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Old 04-01-2011, 07:09 AM   #1
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Default Help Wiring a valve

I am wiring an electric brewery similar to Kal's but with some changes. I am including a valve control circuit that runs a 2-10 volt proportional valve with a 4-20 mA current control PID. I have a 500 ohm resistor across the the 4-20 mA output. Here is the wiring as is shown on the Belimo valve:



My question is how to do this without frying the circuit. My 24 volt transformer does not indicate a hot or neutral wire (and I am not sure such a thing normaly exists after a transformer). If I screw it up and connect the (-) side of the 4-20 mA output to the wrong wire, am I going to fry the circuit? It always seems strange to me to connect a DC wire into an AC wire, but I guess it is on the theory that a neutral is a neutral.

Do I just pick a wire and declare it neutral and the other one hot, and as long as it is consistent not worry about it?

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Old 04-01-2011, 06:02 PM   #2
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Do I just pick a wire and declare it neutral and the other one hot, and as long as it is consistent not worry about it?
Exactly.
The transformer acts like an insulator (I'm not sure this is the correct English term). Take a multimeter, and make sure the secondary windings (24V) on the transformer are NOT connected in any way to the primary windings (120V). This will be the case with most transformers, but in any case, it's good to be sure, as some cheap ones may use a different architecture (Called "autotransformers") in which one winding is part of the other one.
Once you make sure you don't have an autotransformer, you just connect any one of the secondary leads as a neutral. Just make sure, if you use your transformer to power more than one valve, that you use the same neutral lead for all of them.
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Old 04-01-2011, 06:52 PM   #3
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Cool, thanks a lot! I had been way overthinking that.

Klaus

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Old 04-01-2011, 09:06 PM   #4
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Glad I could help.

It's not your fault to overthink it. People are used to AC having a "hot" and a "neutral" wire. In reality, AC doesn't, per se, have them. The neutral is "created" by grounding one of the wires, or, like in street transformers in the US, by using the center connection on a 2x120V secondary, and, of course, the hot wire is created because of that.

Just make sure your secondary windings are not connected to the primary windings, or you're bound to do some damage.

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Old 04-02-2011, 08:54 PM   #5
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ONe question-

I want to meter across the valve so I know what position it is in and to have a better idea what it is doing when it is being automatically controlled. I purchased a DC voltmeter to install on my panel since the 0-10 volt output is DC, but since the "neutral wire" coming from the transformer is actually an AC wire, should I instead have purchased an AC voltmeter? Here's what I mean and how it is wired at the moment (sorry for the amateurish drawing):



The PID is powered by 24v AC. If the transformer provides +12 volts to one wire and -12 volts to the other wire to get 24 volts and then alternates these, there is truly no "neutral wire" in the system, and as such, the voltage at the negative portion of the voltmeter will go from +12 volts to -12 volts. The big issue with this (besides metering) is that the max voltage from the +side of the PID will be 10 volts (0-20mA control over a 500 ohm resistor), so current would flow BACKWARDS when the "neutral" wire reads +12 volts. Not sure if the PID can handle that. If, of course, the transformer makes a hot lead that goes from +24 volts to -24 volts and a true neutral then it is easy. How can I figure out which way the transformer actually works?

Sorry to be such a nob. Not used to doing this kind of thing. I also haven't yet checked the transformer to make sure it is isolated from 120v, but will do that over the next couple days (whenever I can dig through the clutter and track down my multimeter).

I should point out that the transformer in question is a Heath Zenith door chime transformer seen here http://www.amazon.com/gp/product/B000BQY88I. I connected the neutral wire to the leftmost attachment and the hot wire to the furthest right attachment.

Klaus

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Old 04-02-2011, 09:20 PM   #6
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Ok, I'm not sure I understand what you mean, but, if I do, your wiring diagram is wrong. Let me see if I can come up with a way of showing you what I mean. Do NOT connect your equipment like that.

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Old 04-02-2011, 09:39 PM   #7
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Too late! But there has never been any power applied so I can just happily disconnect everything!

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Old 04-02-2011, 11:01 PM   #8
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Wow, you got me scared for a second...

Ok, here's what I understand:

1. You need 24V AC for the PID.
2. You need 12V AC for the valve.
3. You have a 0-10V DC signal between the PID and the valve. I'm not clear whether it's a control signal produced by the PID, or a feedback signal produced by the valve. Either way, it makes no difference.

I modified your diagram, to show you the right way to wire everything. Sorry for the bad quality. Hope you can understand it.



I numbered the connections to make it (hopefully) less confusing.
Notice you will need a 120V/2x12V transformer. For that, you have a couple of options:



Now, I'm gonna start explaining the second option.
If for some reason you can't get a 2x12V transformer, you can just replace it with 2 transformers, 12V each, connect both primary windings in parallel (as shown), and then connect one lead from each secondary winding together. Now, power the transformers up, and with a multimeter, measure the AC voltage between the free leads. If you measure 24V AC (approximately), you're ok. If you measure about 0V, disconnect them from the line, and invert one of the secondaries (disconnect the secondaries, and connect the free lead from one of them, to the lead you had connected before on the other one. Power up and measure again.
Once you have the 24V AC on the free leads, the 2 leads you connected together will give you your 12V AC. One of the free leads will be your neutral (either one, doesn't matter) and the other one will be your 24V AC.

Option 1 is easier: just power up the transformer, and measure AC V between any pair of secondary leads . You will have 24V AC between 2 of them, and 12V AC between one of the leads, and any of the other 2. That lead that gives you the 12V is the important one: mark it. That will be your 12V AC for the valve. One of the other 2 leads will be the neutral, and the 3rd one will be the 24V AC.

Finally, make sure the transformer(s) have a current capacity at least 1.42 times the added consumption of both devices. For example, if both the PID and the valve need 1A each, you need a 3A transformer (or 2, if you use the second option).

Last, you need a DC volt meter.

Hope it helps.

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Old 04-03-2011, 12:20 AM   #9
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Wow, thanks for taking the time to do this!

The only thing on the assumptions is, I must have somehow mislead you, but the valve works on a 24 volt signal, and the PID itself outputs the signal (actually 4-20 or 0-20 mA, which across a 500 ohm resistor should make a 0-10 or 2-10 voltage). That being said, since the valve doesn't need 12 volts but rather 24, could I just split wire 1 and run it to both the PID and the valve, thus eliminating the need for lead 2 from the transformer?

I guess my only real hangup is the "neutral bus." With a standard transformer, if I ran a volt meter between the neutral bus and ground, would it read zero volts, or would it read some AC voltage? I guess as an example what I mean is this. THeoretically, I can stick a white wire from the service panel into my mouth and bury myself in the ground and I shouldn't get shocked because this is a neutral wire. From a transformer, however, I think doing such a thing would be a pretty bad proposition as neither of the two leads coming off the transformer are grounded-- in other words, aren't both wires "hot?"

And if both wires are actually hot and there is no such thing as a true neutral, then the red wire going to the neutral bus will be seeing a voltage from the neutral bus. Then the volt meter will be measuring the potential between the positive DC signal from terminal 2 on the PID and the alternating voltage on terminal 3 of the PID (induced because it is connected to and alternating current "neutral").

Would it matter if I threw a 1 megaohm resistor on the red wire between PID terminal 3 and the "neutral bus?" That way there would be a voltage drop across the line and you wouldn't measure the voltage from the "neutral bus" (which is actually hot). My understanding from the valve actuator literature is that there is a 1 megaohm resistor in the valve between the 0-10v signal and the common lead.

Also, very cool... I had no idea you could link transformers like that to get an intermediate voltage. Pretty cool.

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Old 04-03-2011, 02:09 AM   #10
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Ok, first, forget everything I said on my last post. I don't know why the hell I decided that the valve needed 12V. Your diagram is perfect. You need a DC volt meter.

Now, going to your question. Do this experiment:

Take the transformer, and power it up without connecting anything to the secondary.
Take a multimeter, and measure the voltage between any of the secondary leads and ground.
Now do the same between any of the secondary leads and neutral on the line voltage.
Now do the same, again between any of the secondary leads and the hot wire in the line voltage (be careful!)

You will see that, in all cases, the AC voltage will be "0", or very close to it (there can be some electrical noise in the wires).

That's because the secondary winding of the transformer is electrically isolated from the primary winding. The only voltage you will find on the secondary is between the 2 leads. At this point, there is no hot wire in the secondary.
When you choose one of the leads (either one, doesn't matter) and physically connect it to the primary neutral wire, you're creating the hot wire, because the neutral wire is physically connected to ground, so, that other secondary lead that before had a 24V AC voltage towards the other lead only, now has the same voltage towards neutral (and ground), because your first lead IS ground.

In this case though (the case of your wiring diagram), you're not really connecting the secondary neutral to the primary neutral. In a case like this, the secondary "neutral" is not really a neutral, but it is (as it is correctly named in the valve drawing) called a "common" wire, as it is the shared wire between 2 or more signals (in this case, the 24V AC and the 2-10V DC).
That means that, theoretically, you won't find any voltage between any of the secondary leads and ground, unless there's an internal connection between the GND and neutral terminals in the PID (normally there isn't one).

So, to round it up a little:

* In a pure AC signal, there's no hot wire at all.
* You fabricate the hot wire, when you ground the wire that will be the neutral.
* If none of the wires are grounded (I think the right word is "bonded", but I'm not sure), there is no voltage between either one and ground. Again, that's theory. There's always some residual voltage, due to air conductivity, Foucault currents in the transformer, etc.
* The fact that you're attaching a DC signal to a wire carrying an AC signal doesn't affect anything, as long as the other wires don't touch. If they do touch, you will remember me.
* The neutral bus in your wiring diagram is not really a neutral bus, but a "common bus", as it's not connected to the grid's neutral, or ground.
* You don't need any resistor between terminal 3 and "common. Unless the instruction manual for the PID calls for it, don't use it.
* If the valve has an input impedance of 1 MΩ (for what you said) and you connect a 1 MΩ resistor between terminal 3 of the PID and the valve input, you're effectively cutting the voltage at the valve input in half. Don't do it.
* If, instead of connecting the resistor between 3 and the valve, you connect it between 3 and the common bus, you will be effectively doubling the voltage on the valve. Again, don't do it.
In both cases, you will have a false reading on your volt meter.

I think that about sums it all up. Let me know if there's something else, or if you didn't understand something.

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