Why can't I see my Optical Encoder Output Signals?
October 21, 2014
Why do my incremental encoder signals disappear?
Today I was called to the production floor after one of the test Technicians noticed that the encoder signals disappeared from the oscilloscope when they ran the encoders “unloaded”.
I asked one of the techs if the encoders being tested were of the open collector variety. The answer was “No, they’re line drivers”
I immediately pictured our standard 26C31 line drivers, which are push-pull devices.
When I checked the label on the line driver IC on the encoder. I saw that it was an ET7273. This meant that the right answer was that they were both a line driver and an open collector device. The ET7273 is used in our optical encoder when a customer wants complementary open collector outputs, or incremental side open collector outputs
The problem was that I, as well as the Tech I talked to, am guilty of using the term “line driver” to synonymously mean “push pull” outputs.
Open collector style outputs are used when you want to be able to pull up to a different voltage level from the rest of the system. For example most of the optical encoders we sell are 5V devices, but the open collector output can be pulled up to many times that level. The ET7273 has a max spec of 30Vdc.
So why was she not seeing any outputs when the encoder wasn’t loaded?
The term “open collector “refers to the fact that there is no electrical connection to the collector side of the output transistor. It is up to the user to put the correct value of load resistor between the positive voltage rail and the output in order to make the transistor start switching. The value of this resistor depends on the desired current through the device and the maximum rating of the transistor.
When the Tech was testing the encoder while it was electrically loaded, the resistor internal to the encoder test station was in series with the transistor pulling it up to the positive voltage rail. When the Tech took the loading off, the resistor was removed from the circuit, making an open, and causing the signals to flat line.
The test personnel are used to seeing the 26C31 line drivers day in and day out. Since the 26C31 line driver is a push pull device, signals are present whether or not the encoder is under load.
I am proud of the fact that the test Tech took the time to ask “why” instead of just writing it off as an anomaly they didn’t understand.
I think things like that help to make Quantum Devices the best optical encoder manufacturer around.
Quantum Devices Inc is a leading manufacturer of optical rotary encoders.
Jim can be reached at 608.924.3000