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Topic: Problem using a voltage divider to input a voltage to a low impedance line (Read 3 times) previous topic - next topic

Daanii

Well, I'm clearly not understanding something here.

I tried the TIP120 transistor (a Darlington) using 0 to 5 Volts instead of 0 to 12 Volts. The highest output I got was 3.7 Volts. Then I tried using a MOSFET and the highest output was less, about 3.0 Volts. I can't figure out why I cannot get the full 5 Volts.

What am I missing?

UPDATE: This situation is about what Jackrae said in post #2.

This will limit output signal to around 4.3 volts
But I do not understand why the voltage is limited. I tried using 12 Volts as the supply (rather than 5 Volts) and got about the same results.

jackrae

A transistor used in emitter follower mode has the "load" between emitter and ground.  Therefore "all" of the output voltage appears across this load.  There must always be a drive voltage between base and emitter to produce the base-to-emitter drive current.  Hence the emitter voltage can never be greater than the base voltage (both relative to ground).   If the base voltage is the PWM output of the arduino - a nominal 5 volts, it follows that the emitter voltage can never be greater than the arduino voltage. 

Magician

Quote
I need to send a signal of between 0 and 12 Volt to an electric-over-hydraulic trailer brake.

I stumbled, why not put simple question in subject line "how to switch my trailer brake?"
Low impedance line ? 

Daanii


A transistor used in emitter follower mode has the "load" between emitter and ground.  Therefore "all" of the output voltage appears across this load.  There must always be a drive voltage between base and emitter to produce the base-to-emitter drive current.  Hence the emitter voltage can never be greater than the base voltage (both relative to ground).   If the base voltage is the PWM output of the arduino - a nominal 5 volts, it follows that the emitter voltage can never be greater than the arduino voltage. 


Thanks for the explanation. That makes sense. It's nice to understand why I was seeing what I was seeing. It had me stumped, and I had not yet found an explanation elsewhere.

Now the question is--how can I get the 0 to 12 Volt range that I want? Obviously it is not going to be in the common emitter mode. I'll keep looking for an answer. Any help will be appreciated.


Quote
I need to send a signal of between 0 and 12 Volt to an electric-over-hydraulic trailer brake.

I stumbled, why not put simple question in subject line "how to switch my trailer brake?"
Low impedance line ?   


I didn't mention what the load was because I did not know that information was important. Plus I did not want to spark a lot of discussion along the lines of: "Just use a commercial in-cab brake controller." or "It's dangerous and stupid to use a homemade brake controller on the public roads. You'll kill us all!"

jackrae

Since all you want to do is drive a low impedance load using PWM, why not use a simple FET driver circuit as per this arduino tutorial
http://www.arduino.cc/playground/Learning/SolenoidTutorial

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