Help with MOSFET motor control

[PeteW1959]

I have a problem with a motor control board, and having no experience of MOSFET motor control I am stuck. I don't have a schematic for it, so I have attempted to draw the circuit from the PCB, but I'm having trouble laying it out in a way that makes sense. (PDF Attached)

All the MOSFET motor controls I can find online have either 1, 2 or 4 MOSFETS; this one has 3, so I can't use anything as a pattern.

Can anyone lay this out any better, and perhaps explain how it works? I need to fault find, and I know it is in this part of the board where the problem lies; I am getting the start signal, and the pulse from the 555 timer varies when the speed pot is adjusted.

Any help appreciated.

 

[Harald Kapp]

Welcome to EP.

Please check your circuit diagram. Regardless of layout it doesn't make sense (to me) in the current form. For one the connecting dots are missing, which makes it impossible to decide whether crossinglines are connected or not. Secondly I guess you have switches /START and PWM control.
The circuit looks like it could be a half bridge (Q2, Q3) with an additional on/off control by Q1.
As it is currently drawn with /START being low Q2 will be on, Q3 will be off and the motor will not receive power, regardless of pulses on the PWM input.

 

[Minder]

Agreed, it does not look right, if a reverse engineered schematic you may want to re-check!
M.

 

[Chemelec]

Somewhat makes sense to me:
I Assume Q2 and Q3 is a Flip Flop, (Output to Source of Q1, goes either High or Low) Turning Q1 on and off.
Q1, driven by the 555 is actually controlling the Motor.
So I suggest you try replacing Q1.

Check for a Waveform at the Drain of Q1.

 

[PeteW1959]

Thanks for the replies, much appreciated.

I have double checked my drawing, and in transferring it from pencil sketch to CAD I got the two 1N4007s the wrong way round, and missed a cap from the gate of Q2 to 0v. Sorry, my bad. I've also put junction dots in.

The start signal is from a flip-flop (2 gates of a 4011) and PWM control is a pot in the timing circuit of a 555 timer. Both of these elements are working correctly.

 

[Harald Kapp]

Still doesn't makes sense to me, by the same arguments as above.

 

[WHONOES]

For the motor to run, Q3 needs to be switched hard on otherwise there is no current path to ground for the source of Q1. With Q3 turned on, the source of Q1 is pulled up to the + supply effectively shunting the motor. I think the notation of input signal Start barred is incorrect as that point needs a positive voltage on it to get Q3 conducting. Also, if there is a significant difference between the Regulated and unregulated supplies, Q2 could be partially conducting which would cause problems. Q2's Vgs can be as low as 2V.

 

[Alec_t]

I think the notation of input signal Start barred is incorrect

So do I. My understanding of the circuit intent is that taking Start high turns on Q3 fully so that Q1 can then PWM the motor under 555 control. Taking Start low turns off Q3 and turns on Q2 to provide motor braking (albeit pulsed by virtue of the 555).

 

[WHONOES]

So do I. My understanding of the circuit intent is that taking Start high turns on Q3 fully so that Q1 can then PWM the motor under 555 control. Taking Start low turns off Q3 and turns on Q2 to provide motor braking (albeit pulsed by virtue of the 555).

I agree with you though I doubt if there will be any PWM effect as Q2 will be hard on whilst the drain of Q2 will be high impedance meaning there is no current path at all for the source of Q1.

 

[PeteW1959]

Thanks a lot for your efforts, sorry it's taken a while to reply but I have been away from my bench for a couple of days.

The little information I had on this board indicated that to start the motor, the start signal should go low; on further checking, it does indeed need to go high to start. (Don't trust everything you read, especially hand written notes!).

The 12v unregulated is actually 18v off load, so there is a 6v difference, quite significant.

It does make a little more sense now, and I have realised Q2 is getting quite warm, so it may be at fault. I have ordered some new MOSFETS and will try replacing them.

 

[WHONOES]

Q2 could be getting warm because it is partially switched on courtesy of the difference between the 2 supplies.