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Author Topic: Problem using a voltage divider to input a voltage to a low impedance line  (Read 2649 times)
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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

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.)
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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
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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.
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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
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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.
« Last Edit: July 28, 2012, 10:02:43 pm by Daanii » Logged

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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.

That is normal behavior for many op-amps (esp older types) unless you obtain one that is specified to have 'rail to rail' output voltage capability. So check the datasheet for your specific op-amp type to see what it's output voltage swing spec is.

Lefty 

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Ah, thought it was good, but looked closer and its not rail to rail and also only has 35ma output max so doesn't help much
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That is normal behavior for many op-amps (esp older types) unless you obtain one that is specified to have 'rail to rail' output voltage capability. So check the datasheet for your specific op-amp type to see what it's output voltage swing spec is.

Lefty

Ah, thought it was good, but looked closer and its not rail to rail and also only has 35ma output max so doesn't help much

Oh well, guess I'll have to get a different op amp. At least I got the op amp I have out and gave it a try, and learned something from it.

I'll order a few chips and try a few things. Like the two chips Magician suggested. And a TI chip I found that might work. And I decided to get a commercial in-cab brake controller after all to take apart and see how they built it.

Any other suggestions very welcome. Thanks for all the help so far.
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Ah, thought it was good, but looked closer and its not rail to rail and also only has 35ma output max so doesn't help much

Most op-amp aren't designed to supply much output current, 35ma is probably pretty typical. Most circuits needing more current drive from an op-amp stage just wire in a transistor as a voltage follower that can supply higher output current. There are special op-amps designed with higher output current capability but you will pay a premium for it compared to more standard op-amps.

Lefty
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Most op-amp aren't designed to supply much output current, 35ma is probably pretty typical. Most circuits needing more current drive from an op-amp stage just wire in a transistor as a voltage follower that can supply higher output current. There are special op-amps designed with higher output current capability but you will pay a premium for it compared to more standard op-amps.

Lefty


Yes, it looks like an op amp is not going to help here. I need to send a voltage signal to a low-impedance device (I think it's about 50 Ohms) that ranges between 0 Volts and 12 Volts. That means the current will range between 0 Amps and 240 milliamps. Well outside the range of any op amps I can find.

Some people have had some good ideas here. I'll wait until the stuff I've ordered comes, try it out, see if anything works, and post again.

Thanks for the help.
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You seem to be making hard work out of this project.
You can drive an open collector transistor or FET from the arduino. Your 50ohms load is connected between your 12 volt supply rail and the collector of the transistor.
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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.

If there's a datasheet for the device it would be really re-assuring to see it - I don't know how such a thing works and what sort of frequencies of PWM are suitable.

Also automotive devices often are permanently connected to the chassis/ground and can only be switched on the high-side - if this is so you'll need a PNP or p-channel device to switch and some kind of level-shifting circuit to control this from the Arduino.
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You seem to be making hard work out of this project.

I certainly am making hard work out of this project, aren't I. Two or three times, I thought I had it solved, only to have it not work and have to go back to the drawing board.

You can drive an open collector transistor or FET from the arduino. Your 50ohms load is connected between your 12 volt supply rail and the collector of the transistor.

I can't do that because I only have one wire that goes to my "load." Not two.

If there's a datasheet for the device it would be really re-assuring to see it - I don't know how such a thing works and what sort of frequencies of PWM are suitable.


There is no datasheet. There is a manual: www.championtrailers.com/BRAKERITEMANUAL.pdf The wiring diagram is on page 19 of the manual. I called up Titan and they said that PWM will work fine, at frequencies from 200 Hertz to 20 kilohertz.

Also automotive devices often are permanently connected to the chassis/ground and can only be switched on the high-side - if this is so you'll need a PNP or p-channel device to switch and some kind of level-shifting circuit to control this from the Arduino.

I think that is the situation here. The device is wired to ground and I just have one signal wire that I need to work with. I need to source a signal between 0 Volts and 12 Volts using the Arduino, at currents up to about 240 milliamps. At least that is what I have come up with so far.
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I can't do that because I only have one wire that goes to my "load." Not two.
No you always have two wires to connect to the load otherwise you can't get any current to flow.
Your other "hidden" wire is either the ground or the +ve power supply.
What do you need to do wit this wire to turn it on. If it is supply +ve then the ground is your other control wire.
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I can't do that because I only have one wire that goes to my "load." Not two.
No you always have two wires to connect to the load otherwise you can't get any current to flow.
Your other "hidden" wire is either the ground or the +ve power supply.
What do you need to do wit this wire to turn it on. If it is supply +ve then the ground is your other control wire.

You're right, of course. The "hidden" wire in my case is the ground. What I meant, and should have said, is that I cannot connect my load in series between the 12 Volt power supply and the collector of the transistor because the ground wire must be connected directly to the battery's negative terminal. I cannot control the ground wire.

My device has three wires going into it. (Actually, five wires, but two of them are not important here.) One wire is 12 Volt power, and needs to be connected to the battery's positive terminal. One wire is ground, and needs to be connected to the battery's negative terminal. The third wire is the signal wire, which needs to vary between 0 Volts and 12 Volts.
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