# Problem using a voltage divider to input a voltage to a low impedance line

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?

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)

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?

how "low" impedance? 500? 50 ? 1 Ohm? how "fast" voltage has to be set?

Magician:
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.

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.

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.

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.

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?

MarkT:

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

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.

jackrae:
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.

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.

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 ?

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.

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.

Magician:

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!"

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

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
Arduino Playground - HomePage

The "load" I have is just one signal wire going into the device. Although they share a common ground, I can't connect the load up in series. Is there any way I can use a FET in that configuration? That's what I'm struggling with.

(Thanks for the reference to the tutorial, by the way. I'm just learning about transistors, and that tutorial is a helpful one I had not seen.)

How about an opamp buffer? Just wired as a voltage follower so you'll have 1M impedance to the opamp and depending which one you pick you will have a decent enough voltage source

winner10920:
How about an opamp buffer? Just wired as a voltage follower so you'll have 1M impedance to the opamp and depending which one you pick you will have a decent enough voltage source

That sounds it will work, thanks. The only problem being that I know less about op amps than I do about transistors. But I'll look into it. I even have an op amp, a OPA277P, but the chances that one will work seem small.

Actually that op amp is perfect for this situation since it has a very low offset voltage which will aallow you to go down to almost 0v on the output
lookup opamp voltage follower its extremely simple

I must be doing something wrong. I wired up the op amp as a voltage follower, and used it with the potentiometer wired up as a voltage divider. I get from 0 Volts to 5 Volts coming out of the potentiometer into the + input of the op amp.

But I get .3 Volts to 4.3 Volts coming out of the op amp when I leave the output floating. When I hook up the output of the op amp to the low impedance device input, I get only .3 Volts to 2.5 Volts output.

UPDATE: I should clarify that I am just using a + 5 Volts power source for now. So the 0 to 5 Volts that comes out of the potentiometer is correct. But the output of the op amp has a reduced range, although it seems to vary correctly within that range, smoothly without any jumps.