Is there a dry contact that, when closed, can trigger 24vac?

SweatHog

My indirect water heater is controlled by a temp sensor with dry contact switch leads. Looking at the TACO ZVC-406 instruction sheet, (https://www.tacocomfort.com/documents/FileLibrary/102-397.pdf) there seems to be no way to connect the sensor to the TACOs thermostat priority zone without shorting out the sensor with 24VAC power coming from the TACO board when the DHW sensor dry contact circuit is closed?

Steamhead

What sensor are you using?

Jamie Hall

Say again? A dry contact, almost by definition, is just a switch and it doesn't matter what you hook up to it, so long as you stay within the voltage and current limits.

SweatHog

@Jamie Hall. That's the point Jamie, 24vac exceeds and is different from the volt limit of the switch, which is 5vdc The other issue is arcing across
the internal relay contacts of the sensor switch each time the contacts open. The manufacturer recommends a snubber circuit for AC contact protection but I've tried those and they don't seem to work. Basically I need a device that works the opposite of an RIB2401B: i.e. provide 24VAC power when the dry contacts are closed.

pecmsg

Dry contacts that are rated at 5VDC?

ratio

How 'bout some details on the indirect? 5 volts is about what I'd expect as the source voltage for a thermistor. I can't imagine there's an indirect that can't interact with 24 VAC. That's been control voltage for like basically ever.

HVACNUT

What "temp sensor" is being used? Can you post a pic of the specs?

Jamie Hall

If your rating is correct, you have a real problem. That would appear to be a device intended solely for solid state integrated logic circuits, which do operate on 5 volts DC source. Usually a closed switch of 5 VDC on the output is regarded as a logical true, but the same switch can be used on the ground side to pull down the source to zero, going through a resistor, to represent a logical false.

You might be able to find a relay which will work on your available voltage and current, and use its contacts in turn to switch the 24 VAC, but you may also need to use a transistor or SCR to get enough current to operate a relay. You might take a look at what Digikey as to offer...

EdTheHeaterMan

SweatHog said:

@Jamie Hall. That's the point Jamie, 24vac exceeds and is different from the volt limit of the switch, which is 5vdc The other issue is arcing across
the internal relay contacts of the sensor switch each time the contacts open. The manufacturer recommends a snubber circuit for AC contact protection but I've tried those and they don't seem to work. Basically I need a device that works the opposite of an RIB2401B: i.e. provide 24VAC power when the dry contacts are closed.

You figure out how to have something make any amount of electric energy from just connecting the contacts together, with no apparent source of any form of energy, and you will have saved the world from complete destruction. An amendment to the Law of the Conservation of Energy that states "energy can neither be created or destroyed..." will be approved when @SweatHog releases the new Reverse RIB Power provider.

The amended law is going to say
The law of conservation of energy, also known as the first law of thermodynamics, states that energy cannot be created or destroyed, but can only be transformed or transferred from one form to another. Except when you press these contacts together on the @SweatHog reverse RIB. (aka: the BIR HogSweat) you will make 24 volts happen


Am I being too literal? Or does this guy think that he can just make up the laws of physics as he goes along?

pecmsg

Jamie Hall said:

If your rating is correct, you have a real problem. That would appear to be a device intended solely for solid state integrated logic circuits, which do operate on 5 volts DC source. Usually a closed switch of 5 VDC on the output is regarded as a logical true, but the same switch can be used on the ground side to pull down the source to zero, going through a resistor, to represent a logical false. You might be able to find a relay which will work on your available voltage and current, and use its contacts in turn to switch the 24 VAC, but you may also need to use a transistor or SCR to get enough current to operate a relay. You might take a look at what Digikey as to offer...

AKA
Triac
?

HomerJSmith

Today is my dumb day. What are you trying to do? What does the 5 Vdc sensor do? What ...controlled by a temp sensor with dry contact switch leadsmean? What kind of indirect?

109A_5

Hello @SweatHog,
I get the feeling you are trying to control two isolated circuits, that should remain isolated.

The RIB2401B is a SPDT (Single Pole Double Throw) relay. A 24 / 120 VAC control coil controls two sets of contacts, one NC (Normally Closed) and one NO (Normally Open) contacts to 1 common lead.

The RIB2402B2G is DPDT (Double Pole Double Throw) relay. A 24 /120 VAC control coil controls four sets of contacts, two isolated contact sets with NC and NO to 1 common lead each.

So a switched 24 VAC control power can switch (or control) another 24 VAC circuit and the other set of isolated contacts can switch a 5 VDC circuit.

Or do you need a relay with a 5VDC coil ? I think your application has us all confused as to what you are actually trying to accomplish.





BTW the contact ratings are Maximum that should not be exceeded, less is OK.

The contacts on the RIB2402B2G are rated at 20 A @ 28 Vdc. The RIB2401B does not seem to have a DC Voltage rating. If the situation of your DC load is reasonable RIB2401B probably would work fine if it is the correct relay for the application.

Jamie Hall

I'm still wondering what, exactly, it is you are trying to do. Digikey does have reed relays which operate on 5 volts, and will switch 24 VAC, but not a whole lot of current. You could use one, though, to control an RIB, as the folks above have said.

However, let me be curious: 5 volt DC power is just vanishingly rare in heating and ventilating operations, external to the control circuit boards. What is your power source for these dry contacts you speak of? What do you want to control with them?

109A_5

Hello @SweatHog,
Are you replacing the Aquastat with some other device ? Where does the 5 VDC issue come in ?
The Red annotation is just a thermal switch, closes below a set temperature, opens above a certain temperature. When the switch is open I would expect 24 VAC between R and W.

SweatHog

Sorry for ghosting this post. I'm using these:

https://www.controlbyweb.com/x300

hot_rod

We had some issues come up with the end switch on zone valves when connected directly to the digital controls.
A switch manufacturer suggested we need to consider a reed switch with a better low end capacity. We use a reed switch now in our zone valves. You give up some high end current capacity and need to run thru a relay for loads over 1 A, is the trade-off.

The ratings on some of theses switches show "light load"less than .25A and 6V.

109A_5

Hello @SweatHog,
So if your X-300 is doing the temperature monitoring and telling the TACO ZVC when the tank needs heat. I would just set it up like this. Where does the 5 VDC issue come in ?

SweatHog

@109A_5. That's what I initially tried. However when the x-300 closes the circuit. The TACO sends 24VAC through it. It will work for a while but the voltage surge eventually kills the x-300. They have a diagram in the manual that discusses the issue but the snubber surge protector circuit they recommend doesn't work for me. Using the x-300 on the TACO main isolated end switch works fine since there's no voltage running through it and that's how I control space heating. If I had another isolated switch that would solve the problem. But I guess there is nothing that fits that bill.

Jamie Hall

OK. Is there power on 1NO and NC? The two terminals which the diagram shows connected to R and W on zone 6 on the Taco controller? If so, how much and what kind? If you have 5 volts available there when 1NO is closed and calling -- presumably triggered by the aquastat -- then use a reed relay powered by those two contacts to close the relays dry contacts and close the the circuit to R and W on the Taco.

However, if those terminals on the widget are unpowered, but close when called, the solution is a little more complex. Find a 5 volt power supply which is independent of everything. Wire it and the coil of the reed relay through those two contacts. Wue the dry contacts of the reed relay to R and W on the Taco as above. One output of a USB charger is 5 volts and ground, and with a little work would do just fine.

pecmsg

So

what does the X-300 do that any number of t-stats can’t?

HVACNUT

SweatHog said:

@109A_5. That's what I initially tried. However when the x-300 closes the circuit. The TACO sends 24VAC through it. It will work for a while but the voltage surge eventually kills the x-300. They have a diagram in the manual that discusses the issue but the snubber surge protector circuit they recommend doesn't work for me. Using the x-300 on the TACO main isolated end switch works fine since there's no voltage running through it and that's how I control space heating. If I had another isolated switch that would solve the problem. But I guess there is nothing that fits that bill.

You're on R-W on the ZVC? Not C, right?

109A_5

Hello @SweatHog,

OK, since the X-300 relay contacts seem incredibly wimpy, just buffer it with a more robust relay, like shown in the manual. Here is an example, but a bit pricey, there are many others.
https://www.digikey.com/en/products/detail/sensata-crydom/D1210-10/221770
https://www.digikey.com/en/products/detail/celduc/SO763090/14310770

Inside the Solid State Relay (SSR) they basically just light up an LED on the control side.
DC input controls AC. The nice thing about an SSR is there is no coil, so no inductive load to cause an arc at the relay contacts.
The other thing you may want to understand is the electrical characteristics may not be killing the relay, it may the amount of cycles of operation. In which case buffering it with another relay won't help.

109A_5

Hello @SweatHog,
Not quite buying this,

SweatHog said:

Using the x-300 on the TACO main isolated end switch works fine since there's no voltage running through it and that's how I control space heating.

No voltage, no control, there must be a Voltage of some sort. Maybe the load current is a lot different, the inductance of the switched load is different or the cycles of operation is a lot less. Or by happenstance and the order of operation the more robust end switch takes the abuse.

SweatHog

pecmsg. it can do both time and temperature. So I can shut down the DHW at night or plan ahead for a vacation schedule and change both those parameters and temperature, remotely.
@HVACNUT. yes
@109A_5. I'm going to try your external relay idea.

109A_5

Hello @SweatHog,
Your Delta T isn't too narrow cycling that poor relay to death ?

Joe Mattiello

 Hi, I’m going to try to figure this out..

I believe you’re saying you have a sensor on DHW tank,  when you may have an adjustable temperature control with either mechanical dry contacts or relay dry contacts.

If you truly have just a sensor (thermistor or thermal couple) than it would require it be connected to some type of temperature controller to adjust the temperature and give dry contact output.

Taco zone controls work with dry contact closure between R and W; not a 24-volt input.

If a voltage is measured between R & W, then it is just floating voltage and is an erroneous measurement; 24 volts power is between R & C. Remember to create a call for heat, you short R to W…