Troubleshooting SCR speed controller / 50v DC motor / failed diodes and SCRs

Thread Starter

aspiespot

Joined Dec 14, 2018
17
Hello,

I have a number of speed control boards I would like to try and fix and improve , I am reasonably new to electronics and am keen to learn and understand as much as I can about this type of controller.

The PCB powers a 50v DC brush motor ,low speed 400rpm (15v) through to 1400rpm(40v),(Input is 230v 50hz UK)

*looking at the circuit diagram I assume its a DC output full wave SCR controller ? (confirmation of this would be helpful)
* some have failed diodes D5 and D8 (IN5408)
*A couple have failed SCRs TR1 and TR2 and seems to allow AC voltage into the motor ?(TYN612M)
*I would think that D1 and R5 are for reverse EMF ?


I would very much welcome any help or advice on improvements or upgrades I could carry out to make the PCB more robost / reliable / protect the motor from AC voltage.

Thank you in advance ,Aspiespot.

IMG_0942.JPG
 

Attachments

MaxHeadRoom

Joined Jul 18, 2013
21,643
Looks like something missing for the power supply, I assume it is derived from a CT secondary of a mains transformer?
The GND symbols could be the CT common of the secondary , but are they earth grounded as the symbol implies?
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
21,643
BTW if you want an improved version, IOW, a little more efficient means of controlling the motor, there are many PWM circuits using Mosfets etc, you could use in place of the cruder Triac control.
This method was typically only used using 3phase full wave SCR for uses such as servo control etc.
Now PWM is the more favored way.
Max.
 
Last edited:

Thread Starter

aspiespot

Joined Dec 14, 2018
17
Looks like something missing for the power supply, I assume it is derived from a CT secondary of a mains transformer?
The GND symbols could be the CT common of the secondary , but are they earth grounded as the symbol implies?
Max.
Thanks for your replies , I can confirm there are no transformers , the AC supply goes straight to the PCB. Aspiespot
 

Thread Starter

aspiespot

Joined Dec 14, 2018
17
Tr1 anode and gate are shown shorted together.

D1 is, but I reckon R5 provides a start-up voltage for the MCU. Did you mean R1? That is a current-sense resistor.

Yes.
Be aware that the circuit shown provides no mains isolation, so any part could be 'live'.
Oppps yep sorry , the R1 resister not R5 , thanks for your help.
 
Top