SCR-UJT DC motor speed - Motorola App Note

Thread Starter

AnalogKid

Joined Aug 1, 2013
8,308
I'm trying to repair a baseball pitching machine motor speed control board for a friend. The circuit is based on this schematic, but has an LM723 added in, probably for more precise control of the unijunction transistor.

SCR-Circuit-h.gif

http://www.next.gr/automations/motor-control-circuits/High-torque-motor-speed-control-l12852.html

This is a graphic from an old Motorola application note on using a unijunction transistor and SCR's to implement a phase control circuit for running a DC motor off the AC power line. Does anyone have the original note or a link?

Thanks.

ak
 

ebp

Joined Feb 8, 2018
2,332
I had a root around in my scans of old ap notes, my remaining original paper ones, a power ap book and an old thyristor book and can't find anything (did find an ap note for an integrated amp for mag core memories).

From appearance, my guess is it dates from early 1970s. By about '74 they went to sans font, quit using subscript for numeric part of ident & generally made schematics cleaner. Not much help.

I do have a datasheet for the 2N4870, if that is any help.

What is AAC policy on posting scans of copyrighted material?
 

MisterBill2

Joined Jan 23, 2018
5,219
I sort of recall the circuit as well, but given the HUGE advances there are better ways of providing similar control functionality with a simpler circuit.
But if you are trying to repair a system do you have any ideas as to what has failed, and why? A repair ought to be simpler than a replace. Or is the original control to destroyed to repair?
 

Thread Starter

AnalogKid

Joined Aug 1, 2013
8,308
The motor runs at minimum speed no matter what the pot setting. Setting the pot to mid-range and shorting the wiper contact to either end has no effect, so the pot wiper probably is ok.

The unit belongs to an ME I used to work with. Smart guy, and picked up some electrical knowledge hanging around the EE's, but this is way above his level. I'm working up a true schematic based on photos of the board, and telling him things to try. Until I show up with a scope, the only test equipment is a DMM.

The company refuses to give me any information about the circuit even though it is obsolete in their product line. Besides that, no one in customer support knows anything about the electronics; their only advice is to replace the board ($200).

There is an LM723 that drives the UJT phase circuits. It has a modified current limit function that is derived from the peak motor current rather than the 723 output current. Also, there are two trimpots in series with the main adjust pot, to set the max and min motor speeds. Timing capacitor C2 is charged by the variable 723 output through a fixed resistor. It looks like there are two independent UJT's, one for each SCR. but I have not yet figured out that part of the board.

ak
 

MisterBill2

Joined Jan 23, 2018
5,219
Since you don't mention any obviously damaged parts then the next step is systematic trouble shooting. This would start with checking the various diodes to see if any are open or shorted. Then I would check the whole resistance string that sets the speed, to verify that the over all resistance is within the range that the part values would indicate. After that, tracing out the actual circuit between the motor and the AC line, The reason is that since the motor is running at least one SCR is either shorted or firing. An ope4n or shorted diode or SCR would cause the feedback circuit to back off the speed control because of thinking it was running too fast.
 
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