Can you run 0-20mA signals through dry contacts?
Can you run 0-20mA signals through dry contacts?
(OP)
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.
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.





RE: Can you run 0-20mA signals through dry contacts?
RE: Can you run 0-20mA signals through dry contacts?
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can you run 0-20mA signals through dry contacts?
Also lots of control systems think an open means bad things, so even a brief open could trigger alarms. Check that.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Can you run 0-20mA signals through dry contacts?
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
RE: Can you run 0-20mA signals through dry contacts?
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
http://splatco.com
RE: Can you run 0-20mA signals through dry contacts?
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
RE: Can you run 0-20mA signals through dry contacts?
RE: Can you run 0-20mA signals through dry contacts?
TTFN

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RE: Can you run 0-20mA signals through dry contacts?
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.
RE: Can you run 0-20mA signals through dry contacts?
The universe; running 50' of cable is an EMI risk. Your installation may simply be lucky.
TTFN

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RE: Can you run 0-20mA signals through dry contacts?
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can you run 0-20mA signals through dry contacts?
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 - http://www.flaminsystems.com
RE: Can you run 0-20mA signals through dry contacts?
RE: Can you run 0-20mA signals through dry contacts?
RE: Can you run 0-20mA signals through dry contacts?
RE: Can you run 0-20mA signals through dry contacts?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can you run 0-20mA signals through dry contacts?
RE: Can you run 0-20mA signals through dry contacts?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can you run 0-20mA signals through dry contacts?
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?
RE: Can you run 0-20mA signals through dry contacts?
I do not think that we should do all the electrical classes here. You simply have to believe me.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can you run 0-20mA signals through dry contacts?
We aren't paying the $5000-a-day commissioning guy yet. I was just trying to get installed this as inexpensive way as we can. In the end I suspect all this will come out and will be done via two direct 4-20mA signals that are switched with software. It's a flaw in the design. We want to make that their problem, not ours.
RE: Can you run 0-20mA signals through dry contacts?
Hmm... 5000 dollars per day. I think that I should work more in the US!
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can you run 0-20mA signals through dry contacts?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Can you run 0-20mA signals through dry contacts?
There must be a clean break between (2) independent hardwired control systems, (2) independent hardwired metering sytems and the (2) independent power systems with no chance of backfeed. And then toggle back to the other system when required. That's the project.
"You simply have to believe me." I appreciate the bluntness of this statement.
And $5000 a day is not an exaggeration. That's what it averages for the cost of one week per our Vendor. We have to make best use of their time. They will have to check out the whole system, this being a very small part, and to check the major parts of the system it must be ON and running. And these analog inputs, as with all the others must be there before it can come ON. Oh well I'm probably ranting to myself at this point....thx again.
RE: Can you run 0-20mA signals through dry contacts?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Can you run 0-20mA signals through dry contacts?
And below are the specifications of the analog current sources. There are two of these feeding into the input.
4-20 mA Signal Specifications:
Full-Scale Measuring Range ±5kA to ±100kA
Full-Scale Protection Range ±5kA to ±200kA
Signal Output Calibration Accuracy ±0.75% full-scale
Signal Output Type (configurable) ±20mA, ±1 Volt, or ±10V Full-Scale
Signal Output Maximum Burden 10 volts
Maximum Load Resistance (Current Loop Output) 500Ω44
Minimum Load Resistance (Voltage Output) 100kΩ
Linearity Error ±0.1% of full-scale
Repeatability Error Limits ±0.1% of full-scale
Temperature Sensitivity (Note 1.) ±0.02%/°C
Response Time (td) ≤ 50 μs
di/dt Accurately Followed 500 A/μs
Frequency Response (refer to Figure 3-1)
RE: Can you run 0-20mA signals through dry contacts?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can you run 0-20mA signals through dry contacts?
5mA isn't 4mA. So...
Gold contact ones looks to meet the spec, and since it's got the same base we can go ahead and wire it. I looked online and some of these gold contact relays are like $400 each. Yikes. It would be nice if the spares would do it. I gotta make sure this will work before we buy these things. And I'm not telling anyone there is gold in them!
Any other suggestions or if there is one you typically use in a similar situation would be appreciated.
RE: Can you run 0-20mA signals through dry contacts?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can you run 0-20mA signals through dry contacts?
http://www.gke.org/rapporter/files/SpecialPaper%20...
All names and company identities have been changed to fictious names. The picture with the different contact types is good reading.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can you run 0-20mA signals through dry contacts?
Both sides will be dead. These are hand cranked switches requiring planned outages to operate.
RE: Can you run 0-20mA signals through dry contacts?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can you run 0-20mA signals through dry contacts?
They look like square bases to me.
RE: Can you run 0-20mA signals through dry contacts?
These meters don't control anything. The unit we purchased comes with it's own meter that does that function. They provide the required inputs to their master system and a reliable comparison to the meters onboard the unit.
I think what I'm going to do is wire these to our spares on hand, see if it works, explain the situation to the Vendor and project people and see if they want to invest the $$ for the gold contact ones. It's hard to believe the spares will conduct for 5mA and not 4mA?
RE: Can you run 0-20mA signals through dry contacts?
"Much ado about nothing" (William Shakespeare, English poet and playwright, 16th century)
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.