Dry Contact Basics

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MD88 said:
Is there such thing as a relay where the coil is also a power supply? For example, the coil itself provides the 24VDC used to monitor a dry contact, and when that contact closes, the current in the circuit activates the coil on the relay then closes the dry contact that belongs to the relay? In the end, it would just save getting an external power supply...
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Industrially, we have 2 wire, 3 wire, and 4 wire sensing devices (proximity and photoelectric for example) that use "some" line power. The 2 and 3 wire could both use the "coil" as the power supply. With the 3 wire, the output might either be "sourced" or "sinked", connected to hot or ground. The 4 wire USUALLY are "dry contact".

As others have said, when a device requires a dry contact, it is supplying whatever power it needs. The advantage is that the "relay" can have an AC or DC coil of virtually any voltage. Isolation is assumed.

Whet is not mentioned is that there is a difference in contacts being dry (unpowered) and "dry circuit rated". Often a very low current must be switched; in my electro-hydraulic activities, this is often an analog signal, as into an inverter or servo amplifier. Contacts rated 15 amps usually work for a short period before the resistance becomes an issue. From the old telephone days, "crossbar" contacts, usually of noble metals (palladium or gold commonly) are used ... 1 microamp "wetting" current is not uncommon. Look at, for example, needing to reliably source 10 mV (3 rpm on a 10V=3000 rpm) into a 10k input impedance ... 0.1 microamp ...
 
KentAT said:
MD88 -

What exactly are you trying to do?

Your initial explanations are based on the devices you think might be necessary. There are all sorts of power supplies, relays, and other equipment out there.

Please draw a picture or provide a more detailed explanation of what you have that needs monitoring and what you have that wants the information that the monitored device has changed (devices, voltages, sizes, etc...)

Kent
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I have a 120VAC PLC that needs to monitor a dry contact on a heat tracing control panel that is rated for 24VDC. I guess that's the simplest explanation! So the PLC will be trying to send 120VAC through the contact and will burn it out! What would I need in the middle?
 
MD88 said:
I have a 120VAC PLC that needs to monitor a dry contact on a heat tracing control panel that is rated for 24VDC. I guess that's the simplest explanation! So the PLC will be trying to send 120VAC through the contact and will burn it out! What would I need in the middle?
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Sounds like you can just run your 120volt through the dry contacts (make sure the contacts are rated for the voltage and current) and the 24 volts DC will energize the coil of the relay of which your 120 volts will be connected to the NO contacts of that same relay. There is no electrical connection between the coil and the contacts of a relay so the different voltages won't interfere with each other.
 
But the dry contacts are only rated for 1A at 24VDC, so running the 120VAC from the PLC through it wouldn't work, I imagine (the coil on the relay with the 24VDC rated contacts is energized by the heat trace panel itself and we don't have to worry about that). Just to clarify again... the dry contacts I have to monitor are rated for 24VDC, and the monitoring signal from the PLC is 120VAC. I know I need a relay between them with a 24VDC coil and contacts rated for 120VAC, but this would also require a 24V power supply, which I would like to avoid if possible. I was wondering if there was a relay/power supply combined that would work for this application.
 
Dry Contact Basics

MD88 said:
Hello,

" I'm having a bit of a hard time getting a truly straight answer on what a dry contact is... I'm under the impression it's switch that closes or opens by some external control source but doesn't have a voltage source itself - it basically just connects two pieces of wire together. "

you are correct

and if I may add a bit of hard-learned wisdom from the Broadcast School of SpitzenSparken:

If you choose to use connectors between the dry-contact-relay and whatever it is controlling, arrange them so that the 'PLUG' end is to the relay and the 'SOCKET' end is to the 'controlled' device.

The reason for this is so that when the connection is broken, there is no exposed voltage on the plug contacts

Good luck with your project

Gary
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090223-2024 EST

MD88:

You could have received more useful information quicker if you had stated the details sooner.

A PLC is likely to use some sort of solid state device such as an optical coupler for its logic input interface. These typically require little current. 20 MA maybe. I use a 5000 ohm 5 W resistor for 120 V inputs. This is a peak current of 170/5000 = 34 MA.

If your "dry contact" has adequate insulation and contact gap, then it can probably work at the 120 V level.

If this is a pure mechanical contact, then there is no reason the contact can not switch AC and a resistive load. Even 24 V AC and an inductive load within the contact rating is probably less damaging to the contacts than a DC inductive load.

Suppose you do not want to directly switch the PLC input after further study, then you only need a simple low power circuit to control the PLC input. A resistive voltage divider across the 120 at the PLC can provide a low voltage that you switch with your dry contact to control the input of an OPTO22 OAC5. There needs to be rectification of the voltage divided supply. At the PLC input you will need a 2000 to 5000 ohm pull down resistor of adequate wattage. The OPTO22 relay switches between hot 120 and the PLC input. The pull down resistor is to keep the PLC input low even with leakage thru the OPTO22.

Other gimmicks can do the same thing. Use a small transformer in place of the resistive divider. If the PLC input threshold is maybe 10 to 15 V, then you could use the dry contact to switch 24 V AC, maybe even DC to the PLC input.

You really need to know more about the reason for the dry contact rating.

.
 
MD88 said:
I have a 120VAC PLC that needs to monitor a dry contact on a heat tracing control panel that is rated for 24VDC. I guess that's the simplest explanation! So the PLC will be trying to send 120VAC through the contact and will burn it out! What would I need in the middle?
Click to expand...
so what is the heat trace cable putting out? I thought you already had 24 VDC to your relay coil and you were trying to switch the 120VAC signal through the NO contacts.
 
MD88 said:
I know I need a relay between them with a 24VDC coil and contacts rated for 120VAC, but this would also require a 24V power supply, which I would like to avoid if possible. I was wondering if there was a relay/power supply combined that would work for this application.
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There are self-contained power supply/relay packages; they've been posted in the forum before. Maybe someone will read this a post them again.

Note that the contact's 24v/1a ratings are maximums. Nobody says you have to use a 24v supply and relay. Even Radio Shack has low-cost relays.

As mentioned above, AC is easier on contacts than DC is. You could buy a cheap 12v transformer and a cheap 12vac relay that will switch the 120vac.
 
Hmm... lots of information!! Just in case anyone is unclear, I've drawn a very crude little picture.
 
MD88 said:
Hmm... lots of information!! Just in case anyone is unclear, I've drawn a very crude little picture.
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Ok. You can get a 24 VDC power supply if that it what you are asking. I personally haven't seen them built into a single unit with a relay but I've used 24VDC power supplies before. The kind we used, you just supplied it with 120VAC and it put out 24VDC. Get one of these and a seperate relay for your PLC 120 VAC.

Seems like you already know what you need at this point.
 
MD88 said:
Hmm... lots of information!! Just in case anyone is unclear, I've drawn a very crude little picture.
Click to expand...
That's the right circuit, simply enough. Again, nobody says your power and relay have to be either 24v or DC, as long as they match and don't exceed the contact ratings. You'd have to use a good-sized contactor to exceed the 1a amperage limit.
 
090224-2023 EST

MD88:

Your circuit is not good as drawn, and the contacts on left hand relay, the 1 A 24 V DC are likely to weld at some time. You are too hung up on specifications without additional information.

Add a reversed biased diode in parallel with the 24 VDC relay coil to reduce arcing at the controlling contacts.

Probably better to use an AC coil relay instead of the DC and then you also do not need a DC supply.

Now I have a different problem. You have used the word "contact" with respect to the output of your Heat Trace unit. Is this really a mechanical contact? Or is it some solid state output device?

Assuming it is in fact a mechanical contact, then what kind is it? By chance is it a reed contact? For reference on reed switch capabilities see:
http://www.hamlin.com/technical-detail-reed-switch.cfm

Unless there is very little contact spacing, or very poor insulation I see no reason for a normal electromechanical relay, including Hamlin reed switches, having a rating that prevents use in switching a low current resistive load at 120 VAC 60 Hz.

We need more information on what is the actual output device on your Heat Trace unit.

.
 
090224-2056 EST

iaov:

Until there is some positive evidence of what this output device is I am asking questions because the rating does not make a lot of sense.

.
 
Look at his diagram again. The red components are part of the equipment, including the relay on the left. His propose additions are in black.
 
I don't see why you need a power supply....just parallel the coil of a new relay that has 120 volt rated contacts with the coil of the existing relay that has 24 volt rated contacts, of if the existing relay doesn't do anything else, just replace it.
 
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