-24V NPN

[grunt58]

We work with PNP and NPN signals for robots. How do you detect a NPN -24v signal with a multimeter? With PNP we see 24v on the multimeter so we know we are getting the signal but NPN we see 0. I applogize for the simpilicity of this question but any help would be apprecitated.

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[VEBill]

NPN outputs are often what is called "open collector". To see the signal, one might need to use a pull-up resistor (e.g. 10kohm).

 

[grunt58]

Currently we wire the output into a junction box that can detect PNP or NPN. We thought it would be possible to use a multimeter volt meter to verify the output.

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[itsmoked]

Try using your multimeter to go from your +voltage 12? 24? to the NPN output. Now when the NPN is OFF you should see very little voltage. Less than a 1/2 volt to 0V and when it's ON you should see about your +voltage minus a little.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[grunt58]

We are always 24v. I understand the first part, put the red multimeter lead to a +voltage and the black lead to the NPN out output? What reading on the multimeter am I looking for?

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[VEBill]

That method may work, but it depends on the ratio of the DVOM input Z (probably 10M ohm) to the leakage of the OC output when off.

If you can't be bothered to use a pull up resistor, then try putting the meter into continuity or diode check modes. Try both ways around (red/black) because meters can be designed either way.

I'd use a pull up resistor myself because it reflects the actual application of OC outputs. One could save an entire afternoon messing around...

 

[Skogsgurra]

Are you actually meaning a negative signal? If so, "How do you detect a NPN -24v signal with a multimeter?" should not be a problem, just make sure the DMM is in DC and you will see a minus sign (-) in front of the voltage. If you mean something else, I think you should review your terminology.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

We are always 24v. I understand the first part, put the red multimeter lead to a +voltage and the black lead to the NPN out output? What reading on the multimeter am I looking for?

Always 24V: You should see around 23-1/2V when the NPN output is active and you'll probably see something around 0.08V when the NPN output is not active. The exact values are based on your meter's internals. That's why I can't tell you exactly. It's not important anyway, as a bad NPN output will be markedly different from this description.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[grunt58]

Yes, I just want to check if there is a -24v (NPN) output with a multimeter. We will play around with the DMM and see what we get.

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[xyzz]

There is no -24V in your system.
All voltages must be positive relative to the 0V.
Active PNP output connects the load to the power source voltage (+24V) and is called sourcing.
Active NPN output connects the load to 0V and is called sinking.
Both are subject to voltage drop on the output transistor, and will not be exactly 24V or 0V, but something like 20-22V and 1-3V correspondingly.