Need help! Stepper Driver KEEPS Failing

Thread Starter

Lafaver14

Joined May 2, 2017
7
This is my first post but I am desperate for help.

I have a 24VDC Anaheim Stepper circuit that is getting a signal from the relay output of a Mitsubishi PLC. Power is going through a power supply (have already changed this once to eliminate it from the issue) to get me the 24VDC.

The driver needs a 24VDC signal for direction, but after running for a random amount of time the direction input to the driver fails (two transistors and a Opto-coupler)

Below is the diagnosis from the factory and the driver schematic. I have also attached our wiring diagram.

"It was found that both transistors were shorted and the opto diode was opened. This was mostly caused by an over voltage. I would say the transistors shorted first which then left enough voltage across the diode to open it. I replaced Q3, Q4 ad A5 on both boards. They will finish being tested and then we will burn them again. They will be shipped out tomorrow."

The strangest part is that the clock has not failed.
 

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MrChips

Joined Oct 2, 2009
21,082
Your circuit is all wrong.
You are applying +24V through CR1, to the opto diode, Q1, R7 80.6Ω back to -ve.
This is going to blow everything in its path.

Same with the second channel.
 

Colin55

Joined Aug 27, 2015
519
Draw the circuit and not just provide block diagrams and you will see what I am talking about. If you can't see this, you should not be producing things you have no understanding about.
 

MaxHeadRoom

Joined Jul 18, 2013
20,661
Your PLC designations on the stepper drive do not appear to jive with the PLC output address?
What is the object of the circuit?
Also as an aside, the use of contacts on the RH side of an output on the main schematic, neutral side of the two motor contactors (CR710/CR711) is considered a NO-NO now as a safety hazard.
Max.
 

dendad

Joined Feb 20, 2016
3,517
Has this failed a number of times?
And what makes you say over voltage? Are you getting transients on the 24V line?
Is there any evidence of a fault on the driver PLC an one would think if an over voltage event happened, it may pop both ends.
If it was a once off failure, just replacing the parts should be ok.

If you want to try a mod, maybe you could add a series resistor in the + line to limit the current, say 1K, and a 15V 1Watt zenner diode across + and - terminals, with the cathode (lined end) to the + and the circuit should still work but be protected a bit better.

Really, fixing the over voltage problem is a good idea, if you can figure out what it is from. Industrial environments can be very electrically noisy!
 

Alec_t

Joined Sep 17, 2013
11,274
Your circuit is all wrong.
Are you sure? I see nothing wrong in principle with the circuit (provided the transistor specs are adhered to), which includes a bog-standard current-limiter. The opto-LED current is sensed by the 80R resistor and held at about 10mA when active. Current higher than that tries to raise Q2/4 base, pulling Q1/3 base down and hence tending to reduce the LED current.
 

dendad

Joined Feb 20, 2016
3,517
"Between "+" and "-" you just have diode junctions. It is not wonder the thing blows up. !!!!!!!"
I can't see that. Maybe I'm missing something. As it is almost 3AM here and I'm having problems sleeping that mau be so.
Also,
"Also as an aside, the use of contacts on the RH side of an output on the main schematic, neutral side of the two motor contactors (CR710/CR711) is considered a NO-NO now as a safety hazard."
Yes, switching just the neutral is a BAD idea!
But it looks like CR710 and CR711 are both double pole contactors so are switching the active and neutral.

And I too cannot see anything wrong in the current limiter circuit so it must be a fairly large over voltage to cause the problem. It is reverse polarity protected too. Maybe some one dropped a screwdriver and put mains on the line for a bit???
 

Thread Starter

Lafaver14

Joined May 2, 2017
7
Are you referring to the drive circuit diagram or the wiring diagram?

I am not a EE which is why I am relying on your help. Thanks for remaining respectful
 

LesJones

Joined Jan 8, 2017
2,638
I can see nothing wrong with the opto isolator driver circuit. The two transistors for a constant current driver circuit for the LED in the opto isolator. It will regulate the current trough the LED to about 8.6 mA. With reference to the top driver (Q1, Q2) when the voltage across the 80.6 ohm resistor reaches about 0.7 volts (Vbe of Q2) it starts conducting thus reducing the base drive to Q1. Have you measured the voltage on the direction input of the stepper driver ? I can't find sheet 7 of the schematic which is where the drive signal originates.

Sorry Alec_t. I have just noticed I have said the same thing that you said in post #8

Les.
 
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Thread Starter

Lafaver14

Joined May 2, 2017
7
"Between "+" and "-" you just have diode junctions. It is not wonder the thing blows up. !!!!!!!"
I can't see that. Maybe I'm missing something. As it is almost 3AM here and I'm having problems sleeping that mau be so.
Also,
"Also as an aside, the use of contacts on the RH side of an output on the main schematic, neutral side of the two motor contactors (CR710/CR711) is considered a NO-NO now as a safety hazard."
Yes, switching just the neutral is a BAD idea!
But it looks like CR710 and CR711 are both double pole contactors so are switching the active and neutral.

And I too cannot see anything wrong in the current limiter circuit so it must be a fairly large over voltage to cause the problem. It is reverse polarity protected too. Maybe some one dropped a screwdriver and put mains on the line for a bit???
It has failed 5 times. Even after a complete rewire, power supply change, and going to transistor outputs on the PLC.
 

MrChips

Joined Oct 2, 2009
21,082
Perhaps I don't see the function of the driver circuit.
Why not just a simple 2k2Ω resistor between the 24V signal and the opto diode and be done with it?
 

dendad

Joined Feb 20, 2016
3,517
Hmmm.
If it has failed 5 times, you need to look further. It should not fail at all so I would suspect there is a wiring problem and some sort of "evil voltage" is getting onto the input.
Just as a thought, make sure the inputs are rated to run on 24V, even though they look like they should.

If you put a 100mA fuse in line with the input from the PLC, and a transient diode across the inputs at the drive, Cathode to +, anode to -, like above. Then look to see if the fuse blows. If it does, you have an extra voltage problem. Find and fix that!

https://www.digikey.com/product-detail/en/littelfuse-inc/1.5KE33CA/1.5KE33CALFTR-ND/688026

If the fuse pops, make sure you check the 1.5KE33 tranzorb too as it may have shorted.
 
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Thread Starter

Lafaver14

Joined May 2, 2017
7
Hmmm.
If it has failed 5 times, you need to look further. It should not fail at all so I would suspect there is a wiring problem and some sort of "evil voltage" is getting onto the input.
Just as a thought, make sure the inputs are rated to run on 24V, even though they look like they should.

If you put a 100mA fuse in line with the input from the PLC, and a transient diode across the inputs at the drive, Cathode to +, anode to -, like above. Then look to see if the fuse blows. If it does, you have an extra voltage problem. Find and fix that!

https://www.digikey.com/product-detail/en/littelfuse-inc/1.5KE33CA/1.5KE33CALFTR-ND/688026

If the fuse pops, make sure you check the 1.5KE33 tranzorb too as it may have shorted.
This is exactly what Anaheim just recommended/provided
 

dendad

Joined Feb 20, 2016
3,517
Perhaps I don't see the function of the driver circuit.
Why not just a simple 2k2Ω resistor between the 24V signal and the opto diode and be done with it?
The use of a constant current circuit allows for consistent operation over a wide range of input voltages. This drive unit would work quite well with 5V inputs I would think. If it was just a 2k2 resistor, it probably would not.

(It is particularly useful in LED lighting applications so to limit flickering.)
 
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