Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
HobbyCNC said:I have built a stepper motor driver using PIC PWM to control current but the bridge low side is hot while the high side is cold.
but the bridge low side is hot while the high side is cold
I hope to have explained clearly
Nigel Goodwin said:OK, although I don't really see the need for PWM? - but regardless, the H-bridge looks fine - although it's ABSOLUTELY CRUCIAL that you don't accidently turn both PIC outputs HIGH at the same time!.
HobbyCNC said:Do you think that chopper works better than PWM ?
motion said:When you mean "hot", do you mean the temperature is high? What does "cold" mean?
motion said:How do you control the bridge using PWM?
motion said:What happens when the PWM is in the OFF state?
motion said:Do you know the terms fast decay/slow decay?
Nigel Goodwin said:I just don't see why you need to do anything? - a stepper moves a single step (or a half step) at a time, reducing the power to the motor can either move exactly the same step, or be too low to move at all. So I don't see the advantage of lowering the power available?.
Douglas W. Jones - Stepping motors tutorial said:There is good reason to run a stepping motor at a supply voltage above that needed to push the maximum rated current through the motor windings. Running a motor at higher voltages leads to a faster rise in the current through the windings when they are turned on, and this, in turn, leads to a higher cutoff speed for the motor and higher torques at speeds above the cutoff.
Microstepping, where the control system positions the motor rotor between half steps, also requires external current limiting circuitry. For example, to position the rotor 1/4 of the way from one step to another, it might be necessary to run one motor winding at full current while the other is run at approximately 1/3 of that current.
Nigel Goodwin said:So you're wanting to try micro-stepping then?.
Styx said:The FET's you are using, do they have intrinsic diode?
the CCT you are showing has no free-wheel diodes
Why would the gate voltage not go to VMOTOR when the NPN turns off?Styx said:What voltage is VMOTOR? This will help
I am suspecting that when you are ment to be actually turning OFF the upper FET's, the gate potential still has enough for conduction to occur, be it very slightly it is still an ACTIVE-region current and very lossy
The GATE of the top FET's need to be pulled upto VMOTOR to turn them off
Ron H said:Why would the gate voltage not go to VMOTOR when the NPN turns off?Styx said:What voltage is VMOTOR? This will help
I am suspecting that when you are ment to be actually turning OFF the upper FET's, the gate potential still has enough for conduction to occur, be it very slightly it is still an ACTIVE-region current and very lossy
The GATE of the top FET's need to be pulled upto VMOTOR to turn them off
Ron H said:You are driving the lower gates with emitter followers, so the gate drive will be 0.7V lower than the output voltage from your PIC. This may be too low to get good on resistance. The slow gate drive risetime also contributes to heating.
Styx, you need a nap. :shock:Styx said:Ron H said:Why would the gate voltage not go to VMOTOR when the NPN turns off?Styx said:What voltage is VMOTOR? This will help
I am suspecting that when you are ment to be actually turning OFF the upper FET's, the gate potential still has enough for conduction to occur, be it very slightly it is still an ACTIVE-region current and very lossy
The GATE of the top FET's need to be pulled upto VMOTOR to turn them off
Because the NPN need's to be turned-ON for the top FET to turn-OFF.
The Top-FET is a P-Type.
Without knowing what the VMOTOR is w.r.t. that Zener then dunno what is happening, might be nothing, but for just the TOP-FET's to be getting warm means they are drawing more current then they should be.
When the top-FET's turn-ON they will be saturated (just like when the bottom FET's turn-ON), thus the only time that ONLY the top-FET's can be dissipating more power is if they are not really OFF