POWERING IC's FROM PWM SOURCE

Thread Starter

dandy1

Joined Sep 30, 2017
112
I would like to power a 3144 hall sensor from a 12 volt PWM motor driver circuit with a minimum duty of 20% (1Khz)
ss1.jpg I do not want to waste power keeping the sensor active and is only required to detect the end of travel on an actuator.
does this circuit look feasible to power the sensor? or could use a zener regulator circuit instead?
Both regulator and sensor have about 5mA Quiescent current.

The 3144 states that its input only requires an unregulated power supply. Would the PWM driver qualify as an UNREGLATED power supply?
If so maybe better to skip the 78L05 and just clamp the output to suit uC.
 
Last edited:

MrSalts

Joined Apr 2, 2020
1,762
What's the frequency of the PWM?
Can you place a resistor between C4 and the regulator to drop about 4 volts at 5mA (680 ohm).
If the PWM is low frequency, change C4 to about 1000uF. If PWM frequency is over 5kHz, you'll be fine with what you have.
 

DickCappels

Joined Aug 21, 2008
8,708
I hope you realize that the diodes will peak detect the PWM signal, and that is good. The circuit should work ok whenever the PWM signal is present (above 9 volts) for a sufficient amount of time to keep C4 charged.
 

Thread Starter

dandy1

Joined Sep 30, 2017
112
I hope you realize that the diodes will peak detect the PWM signal, and that is good. The circuit should work ok whenever the PWM signal is present (above 9 volts) for a sufficient amount of time to keep C4 charged.
I don't understand what peak detect is or What is the significance of this?
The diodes are in place to prevent current from one leg of the motor driver to the other. A small convenience I suppose this will help prevent ripple on the regulation
 
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Thread Starter

dandy1

Joined Sep 30, 2017
112
What's the frequency of the PWM?
Can you place a resistor between C4 and the regulator to drop about 4 volts at 5mA (680 ohm).
If the PWM is low frequency, change C4 to about 1000uF. If PWM frequency is over 5kHz, you'll be fine with what you have.
Would an inductor replace purpose of the capacitor, (space constraints)
 

Papabravo

Joined Feb 24, 2006
18,422
I don't understand what peak detect is or What is the significance of this?
When the input goes one diode drop below the voltage on the capacitor, the diode will stop conducting, and the PWM can no longer hold the voltage on the capacitor and the capacitor will discharge through the regulator. As long as the PWM signal comes back reasonably quickly the capacitor will hold its voltage, but with some ripple.
 

crutschow

Joined Mar 14, 2008
29,792
Here's an LTspice simulation of the rectifier-capacitor with a 10mA load.
The capacitor charges to one diode-drop below the 12V peak PWM signal.
The voltage ripple is <100mVpp, so will have no significant effect on the regulator output voltage.

1647912452789.png
 

DickCappels

Joined Aug 21, 2008
8,708
I don't understand what peak detect is or What is the significance of this?
The diodes are in place to prevent current from one leg of the motor driver to the other. A small convenience I suppose this will help prevent ripple on the regulation
The to what "peak detect" means is given in @crutschow 's post above. The diodes will charge the capacitor up to the peak voltage of the pulse. The duty cycle will have very little to do with voltage across the capacitor.
 
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