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Author Topic: Problem using a voltage divider to input a voltage to a low impedance line  (Read 3164 times)
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I had a problem using a voltage divider to input a voltage (between 0 and 12 volts) to a low impedance signal line. I used a potentiometer as the voltage divider, and as I turned the pot the signal voltage would suddenly jump from around 1 volt to over 10 volts. So that did not work.

My new plan is to use a TIP120 transistor to send the voltage signal. I'll use an Arduino analog PWM output to the transistor's gate, and put ground and +12 volts through the transistor. By varying the output from the Arduino from 0 to 255, I hope to get the whole range between 0 and 12 volts. 

Will that give a good voltage signal on the low impedance signal line?
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For low impedance driving you'll need the transistor in common-emitter mode (resistor between emitter and ground) with output taken from emitter terminal.  This will limit output signal to around 4.3 volts but one major advantage is that the output signal will be in-phase with the arduino PWM output.  The emitter resistance should be the same as your "low impedance" load (50ohms or whatever)
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Thanks for the reply. I need to get the full 0 to 12 Volt range on the output line. Is there any way to do that?
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how "low" impedance? 500? 50 ? 1 Ohm? how "fast" voltage has to be set?
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how "low" impedance? 500? 50 ? 1 Ohm? how "fast" voltage has to be set?

About 50 Ohms. Voltage does not have to be set quickly.
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I'm thinking about 2 options:
1. Motor driver L293/L298 or shield, powered 14-15 V DC ( to have some room for saturation, if load needs exactly 12V), than you PWM input as a motor driver, and set RC filtering at the outputs . So problem with this concept, could be high value of capacitors.
2. Audio Power amplifier (TDA2030 or similar), with RC filter at the input, and load connected directly to output. Basically, OPA with high load current capability. Regular OPA buffered with couple complementary darlingtons at the output will also works.
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Thanks for the suggestions. I'll look into them. One thing is that I do not need to filter the output to get rid of the PWM. The output signal line can take PWM fine.
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I decided to give the TIP120 transistor a try. It looks like it should work. It does have a voltage drop of about 1.3 Volts. But it gives a nice voltage range without the big jump I had when using the potentiometer.

I'm got another circuit with a MOSFET that I think I will build and try out. That may give me close to the full 0 to 12 Volt range.
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If you don't need to filter the PWM then you just need a circuit to switch 12V at upto .25 A.  What is the load?
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What is the load?

I need to send a signal of between 0 and 12 Volt to an electric-over-hydraulic trailer brake.
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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.
« Last Edit: July 27, 2012, 10:00:06 pm by Daanii » Logged

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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. 
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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 ?   
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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!"
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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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