[SOLVED] DC motor circuit control for push the wire in a MIG welder. Help to understand it

[danielr]

I have surveyed the attached circuit from a MIG welder PCB board. I have copied only the circuit that control the push wire motor.
I understand it almost all except the (protection?) circuit constituted by SCR3 and T3.

Please let me your explanations.

 

[chuckey]

I can't see SCR3 and T3 in your sketch. I would think that the motor is started by the striking of the arc. Could this be by the change of voltage on the transformer?
Frank

 

[danielr]

Mr. Chuckey, please excuse me, I have attached the scketch without the references. I have edited it and attached the scketch with references.
Struck me that nobody answered my thread.

 

[chuckey]

All the time the base of T3 is above .8V, its switched on so there is a short from collector to emitter. This short diverts the current from the 330 ohm resistor to earth, so the SCR is off. once the voltage fall below .8V, T3 goes off the 330 ohm current goes into the gate of the SCR and it turns on, diverting current from the motor feed resistor (2 X 4.7 ohms?) to earth, so the motor stops/slows down.
Frank

 

[danielr]

Mr. Chuckey,
Thanks for kindly your answer.

2x4.7ohms? this resistence is in the welder chassis and not in the board sketched by me, was documented in the service manual as 2/4.7ohm. In this moment the MIG machine is in other address and now I can not check if are 2 resistences the 4.7 in paralell or in serie setup.

I understand the electronic circuit behavior that you have explained me. I do not understand if the T2, T2, SCR3 circuit is for protect the SCRs and Diodes of the bridge or is to stop the motor, etc.

I think the follow, normally the circuit functions as you have explained. But if the motor is mechanically blocked there is not emf and the potential drop is aproximately 0 V.
SCR3 can not be fired by the 330ohms and T2 remains all the time OFF, letting the the bridge SCRs be triggered in any moment.
This explanation do not have any sense for me, because I do not discover the objetive. Maybe that I have forgot to verify some connection.

I wish to think that this circuit is for protect the bridge but I do not understand how.

Regards,
Daniel

 

[chuckey]

I have re-drawn the circuit that feeds the motor. The motor speed is controlled by the pair of SCRs. There is no back emf sensing circuit or current sensing to cope with motor jams.
Frank

 

[danielr]

Frank

Very thanks for your effort.

The following are a link to the MIG manual, in the page 17 you can see the external conexions to the board.
**broken link removed**

The following is a schematics of the board that I have found in internet, please notice that the T3 have not the collector resistor. In my survey has a 330ohms to 12V
**broken link removed**

Seems that the board has not protection for the SCRs and Diodes of the bridge, and we can not discover the true function of the T3 and SCR3 circuit.

Regards, Daniel