Who can explain my encoder problem?

jdolecki

I just got a bridgport 3 axis cnc machine. My new drives are reporting an encoder problem. I'm swapping the controls to Mach 3 control.

The encoders are powered by 5 vdc. When I put them on the scope it shows A channel at 5 v and B channel at 20 volts.

First I belive the machine was hooked up to 480 when it was wired for 240. So I belive that's why the power supply I'd bad.

But I don't under stand why I get 20 v on the one channel when there is only 5 v supplying it.

I have the encode apart and there are 3 ic in it and 2are the same the rest are resistors and caps.

I'm sure bei won't give me a schmatic to fix it. But they want to sell me 3 new ones at 600 each.

thanks john.

T Chap

An encoder cannot boost a voltage. Disconnect the encoder wires at the encoder and check the voltage again. If you have 20 volts on A or B and 5V powering the encoder, then you have 20 volts getting on the encoder output somewhere else.

jdolecki

I agree that it cant boost voltage, unless im reading the scope wrong.

the encoder is disconnected on the bench. I have my bench power supply set at 5v dc.

Scope leads are on the A channel and B channel. and scope lead grounda are attached to 0v.

A is set for 5v scale and produces a nice square wave.

B set at 5 v give a square wave 4 times a large as A channel. or if i put it on 20 v divisions the wave is the same size.

I could still be using / reading the scope wrong who knows.

I going to replace them anyways because they are only five 250 ppr and new open end ones with 1000 ppr are cheap.

But I would still like to figure out whats wrong with these and fix them as a learning experience.

the 2 Ic are labeled SN75183N Dual Differential line driver (2 each) and

1 LM3302N Quadruple differential comparator

thanks

T Chap

It takes about 3 seconds to swap the leads from the scope so that you can determine if there is a difference in scope settings or leads. Maybe the lead has a 4x? Usually they have 10x.

Disconnect the a and b wires at the encoder. Read the a and b direct at the encoder with a volt meter. Read the wires that are disconnected.

Duane Degn

jdolecki wrote: »

the 2 Ic are labeled SN75183N Dual Differential line driver (2 each)

I'm not positive, but I think you should be measuring the voltage across some sort of resistor if you're looking at the output from these line drivers.

While "normal" encoders wouldn't boost the voltage, these line drivers might.

I'd suggest using a resistor value around 125 ohms and measure the voltage drop across the resistor. The driver is trying to drive the line with 40mA and without a resistor, the voltage may be much higher than 5V (I think). Edit: I thought wrong.

tomcrawford

T Chap wrote: »

It takes about 3 seconds to swap the leads from the scope so that you can determine if there is a difference in scope settings or leads. Maybe the lead has a 4x? Usually they have 10x.

Disconnect the a and b wires at the encoder. Read the a and b direct at the encoder with a volt meter. Read the wires that are disconnected.

Like the man said. I can promise you that a 75183 connected to 5V is not outputting 20V. It is just a plain ol' totem-pole output (well, two totem-pole outputs, one inverted.)

GordonMcComb

At the risk of mentioning the obvious, have you switched connections on the scope? If you do, and Channel 2 (now reading the opposite encoder output) is at 20, then there's something wrong with the probes, or the scope. A lifted ground in the probe might give an abnormally high reading, but usually also shows a lot of noise. Still, it's worth checking.

Most scopes have a calibration terminal on them. Mine is set to output 1V. So if you just dial your scope to 1V per div, it's pretty easy to check if there's an issue with the probe or scope. Does your scope have such a terminal?

evanh

Yeah, could be it's really 2 volts and you've got your scope scaled wrong. The x10 switch on the probes can be bumped. Of course, 2 volts is not a good result either but at least it's more feasible.