Triacs in modulating controls

[dave1234]

Im having trouble understanding how modulating proportinal controls that utilize a 24V AC signal to open and close work exactly. Case in point J/C VG 7281GS + 8020G zone valve has 3 control wires up, down and common- there is 24V present at all times to both the up and down to common. Ive been told this is do to the use of a triac which allows voltage but no current Im having trouble wrapping my head around that so if anyone could explain it to a simple mind like myself that would be much appreciated.

[JRINJAX]

A Triac [AC transistor switch] will show continuious output voltage as long as there is no load on the output. Your meter reads voltage since it does not load down the circuit. If you read the output with the actuator motor connected, the continuious voltage is not there.
Clear as mud?

[dave1234]

Im pretty sure I checked voltage at the atuator with it commanded all the way open and closed at one point with a known good actuator and found there to be 24v to com at both up and down. Thats what peaked my curiousity.

[qtip]

Im pretty sure I checked voltage at the actuator with it commanded all the way open and closed at one point with a known good actuator and found there to be 24v to com at both up and down. Thats what peaked my curiosity.

Power feeds to COM through both motor windings and the Triac shorts to ground to complete the circuit.

[rad1]

Put the coils of two relays on the outputs (Open & Close), then command the outputs and watch your relay energize. With no command you will still see the 24v with your meter, but no relay closure. I use a meter that "dampens ghost voltages" to read circuits controlled by Triacs. I use the Fluke 16 with Lo-Z.

[azzad]

I have a couple of relay's with tails on them in my back for testing triac outputs.

Dazza

[rad1]

I have a couple of relay's with tails on them in my back for testing triac outputs.

Dazza

That works too!

[Cagey57]

I have a couple of relay's with tails on them in my back for testing triac outputs.

Dazza

As said, Triacs "Leak voltage".

I have relays (RIBU1C/V-100) with pigtails on them just for such an occasion, same as azzad.

yllekicc (not as cool as "Dazza" but hey, I had to do something. Almost looks like a disease.)

[azzad]

Oops, I guess it goes with my 2 heads

[Cagey57]

Sorry azzad,
What I ment was...

yllekicc is (not as cool as "Dazza" but hey, I had to do something. "yllekicc" Almost looks like a disease.)

[azzad]

Its all good; I knew what u meant.

[kostasgreece]

If you use a multimeter you will actually see that when triac (output) is off the voltage is slightly different than when output is on (if I can recall right about 21.5 V in of where 24V in on).
But this is always relied on triac leackage as mentioned.

A relay or in my case a 24VAC lamp with clamps is better to be used.

[crab master]

FYI - there are voltage sourcing (power switched) and voltage sinking (common switched) triacs. Some triacs have you wire both sides of the load back to the triac points and some have you wire one side to the triac point and the other to the common side. Hook it up wrong and you'll burn out the triac. By know means do I feel like an expert on triacs, but working with both voltage sourcing and sinking I've burnt up more than one triac, due to voltage sourcing vs. sinking and not looking closely enough at the wiring diagram.

[orion242]

This topic has been beaten to death already.

http://hvac-talk.com/vbb/showthread....floating+point

[printer2]

This topic has been beaten to death already.

http://hvac-talk.com/vbb/showthread....floating+point

Wonderful. Could you not have posted earlier. Well since I went through the trouble I might as well post it. I may have to come back and edit after giving a day and rereading it but here is my take.


http://i406.photobucket.com/albums/p...tactuator3.jpg

The first thing you have to realize about a triac is that it can be thought of as a pair of relay contacts that has a high value resister across the contacts. In the diagram where it says drive module I have a dashed line around a normally open relay that has a resister across the contacts. If there was no load hooked up to the output and you put a DMM from the output to ground you would read a voltage, in our case 24V. Every triac has some leakage current and you can get fooled into thinking that they are turned on.

Now when we put a motor or relay across the output you end up with a voltage divider. Since the resistance across the triac is high compared to the coil or the motor resistance, most of the voltage will stay across the triac resistance and a minimal voltage will be across the coil or motor. So you will not see a voltage at the terminal.

Now when you turn the triac on you reduce its resistance down to a fraction of an ohm and all the voltage will be across the coil/motor and current will flow. You will see the full voltage on the output terminal.

(One mistake that can be made is to use a device that has a high input resistance on a triac. The device ends up always being switched on because the leakage current through the triac will allow a voltage to appear at the output terminals.

It acts the same way as when you have no load and you use your DMM across the terminals. With an actuator that needs a fair amount of current to drive it this is not an issue.)

Now to the floating point control

There are two ways of controlling a floating point actuator, sourcing and sinking. Each gives the same control but voltages get swapped around.

If you look at the diagram the top left diagram is a source circuit (Both circuits are being supplied by an internal AC power supply.) The power goes through the triac, through the coil (Used a relay coil here.), and returns to the power supply. The return leg of the power supply is grounded in this circuit.

The circuit to the right is a sink circuit. The power supply goes to the coil, then returns to the triac, and then goes to the grounded side of the power supply. Really the only difference between the two circuits is where the coil and the triac is in relationship to the ground. In the sink circuit you can see that if the power is always attached to the one side of the coil and if the other side accidentally gets grounded that it does not matter if the triac is turned on or not you will get current through the coil.

(In the source circuit the one side of the coil is grounded and if the other side accidentally gets grounded you will not turn on the coil. When the triac is turned on you will have the output short to ground but the coil will not turn on.)

Knowing how thing work in different hookups helps you with troubleshooting if you have problems. For now let us forget about that and look at our drive/actuator circuit.


Both switches are shown normally open and if you have a relatively low load impedance you should not see any voltage on the A and B terminal. Switching on one triac will turn the motor in one direction and switching the other on will turn it in the other direction.

If you are not familiar with the motor circuit when you have the A output switched on, current flows through the coil that is attached to the corresponding terminal and through the capacitor and the other coil. It then combines and goes back to the transformer.

In my circuit I labeled a few things that may look confusing at first but with a little explanation it may become clear.

With point E grounded (The ground being our reference point.) you have the transformer with 24 volts going into the motor and if you used your volt meter and measured on the A and B lines you would find you have 24V there. (Ignore the Sources designation and arrows as we are hooked up in Sink mode.)

With the triacs turned off you would not have any current flow, as soon as you turned one on (Lets do A) it acts as a switch and point A is effectively at the same potential as the grounded side of your transformer. On terminal B you should see a voltage less than 24V, what voltage you see here is dependent on the size of the coil and capacitor the manufacturer used.

Now if we disconnect the ground at point E and hook it up at point F the common terminal of the actuator will be at ground potential when the actuator is driven on or off. We are now using the Source configuration.

Terminal C would be your 24V terminal into your module. With the triacs off you would not have any voltage at terminal A and B. Say we turn on the terminal A triac we then would read 24V there and into the actuator. On the B line you might again see a voltage less than 24V for the same reason as in the sink method with a triac on.

If you got this far and it makes sense to you, well I got lucky.

There, now severely beaten.