Transistor Inverter Question

[pittuck]

Hi,

I am using a NPN type transistor to create a inverted version of a PWM signal, i am using a 2n2222a with base connected to the PWM output of a PIC though a resistor (1K).

The emitter to GND and a 4.7k resistor from the collector to 5V and the inverted output will be taken from between the collector and resistor. BUT i also need a buffered version of the original signal, is it possible to get this from the one transistor??

I am modifying my design for a motor controller a bit, i used to use one PWM signal on the enable line of the H-Brisge IC the turn the bridge on/off quickly creating the average voltage over the motors to control speed.

But no i am going to keep the enable high, and put PWM signals on the input pins (2) so i can use a 50% duty to have 0 speed and then control speed and direction through the one 10-bit PWM duty register. It uses more current but should allow much better control over speed!

Regards,

- Martyn

 

[audioguru]

Hi Martyn,
You could put a 4.7K resistor from the transistor's emitter to ground to get a buffered input at its emitter, but it and the collector's amplitude will be half.

A 50-50 duty-cycle for the motor at idle will create a lot of heat. Check-out the way TI idles their PWM in their class-D audio amps. The pulses sort-of overlap. www.ti.com look at Products, audio power amplifiers.

 

[pittuck]

Ok thanks,

Humm, ok.

I was thinking the max difference between base going high and the transistor turning on/off is 300ns, do i have to buffer it? or are there any other inverters i could use (I have quite a few logic chips around, i know i have some hex inverters somewhere, could i use this???)

I will have control of the enable pins, so when idle state is needed i can turn the h-bidge off. The only problem was with the PWM on the enable pins i had to put pwm to 100% and put the direction pins to both on / off which meant extra overhead.

Putting PWM to 50% and then putting a pin low is not as difficult, anyhow, there should be no real problems with power draw when idle.