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powering a rotary encoder



I am putting together a measurement system using DAQ(USB - 6009). I need to power an optical encoder (here) that operates at voltage 5V and current 20mA - 40mA. I would like to use the DAQ's 5V outoput port to power the encoder. Here is where I am stuck because I need the DAQ to supply 5V at 20-40mA. When I read the currnet coming out of the 5V port on the DAQ it was ~400mA. How can I divide the current if I do not have the circuit diagram of the encoder. My other problem is that I do not know how to inspect a circuit with a given current and voltage source on the same wire. I tried to solve a current divider but it did not make sense. Please see pictures for details.  





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Message 1 of 12

Hi sorry about the missing images here are links to the (DAQ + optical encoder circuit) and (optical encoder, measured R)




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Message 2 of 12
Please don't put your pictures on a 3rd party website.  Attach them to your message.
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Message 3 of 12

Just a few points:


1. The 400mA is the max current output of the DAQ's 5V supply, while your circuit will only draw as much current as it needs to operate - that is 20-40mA for the rotary encoder, plus 5/R (I  = V/R) for the resistive load in this case. As long as the circuit current draw is smaller than the maximum current output of the power supply, things should be OK. In this sort of analysis you wouldn't consider the maximum current output of the power supply as a fixed current source. I would suggest finding a book or some other resource on circuit analysis to help you here.


2. The optical encoder does not measure rotation by resistance, instead by examining the waveform out of the CH A and CH B pins. See the 'Output Waveforms' on the datasheet you linked and these articles: ,


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Message 4 of 12


Thank you for the advice. I really appreciate it. I will proceed with my setup using the 5V port & connect it directly to the encoder (Vcc).


Raven Fan:

Sorry about the third party link. I am having trouble copying and pasting from word to the forum. I copied an image from a pdf (used the snapshot tool) to microsoft word. I do not know why it does not work.



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Message 5 of 12

One problem you had in your original message is that the image you inserted was actually located on your hard drive.   file:///C:/DOCUME%7E1/MIRIAM%7E1/LOCALS%7E1/Temp/msoclip1/01/clip_image002.jpg


For obvious reasons, we don't have access to that.


1.  Attach the images (never .bmp, preferably .png, but .jpg is ok too) to your messages.  (Add attachments link is below the editor)

2.  Submit your message.

3.  Edit your message.

4.  Right click properties on each attachment.  Copy the network path of the attachment as it points to the NI server.

5.  Insert the image and paste the link obtained in #4.

6.  Submit the message again.

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Message 6 of 12

frostkiller wrote:



I am putting together a measurement system using DAQ(USB - 6009).... to supply 5V at 20-40mA. When I read the currnet coming out of the 5V port on the DAQ it was ~400mA. How can I divide the current if I do not have the circuit diagram of the encoder.




We do need your presence up here if you're as good as your name implies.


I have had problems with the output of some USB boards before.  If you attempt to switch the power on and off through one of the digital lines the output may not be sufficient for your needs.  In that case you will have to use the digital channel to control a solid state relay which will control the 5V DC output.  If you intend to leave the encoder powered up whenever the USB board is activated this is a moot point.


If you are worrying about protecting the encoder you can always put a fuse to limit the current going to the device.



Message Edited by RL on 12-22-2008 12:02 PM
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Message 7 of 12


Thank you for the instruction. My next post will be far more illustrative.



I like my name too.

Thank you for the tips. I will keep the encoder powered, uninterrupted. I never thought to use a fuse. I should do more practical projects over vacation period. In intro to circuits class I learned how to analyze circuits, but not how to "play" with them.


I will give kudos after I have had a chance to actually power up the device. I hope to do this tomorrow. I will record how it goes.

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Message 8 of 12


I really need help. I have a quadrature encoder and a NI 6229.  After doing the settings in labview . it shows me 3 pins for A, B and Z. but what about the A-, B- and Z-? also what about gound and VCC? Should I have another power supply(not the DAQ itself) to connect to  vcc and ground of encoder?

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Message 9 of 12

What does "After doing the settings in labview" mean???

How can we help you connect an encoder if we don't even know what model it is???


You must be more specific...

LabVIEW 8.5.1 - 2019 Pro Dev
Message 10 of 12