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Red hot steppermotors and jerky movement


Richard CSL

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I have a Scanic Astute 150 on the bench and it is baffling me, I have swopped all the chips with working units to no avail.

 

The processor is the usual /Atmel chip, as far as I can tell buffered by 2 74HC573N chips, in line with these two is a 74HC240AN , I have no idea what function this chip performs, there is also an LM324 quad op amp connected to the 2 pan and tilt stepper driver chips which are NJM 3771D2.

 

as soon as the power is connected the head moves but makes a stepping noise and a wobble can be seen, also after about 20 mins the stepper motors and the chips get red hot. (not the case on normal working units)

 

 

any ideas greatfully appreciated, as there is no support fro Scanic.

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When I put the CPU in a working board, the steppers work fine, so it has to be something on the board. I have traced all the trackwork, (asI have had problems in the past)

 

This one has really got me baffled. I will hook up the power to an oscilloscope , to see if the waveform is distorted. All the other motors work fine (ie co,l gobo, rotate, shutter), but they are on the 12 volt supply, only pan and tilt are on the 24 volt rail. So I am sure it has to be something on the power, but why would the steppers overheat, (to the point where you can't touch them or the heat sinks on the driver chips) The control must be fine too or the CPU would not fire up, so I think that eliminates the 5v supply (derived from the 12 volt, via a volt reg).

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When I put the CPU in a working board, the steppers work fine, so it has to be something on the board. I have traced all the trackwork, (asI have had problems in the past)

 

This one has really got me baffled. I will hook up the power to an oscilloscope , to see if the waveform is distorted. All the other motors work fine (ie co,l gobo, rotate, shutter), but they are on the 12 volt supply, only pan and tilt are on the 24 volt rail. So I am sure it has to be something on the power, but why would the steppers overheat, (to the point where you can't touch them or the heat sinks on the driver chips) The control must be fine too or the CPU would not fire up, so I think that eliminates the 5v supply (derived from the 12 volt, via a volt reg).

 

The 3771 drivers are PWM drivers which provide variable drive current to the stepper coils by rapidly switching the 24V. If there is a problem with those, they can just stick 24V straight onto the steppers - I would guess this is why they are getting really hot. They use 2 sense resistors to set the current - if I remember, if these go open circuit then they put the full 24V onto the steppers. Download the data sheet and locate which pins the sense resistors are on.

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so does the CPU chip generate the pwm, or the others . This looks like the right place to look. The motors do move so not locked on the 24 volt rail, but there is no accelloration / decelleration between steps. So I am guessing the chip that should supply the pwm is not working properly.

 

Cheers Tim.

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so does the CPU chip generate the pwm, or the others . This looks like the right place to look. The motors do move so not locked on the 24 volt rail, but there is no accelloration / decelleration between steps. So I am guessing the chip that should supply the pwm is not working properly.

 

Cheers Tim.

 

The 3772 generates its own PWM. The micro just sends a voltage to tell it how much current to put into each coil.

If the sense resistors are failing/gone it could be just switching the 24V on and off which would still step the motor but would put far too much current into it.

 

Looks like the resistors are on pins 10 and 6, to ground, and should be about 0.5 ohms

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