Is there a reason you wouldn't do this? Could the contacts degrade the 4-20mA signal? There may be an easier way but I'm trying to work with what we have not buy new stuff or have the PLC re-programmed. Thanks.
 
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Yes, I once had a small PLC where I maxed out on analog inputs. I needed to add 8 new well level inputs so I multiplexed the signals with a discrete relay output module and fed them into one analog, it worked great and would read all 8 in less than a second although I had it much slower than that.
If you are switching your analogs you need an arrangement to maintain the current through the unused analog otherwise it takes a second or two to power up and read the normal value. In the case I mentioned the inputs were 0.2 - 1 Volt so I just used a couple of diodes in series with each input.
If your input is 1-5 Volts you could short the inputs out with the relay contact and open the relay of interest, with a steering diode to send the signal to the analog input.
Hope this helps
Roy
 
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The contacts in a mechanical relay or switch consist of two pieces of metal that touch each other so that electricity can flow between them. However, under certain conditions metal contacts can become coated in insulating sulphides and oxides due contamination from the air. This can prevent the contacts from conducting electricity when they are meant to.

When the contacts are controlling high power loads like light bulbs or motors this is not noticeable, because there is enough voltage and current in the circuit to blast away the thin insulating coating. In low power electronic circuits, however, say a digital limit switch input to an electronic controller or PLC, there may not be enough voltage and current to clean the contacts. The circuit will then fail to detect a contact closure, This has been cause of many mystery system failures. The consequences can be very serious indeed.

The solution is to ensure that the contacts' minimum voltage and current ratings match the voltage and current presented by the controller input. This means you don't select a nice rugged tungsten plated 20A switch but one designed specifically for low power operation, say bifurcated gold plate over silver.

David Stonier-Gibson
 
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Hello bdn2004,

From your thread, what I got, you need to use one Analog Input for multiple reading.

Instead of relays,why you can't use Analog Multiplexer Integrated circuit, like ADG506A/507A... from Analog Devices..?

Only thing is need to play little with electronics. No worries on contact,tear/wear..etc

Regards

Shaji Narayanan
 
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I don't think I was clear in my original post...What I was wanting to do is wire the analog switching through dry contacts on limit switches in the field...not through a contactor or an interposing relay. It amounts to running about 50' of analog cabling. The replies here seem to indicate that it should go through an interposing relay with gold contacts. Is what I'm wanting to do bad design? It's the same signal and no power required..
 
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It's going to be a noise problem, not a switch problem

TTFN
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7ofakss

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What is generating the noise? Are the limit switches...they just too clanky? Do they bounce or something? They don't seat well enough for this sensitive circuit?
There is 4-20mA analog cabling all over the place here, and it's described as a very hearty signal with or without the shield and on very long runs, close proximity to higher the whole bit and there is not complaints. Not argumentative, just want to know.
 
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"What is generating the noise? "

The universe; running 50' of cable is an EMI risk. Your installation may simply be lucky.

TTFN
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7ofakss

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ShajiNarayanan (Petroleum)
I think that you should check the specs for those ADG506A/507A gates.
RON is 280 ohms typical and 600 ohms maximum. That, plus a typical receiver having 500 ohms, you are very close to, or above what a 4-20 mA transmitter can handle. There is also a restricted voltage handling capacity (typically 10 V) so a 24 V source will be way outside what the switches can handle.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
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bdn2004; Long term project huh?
2pt4b3o.gif


Anywho, I can't begin to imagine why someone would want to run analog signals thru "limit switches" but if this really is what you want to do you still have to adhere to the gold contacts. You'd need to make sure whatever switches you use include that feature.

Keith Cress
kcress -
 
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Doing the detailed design on a fairly large but seemingly straight forward project by yourself is not that easy i,ve discovered. Deficiencies in the original design consume your time. I'm going to change this over the long holiday based on the comments in this forum. thx all. gold contacts it is.
 
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They don't look right. Definitely load switches. Instead, I would use reed elements atuated by magnets on that red thing. And I would use two in parallel for best reliability.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
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The DC spec is an entirely different thing. It is about arcing and inductive loads. Has no bearing on dry switching.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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Skogsgurra,

0.27A at 250VDC is the maximum load that could be put to this switch. That's not right?

If you notice, there are eight (8) of these limits that change state as that cam turns. They are going to different circuits - including a 48VDC computer and a 125VDC hard-wired shutdown circuit. The hardwired circuit will hold closed a bunch of interposing relays when the switch is on. This is an inductive load so it can't add up to more than 0.27A at 250VDC or .54A at 125VDC? correct?
 
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