Need help determining PWM

panic mode

Joined Oct 10, 2011
1,782
to turn transistor on, you need some signal. for BJT that would be some base current, for mosfet that would be some gate voltage...

increasing (changing) that signal means transistor would conduct more current (this is used in amplifiers), basically transistor becomes controlled resistor. current through it becomes in some way related (proportional) to control signal.

but if we don't want this analog response. if we want transistor to work as a switch (full current), then we need larger input signal....

for power mosfets this usually means some 10-12V and maximum survivable gate voltage is usually some 20V. your mosfet is one of them. also the Vcc supply used for driver circuit is also 12V....

btw. few special mosfets are designed to be turned fully on using much lower voltage (3-5V) so they can be driven straight from logic gates (most logic gates operate at 5V or less). but not this mosfet...

now this explains what is required to turn the mosfet on, but there is more...

BJTs are connected in CC (common collector) configuration. this means the voltage gain is less than 1. in other words to get 10V out, you need to bring a bit more than 10V in (to base). although the voltage gain is less than spectacular, current gain is large and output impedance is low. this is exactly what one needs to charge/discharge gate capacitance fast.
 

Thread Starter

Rickmc3280

Joined Mar 9, 2014
13
Heya Panic. No progress on this, but i've been reading my electronics book and this makes more sense now. Also I see the datasheet was for the Mosfet not the transistor like I had thought.

In my mind, when looking at this motor controller, I know that the motor is rated for 90 volts dc and we ran it using a battery and it was extremely slow, so my mind if trying to figure out how the Mosfets are controlling the board, but I remember now that the motor is being driven post the bridge rectifier, but because I do not fully understand how all the components work, the schematics look like a foreign language with funny pictures of Animals?

So what I think I understand is that the mosfets need 10.1 ish+ but below 12v to run the gates? Then the transistors that "release" the electron flow need generally 5V+, so in theory regardless, that pwm signal coming out of the microprocessor is probably greater than 5V?

My concern is and has been I dont want to blow any of the components while learning & diagnosing this board in order to figure out the best way to control the board via a computer/arduino.

So at this point... what would be the best way to determine the voltage fluctuation of the PWM signal? Also have you heard of PC oscilloscopes? - basically you put a resistor in the speaker wire and attach it to an alligator clip or probe and let a program show you the wave forms when you attach it. I put a diode on mine in order to prevent a signal being sent back to the board and possibly frying it, but I have lately been thinking that its possibly a bad idea to add in the diode and it may prevent me from seeing the signal? Any thoughts (besides quit now before I hurt myself lol)?
 

Alec_t

Joined Sep 17, 2013
10,359
I have lately been thinking that its possibly a bad idea to add in the diode and it may prevent me from seeing the signal?
Remove the diode, but keep the resistor, if you are using the PC as an oscilloscope. Be aware that the PC soundcard probably has a 20kHz bandwidth only, so high frequency PWM signals won't be represented accurately.
 

Thread Starter

Rickmc3280

Joined Mar 9, 2014
13
Thanks Alec, Going to get my spare computer connected out with the treadmill later this evening to test it all out.
 

Thread Starter

Rickmc3280

Joined Mar 9, 2014
13
Took the diode out and reduced the resistance a little bit, and I got a square PWM frequency of about 88.93 hz coming out of the PIC12F508 chip which feeds into the microprocessor which then goes to the "other" pwm output which goes to the motor (PNP/NPN Transistors - Mosfets).

If I am not mistaken I believe it was 5V on GP2 which is the output. GP5 and 4 are the speed up and down button inputs to the chip.

Feeling pretty good about this because the Chip is not soldered in and I can easily remove it and test my own signal once i learn a little bit more about the board.

The Scope program which I used read 1.9v at full speed and then it went down to 1.5v. I am thinking the voltage is so low due to the resistance which I used a 100k resistor and a 10k (in series) so 110k ohms. Also since I used a "Line Input" on my audio card, I dont think its very accurate. I am afraid to put that pin to ground also. Any ideas? I am pretty sure that that it is 5V because before I knew anything about grounding I think I touched it to ground and got 5V?! Which would explain why it does not fully shut off once you stop the treadmill... works fine and doesnt start until i actually start it, but when it is supposed to stop it keeps going at about .1 mph which isnt too bad. (shrug).
 
Top