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Old 02-03-2011, 03:28 PM   #11
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It sounds like a Belimo valve is going to be used for the proportional valve, which have a high impedance 0 - 10 VDC signal connection, and a separate 24VDC input for motor drive. Since I have 5 of these in my plc controlled brewing system and work with similar valves in industrial control installations the resistor method should work well as it is used for similar applications for converting current signals to voltages for inputs.
Yeah I imagine the resistor would also work as Integral action from the PID will get the valve opened to the right flow regardless of whether the signal is scaled properly or not assuming its in the ball park. If the input is indeed high impedance, the losses that might cause span issues would be minimal.
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Old 02-03-2011, 03:33 PM   #12
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Look at the Belimo LR24-SR actuator information, it shows the use of a 500 ohm resistor across the input for 4-20 MA loop systems.

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Old 02-03-2011, 10:55 PM   #13
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Excellent, thanks! So it sounds like people have used a resistor successfully, then. A signal conditioner looks like it will bump the price well above the $100 mark. The two models of PID I have seen that are candidates are the Love 16B-63 and the Omega CN4316-F1-R2. Both controllers have dual outputs (SSR and 0-20mA for the Omega controller, and for the love controller it is SSR and selectable 0-5 and 0-10 volt). The price difference is 30 dollars, plus the cost and effort of converting 0-20mA output to 0-10v output.

I assume the 500 ohm resistor has to be wired in series, to convert the signal, right? How sure is everyone that it will work? I can probably wire a single resistor in series even with my meager knowledge, but is it really that simple? And how do you know that 500 ohm resistance is needed? Doesn't it depend on whether the circuit varies the voltage or the resistance of the circuit on what the converting resistor's value is? Can anyone explain the math of figuring out the resistance required?

At the moment I am leaning toward spending the extra 30 bucks to avoid the hassle, but could certainly be convinced otherwise. Always nice to save an extra 30 bucks.

Klaus

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Old 02-03-2011, 11:17 PM   #14
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wire 2 - 1K 1/4W resistors together to hit 500 ohms, one end needs to be at - lead for valve and - output of controller, other end is where the valve signal line and + output of controller go. The Belimo valve still needs 24VDC to power valve motor on + connection of valve, - side of power supply goes to the - side of the valve. You can see a diagram on the Belimo website for the proportional actuators if you have difficulty with these instructions. The resistor method is not uncommon with the belimo and johnson control actuators and 4-20 MA controllers.

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Old 02-03-2011, 11:19 PM   #15
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Looking a bit further, I suspect a simple resistor may not be the right answer. It seems there are entire product lines devoted to converting between 0-20mA output and 0-10v output. Here is an example: http://www.crouzet-usa.com/downloads...ter_Module.pdf

It seems to me that if it was as simple as a resistor then there wuld be no need for these kind of products, right?

Klaus

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Old 02-04-2011, 12:37 AM   #16
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Actually the resistor method should work as the input to the valve is High Impedance as someone mentioned earlier. We use resistors all the time at work to convert 4 - 20 to a voltage. You end up with a zero offset using a resistor though. Still not a really big deal. Some of the controllers will allow you to set output to 0-20ma.

The conditioners are typically used if you want to have isolation and also have zero and gain adjustments. The resistor you have to size to get your span and endup with a zero offset.

I'd go with the 0-10 VDC PID and call it a day, but either should work.

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Old 02-04-2011, 01:42 PM   #17
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Well, looks like the decision has been made. I put a bid on a Honeywell UDC3300 on ebay and just won for 36 bucks. This is a single output with 4-20mA signal, an alarm and a second "auxiliary output" that apparently can be set as a second 4-20 mA controller for cooling. The actuated valve can be set with a dip switch to either 0-10v or 2-10 v, so I imagine the resistor should still work well when switched to 2-10v mode. I think for 36 bucks, the UDC 3300 is a damned STEAL. Seems like a very capable (but overly complex) unit.

So, I guess in the end I will try to 500 ohm resistor method! Thanks everyone for your input. I'll try to wire it like in the Belimo diagrams (even though it is not a Belimo valve).

Klaus

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Old 02-04-2011, 01:51 PM   #18
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please do a write up on your findings and a build if possible. Im sure others would like to see how you did it.

Good luck

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Old 02-04-2011, 03:35 PM   #19
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Well, looks like the decision has been made. I put a bid on a Honeywell UDC3300 on ebay and just won for 36 bucks. This is a single output with 4-20mA signal, an alarm and a second "auxiliary output" that apparently can be set as a second 4-20 mA controller for cooling. The actuated valve can be set with a dip switch to either 0-10v or 2-10 v, so I imagine the resistor should still work well when switched to 2-10v mode. I think for 36 bucks, the UDC 3300 is a damned STEAL. Seems like a very capable (but overly complex) unit.

So, I guess in the end I will try to 500 ohm resistor method! Thanks everyone for your input. I'll try to wire it like in the Belimo diagrams (even though it is not a Belimo valve).

Klaus
Its great that they have the selection on the valve to change it to 2-10v. That solves the zero offset problem. Good luck
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Old 02-04-2011, 03:59 PM   #20
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Man, I don't know why you chose to ignore kladue. If your PID has a 0-20mA output a 500 ohm resistor would have been perfect. I work for a company that specializes in data acquisition equipment and kladue's suggestion is perfectly valid.

The reason they have products like that is because they usually offer isolation as well. Isolation would be good if the signal is coming from an electrically noisy environment and you want to keep that noise out of your panel. Or they are used to repeat the signal if their is too much of a burden on the current analog loop.

Dont chase the bid on that thing too hard, you really dont need it.

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