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Old 02-02-2011, 07:25 PM   #1
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Default Anyone know a cheap PID with analog output?

Hi everyone-

i am attempting to go bonkers and modulate the flow rate of my wort through the plate chiller so I can get to a specific wort temp directly into the fermentor as fast as possible. I know there are actuated valves that are more than just open or closed, but can be opened to variable degrees by applying a varying voltage.

The only question is this- all the PIDs that i know output a digital on off signal to an SSR or a contactor, which would not allow control of these valves. Does anyone know where I could get a relatively cheap PID that has analog output (preferably 0-10 volts)? I have seen one that is quite cheap that can put out 0-20 mA, but not varying the voltage, and I don't think this would control a 0-10v actuator. Is there a way to convert the 0-20 mA variable output to 0-10 volt output? The cheapest valves I have seen that are modulating are voltage modulated, not current modulated.

Klaus

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Old 02-02-2011, 11:31 PM   #2
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Are you saying you DID find one that provides a variable current output?

If so, I believe you can build a current controlled voltage source with a few simple electronics parts.

I'm digging WAY back in my brain for this, but I think a couple of op-amps can do it.

Let me look at some old college texts to see if I have an example.

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Old 02-03-2011, 10:06 AM   #3
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Are you saying you DID find one that provides a variable current output?

If so, I believe you can build a current controlled voltage source with a few simple electronics parts.

I'm digging WAY back in my brain for this, but I think a couple of op-amps can do it.

Let me look at some old college texts to see if I have an example.
Yes, I found one PID for $69 that will do 0-20mA output, but the valve I ordered requires 0-10 volts DC for proportional control. It seems to me back in my high school electronics days that V=IR... so maybe a resistor might work, but the problem is I have no idea what voltage the 0-20mA signal is or what size resistor to put in line. Also, does the PID vary the voltage or the resistance to change the current from 0-20 mA?

As it turns out, after looking for a long time today I seem to have found something that will give 0-10 v, but it is $100. It is a love controls 16B-63. The other option is the 16B-62, which also offers "voltage pulse output" instead of SSR output, but I have absolutely no idea what voltage pulse output would mean.

Op-amp.... wow. Seems fairly advanced for my meager electronics skills. I had to look up what they were, but it sounds like they are little computer circuits. If you find some way to convert the signal I would be very interested!

Thanks.

Klaus
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Old 02-03-2011, 10:30 AM   #4
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All that is needed is a 500 ohm resistor, 20MA through 500 ohm = 10 volts drop, this is similar to what we do with 4-20MA signals in PLC's with a 250 ohm resistor to get a 5 volt signal. The PID varies the current in the loop, usually with a 24VDC supply driving the loop so it can handle up to 600 ohm loads.

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Old 02-03-2011, 10:48 AM   #5
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I don't think it's going to be that simple.

I gree that putting the resistor on there will show you a variable voltage drop across that resistor, but the problem is that once you connect the valve he wants to control, the resistance seen by the PID changes.

Also, I don't know if there will be enough current available to even operate thew valve. That PID will limit the current.

Might have to use op-amps or something, along with another DC supply, to get a proper driver for the motor that can provide the current the valve needs.

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Old 02-03-2011, 11:22 AM   #6
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We used to use a product called Action Pak. They had conditioners that you could configure the inputs or outputs to voltage or current.

There are other name brands that I cant recall at the moment.

Here is one link -> http://www.actionio.com/products/act...nd/ap4300.html

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Old 02-03-2011, 11:24 AM   #7
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The ones we used were selectable for Voltage or current on both input and output with a zero and span adjustment so you could set to whatever you wanted for range. They were 120VAC powered units.

Not sure what kind of current your proportional valve needs.

edit Our use was 4-20ma to 0-10 vdc.

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Old 02-03-2011, 11:45 AM   #8
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Quote:
Originally Posted by kshuler View Post
The other option is the 16B-62, which also offers "voltage pulse output" instead of SSR output, but I have absolutely no idea what voltage pulse output would mean.
Voltage Pulse Output is used for SSR and SCR Switch style power conductors that are used in Heat control systems.

Pulse output is a time based proportional output. Duty Cycle ie. Percent of time on vs percent of time off.

The 4-20 and 0 - 10 outputs are Magnitude proportional devices. The outputs are typically used with Phase Angle SCR power controls.
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Old 02-03-2011, 12:49 PM   #9
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Quote:
Originally Posted by kshuler View Post
Yes, I found one PID for $69 that will do 0-20mA output, but the valve I ordered requires 0-10 volts DC for proportional control. It seems to me back in my high school electronics days that V=IR... so maybe a resistor might work, but the problem is I have no idea what voltage the 0-20mA signal is or what size resistor to put in line. Also, does the PID vary the voltage or the resistance to change the current from 0-20 mA?

As it turns out, after looking for a long time today I seem to have found something that will give 0-10 v, but it is $100. It is a love controls 16B-63. The other option is the 16B-62, which also offers "voltage pulse output" instead of SSR output, but I have absolutely no idea what voltage pulse output would mean.

Op-amp.... wow. Seems fairly advanced for my meager electronics skills. I had to look up what they were, but it sounds like they are little computer circuits. If you find some way to convert the signal I would be very interested!

Thanks.

Klaus
Go with the Voltage output Controller. You will be over the $100 by the time you get a signal conditioner to convert the 4-20 to voltage. With out the Valve specs, it would be hard to tell if the
output from that controller will have enough current or not.

Make sure you check the current capabilities
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Old 02-03-2011, 03:21 PM   #10
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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.

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