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I have a superheat controller which outputs 24 V PWM. I'm using a voltage divider to bring it down to 5 V so I can interface it with a pump controller.

The superheat controller has two PWM outputs: PWM + and PWM -

The pump controller I'm hoping to work with has two inputs denoted as 0 V. My question is where should the PWM - output go? To the 0V pins or somewhere else?

And here is the link to the superheat controller datasheet

Here is the pump controller datasheet

Do I need to worry about the voltage differences? Can I connect PWM- to GND?

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    \$\begingroup\$ Let me use my mind-reading powers to read your mind... wait... no.. sorry, I can't detect which "superheat controller" you own. \$\endgroup\$
    – pipe
    Apr 6, 2017 at 23:03
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    \$\begingroup\$ Lol @pipe how was THAT trip/.. \$\endgroup\$
    – Trevor_G
    Apr 7, 2017 at 1:25
  • \$\begingroup\$ Data sheet ref would help IMMENSELY. Answer may be ground OR O/C or you use both, depending on the controller. PWM- MAY be inverse of PWM + or the PWM signal may float and need the PWM - connected to a reference eg ground or ... \$\endgroup\$
    – Russell McMahon
    Apr 7, 2017 at 1:36
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    \$\begingroup\$ Sorry about that, I've edited my post to include the datasheet. Your help is very appreciated! \$\endgroup\$
    – ADelapore
    Apr 7, 2017 at 11:52
  • \$\begingroup\$ Judging by the wire types, PWM- is a ground reference for PWM+ (or vice versa, if you want a negative signal). DGND pin seems to be unrelated to PWM. \$\endgroup\$ Apr 7, 2017 at 12:39

3 Answers 3

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It looks like the superheat controller PWM signals are powered outputs to drive their valves. So if I had to guess they're each the outputs of a H-bridge. If that's the case then there at probably two ways they drive them.

1) My usual way for similar designs is to have each output sit at 50% duty. Then to create a positive potential across the valve you can move the '+' signal to a higher duty cycle (say 75%) and the negative down (to 25% for example). You can do the reverse to create a negative potential.

2) Leave both outputs at 0% then if you want a positive potential set the duty cycle of the '+' output. Again do the reverse for a negative potential.

So if it's case 1 then you're probably totally out of luck. You might get what you want with case 2 by just ignoring whichever output makes your system unstable. You will need to use an oscilloscope to be sure. But in either case it looks like a bad idea to try and attach the two unless you're in a real pinch to get things working.

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The data sheet notes the PWM + and PWM - pins as outputs. One might be the inverse of the other. In other words, if it were to output a 30% duty cycle wave, you might see a small positive output on the PWM+ terminal (say, high for three ms and low for seven ms) and an equivalent but inverted signal on the PWM- terminal (low for three ms, high for seven ms).

I would directly email the manufacturer or supplier with this question. They will give you an answer you can trust.

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Differential signals give better noise immunity for PWM. THe Pump controller must have the option for opto-PWM. This usually only needs a single ended input with a series R and V+ for the Optocoupler just to bias the switch on with the correct polarity.

The details are sadly lacking so only the OEM knows for sure. Go thru purchasing, Distributor or OEM tech support.

Grounding the PWM- signal is not wise. That shorts the logic driver.

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