Discharge time in a transistor? sending pwm signal and using transistor as switch

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#1
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Hello, 

 

Im sending a PWM signal through a singular pin on my microcontrolller and then using several transistors as switches to allow me to control where the flow of the pwm signal goes on that line.

 

My main issue is that the PWM signal is distorted after the transistor. Before the transistor it is a very clean signal however after it, the signal looks as in there is a capacitor there discharging? here is what I mean. Im using a simple common collector setup on my bc547 transistor with a base resistor of 1k, and a emitter resistor of 100k to gnd.

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Last Edited: Sun. Mar 10, 2019 - 06:19 PM
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Reducing the value of the emitter resistor will speed

up the discharge. What is it connected to? It must

have some capacitance.

 

When your signal drops, the transistor shuts off

and because the emitter becomes reverse-biased

no current can flow back into the pin. All of the

charge stored in the capacitor has to dissipate

through the emitter resistor.

--Mike

 

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Even if i completely remove the emitter resistor and create an open circuit I get the exact same waveform. It also is not connected to anything right now. it was connected to an LED strip before hand however, i was getting weird results so I began troubleshooting. So, as of right now there is nothing connected to the other end other than my oscilloscope.

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Show your SCHEMATIC how do you expect help if we don't know have your hookup.

you haven't even mentioned what you've done with the collector.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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AH... All of my brain cells jumped out of my head for that last response. I removed the resistor and as expected same thing. but thats because I had read your statement wrong. I tried it with a 470 ohm resistor instead and it works phenomenal. check it out.

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Your oscilloscope probe has some capacitance.

 

An open circuit is worse since there's nowhere

for the current to go. Try 10K instead of 100K.

 

--Mike

 

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sir, calm down. A common collector setup means the collector is where the source is connected. So my pwm signal would have to be connected there. I guess i shouldve stated it instead of leaving you to guess that the only thing left to connect was the pwm source.

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check reply#5. I had misinterpreted your statement. Im using a 470 OHM resistor and everything looks identical to the source. thanks so much for your help.

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It's an emitter follower, right?

Transistors may take a while to come out of saturation. Maybe a speed up capacitor or Baker clamp would help, although the problem seems to be solved?

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the collector is where the source is connected. So my pwm signal would have to be connected there

Your schematic should always be provided to describe all 3 pins.    You could say common collector & we later learn you had a common base circuit all along.  

 

A common collector setup means the collector is where the source is connected. So my pwm signal would have to be connected there

No, you better try again

 

 

 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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I interpreted common-collector to mean an emitter follower.

The common-collector amplifier shown is for analog signals.

 

--Mike

 

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Also 470 ohms is not much. This will draw a lot

of current unnecessarily. Try 10K. If you don't

like the looks of it then try 4.7K, 2.2K, etc.

 

--Mike

 

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Since this PWM signal as described is actually a data line to trigger on and off leds, the current supplied is very small. Thanks so much for the advice.

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just to clear things up... im pretty sure you might be misinterpreting me. Essentially i have the base connected to my microcontroller which has a series resistor to create current to allow me to turn on and off the transistor. I then provide a PWM signal at the collector and I use my  microcontroller to allow me to switch on and off the flow of the PWM through the BJT by creating a current in its base. I then place a resistor at the emitter to allow the PWM signal when it goes low to provde a GND like state and not just be at the same level as the noise.

Last Edited: Sun. Mar 10, 2019 - 07:09 PM
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The common-collector amplifier shown is for analog signals.

It is shown because this  OP statement is confusing

 

A common collector setup means the collector is where the source is connected. So my pwm signal would have to be connected there 

 

The input of a common collector config is the base.  But apparently he does not intend the PWM be the signal source.  Again, reason for a schematic.

 

 

When your signal drops, the transistor shuts off

and because the emitter becomes reverse-biased

no current can flow back into the pin. All of the

charge stored in the capacitor has to dissipate

through the emitter resistor.

--Mike

 

Apparently he is not PWMing the transistor...it stays "ON" or "OFF", so the transistor charge is somewhat irrelevant.    I think he is just trying to build a mux, though he didn't use the that term.

Of course as the PWM occurs, the transistor operating point may sway around somewhat (not full on, or full off) 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Sun. Mar 10, 2019 - 09:23 PM
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I then place a resistor at the emitter to allow the PWM signal when it goes low to provde a GND like state

I'll just mention that it appears that you are using the transistor to switch on and off the PWM signal.

 

You can certainly use the NPN transistor that you are currently using.

 

You could also use a small signal NFet, such as the 2N7000 / 2N7002.

This doesn't require the series base resistor, and the voltage drop across the transistor is much lower than the NPN, when the transistor is turned on.

Your signal will, therefore, be closer to ground.

 

In the off state, both don't conduct, and the signal state is essentially the same.

 

A pull-down resistor on the Gate is still a good idea to hold the NFet off during power up of the micro when the I/O pin is an input, prior to your code making it an output, and setting it to high or low.

 

JC 

 

Edit: typo

Last Edited: Mon. Mar 11, 2019 - 11:50 AM
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kepe88578 wrote:

then using several transistors as switches to allow me to control where the flow of the pwm signal goes on that line.

 Thats what I was trying to explain here. I just could not get the proper terms out at the right time. Thanks for clearing everyone up.