PWM control circuit design

[2PAC Mafia]

Hi guys,

I receive sometimes a treadmill board to be repaired because it´s obsolete and the brand doesn´t exist anymore. I know the problem is usually in the SEALED PWM control circuit which is unrepairable. It controls a 180Vdc motor. Board isn´t complex:
-Transformer with 2 AC output 10V (directly to power upper console) and 17V rectified and regulated with 7812 and 7808. 7812 is powering the PWM sealed PWM control circuit and 7808 is going to console through a potentiometer coming back to PWM c0ntrol circuit as speed voltage control (0 to 8V control voltage = 0 to 180V motor output)
-IRFP460 mosfet and motor diode protection
-Bridge rectifier and capacitor for 300Vdc

I would like to create a PWM control circuit making the same function of this one. It uses only 7 pins. 1,2 is 12V power supply, 3 is 0 to 8 V speed control voltage, 4 is 10K potentiometer to control amp output, 5 is 20K potentiometer to control speed output, 6 to source of mosfet (power resistor also there probably for current sensing), and finally 7 is gate PWM output through a 47 ohm resistor.

Could you help me to design a simple circuit which solve this problem? Regards,

 

[alec_t]

What is the PWM pulse frequency?

to control amp output

Do you mean 'amplifier' or 'Amperes'?

20K potentiometer to control speed output

What does this do that the 0-8V speed control voltage doesn't?
Is each potentiometer wired as a 3-terminal device (potentiometer) or as a 2-terminal device (rheostat)?

 

[2PAC Mafia]

Hi Alec_t,

What is the PWM pulse frequency?

I don´t know, I have to receive a good one then I can measure it with scope

Do you mean 'amplifier' or 'Amperes'?

º

Amperes

What does this do that the 0-8V speed control voltage doesn't?
Is each potentiometer wired as a 3-terminal device (potentiometer) or as a 2-terminal device (rheostat)?

I don´t know, I can´t check what they do at sealed circuit. They are installed as reosthat.

 

[alec_t]

So, if I understand correctly:
1) both rheostats are at the console,
2) each rheostat has only a 2-wire connection back to the sealed unit (ground being one wire?),
3) the 10k rheostat controls current-limiting,
4) the 20k rheostat connected to pin 5 controls speed in some way, but is not the main speed control? I'm puzzled as to why there are two speed controls.

Is the 10k a factory-preset or a user-operated control?
Is the 20k a factory-preset or a user-operated control?
Can you post a link to the treadmill manual or spec?

 

[2PAC Mafia]

Hi Alec_t,

So, if I understand correctly:
1) both rheostats are at the console,
2) each rheostat has only a 2-wire connection back to the sealed unit (ground being one wire?),
3) the 10k rheostat controls current-limiting,
4) the 20k rheostat connected to pin 5 controls speed in some way, but is not the main speed control? I'm puzzled as to why there are two speed controls.

Is the 10k a factory-preset or a user-operated control?
Is the 20k a factory-preset or a user-operated control?
Can you post a link to the treadmill manual or spec?

1) The rheostats are at the controller board, not console, I only have the motor controller board.
2) The AMP rheostat is going to PWM module and to ground. The Speed rheostat is going to PWM module one pin and the other pin to a R5 and from there to plus + bridge rectifier and also R5 which goes to ground.
3) Yes
4) Yes, I don´t know why. May be they are not necessary in the new design.

In theory user doesn´t change any value at rheostats. I don´t have information about the treadmill or the board, the brand already disappeared.