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Help with SCRs and vending machine motors.

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GTechno13

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Hello,
I am redesigning a vending machine control system and am working on the motor control. The machine uses those augers/spirals to bring the product forward. My issue is with controlling an exact rotation for each motor. The motor is a 24VDC motor attached to a gearbox whose output shaft is D shaped around the outside. A SPDT snap action switch rides on the D shaft. The NO and NC outputs of the switch are connected to each other and those and the COM appear to be in series with the motor. There is also a diode and resistor present inside the gearbox but it is difficult to tell what they do. The diode appears to be reverse polarity protection and not reverse parallel with the motor. Removing the switch from the gearbox I was able to determine the switch spends about 1.3ms in a break before make state during the transition on both edges of the "D".

When applying 24V to the motor it continuously turns releasing and repressing the switch each rotation with no noticeable effect.

I have never used a SCR before so that it why I'm asking here. RadioShack doesn't sell any and I don't want to order just one to try it out.

What I would like to know is if a small SCR would be a good way to control these motors on the low side? I could hold the gate high long enough to get past the second transition of the switch then release it and the motor would stop when it finishes its rotation and reaches the first switch transition again. (This is when the switch transitions into the extended position when reaching the flat part of the "D".) The motors draw about 120mA so most SCRs I saw on mouser seemed plenty capable.

Thanks for your understanding and help. I don't want to order a dozen SCRs and find I had the concept wrong.

-GTech
 
An SCR generally only is used for AC since it requires the current through it to go to zero for it to turn off.

You can use a MOSFET to control the motor. An N-channel MOSFET can control the low (common) side of the motor. You need about 10V to fully turn on most MOSFETS (unless they are logic level types).

Make sure there is a reverse parallel (cathode to positive) diode across the motor to protect the MOSFET against transients (are you sure that's not what the resistor/diode are for?).
 
Let me rephrase a bit. I need to know when to shut the motor off, or in how I imagine the SCR to work, the motor will shut itself off. A SPDT break before make switch with its normally open and normally closed terminals tied together is placed in series with the motor. Because I have no way to scope current I assume for the 1.3ms the switch spends in transition, there is no current flowing through the motor. How long does current in a SCR need to be zero for it to shut off? I assume its a lot less than 1ms.
 
Ok, now I think I understand what you are doing. You want to put the SCR in series with the motor and the switch, not have the switch control the SCR. In that case, yes the 1.3ms interruption of the SCR current should readily shut off the SCR.

But my comment about the protection diode across the motor still holds. Otherwise the SCR may blow or not shut off.
 
With everything assembled, I have an issue. I set up a demo program to just turn on a channel of my high side driver and one of the SCRs. The motor begins to turn. After a delay of 1 second the SCR gate is brought back low. This allows for the second transition of the button which rides the D shaft to pass without shutting off. The motor completes its cycle and stops when the first transition of the switch occurs. The high side driver is then shut off. My problem is that about 1/4 of the time the motor stops when the SCR gate is brought low. Does anyone have an explanation for this? I have a reverse diode across the motor as should be and placed a small ceramic cap across the motor. This did not seem to help much. I then placed a .1u electrolytic across and it tested fine for about 10 cycles but I can't just assume this is the solution. (And I don't want to solder in 80 large caps) I am driving the SCRs with 74hc595 shift registers so the gate is brought to ground. I thought maybe the noisy motor could be pulsing the current low enough to shut off but it only occurs when the gate is brought low, never after. The motor draws about 120mA and the holding current is 5mA. Maybe bringing the shift registers to tristate would fix it but then I'm worried about stray current triggering the SCRs plus my pcb is not wired to do so.

Also I have 4.7k gate resistors as the trigger current is only 200uA. The output to these is 3.3V. I don't think the resistors are too big as it has no trouble turning on.

The motors currently have an internal reverse polarity protection diode but not a reverse parallel diode so it looks like I will have to solder in 80 diodes already but adding 80 caps is more work and more money. Is there an alternate solution. I'm not even sure if the caps are the solution without hours of testing.

Summation: What would cause a SCR to shut off when the gate is brought low other than current falling below holding?

Thanks

EDIT: I have three shift registers to control the 10 SCRs and 8 channel high side driver. They are cascaded but I do make use of the latch clock so the bits are not shifted through at the outputs. Is is possible that, regardless of this, that the shift register is toggling the high side driver even though the bits have not changed? ... I need to get to an oscilloscope.

EDIT 2: Sorry I'll stop after this. I placed a 10uF cap between the motor positive (high side driver output) and ground instead of across the motor so therefor I would only need 8 for the whole rig and it still stopped. I thought for sure that would fix it.
 
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You might try a small capacitor (say 0.1µF) across the SCR.
 
A to C or G to C?

Also, what is the best type of diode to use reverse parallel across the motors? I've always use very standard 1n400x type diodes but are any of the other types better suited?

EDIT: Just watched SparkFuns friday new product post and apparently Schottky diodes are best because of recovery time.
 
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Connect the cap A to C.

You don't need Schottky diodes across a motor since reverse recovery time is of little concern for that application. Standard 1N400x type diodes will be fine.
 
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