Help with Amplifiying the Arduino AnalogOutput(0-5v) into 2-10v

I have a modulating valve I need to control that takes a 4-20 mA signal, or 2-10 vDc.

I am trying to use a transistor circuit to provide the 2-10 vDc from the arduino output, but obviously I am misunderstanding something in the math.

I’ve used a couple websites to try and understand it, but every page has different info. The best two I’ve seen are here:
https://backyardbrains.com/experiments/transistorDesign
http://www.learningaboutelectronics.com/Articles/Transistor-voltage-amplifier-circuit.php/

Here is my thought process.

I have a 12v source, and want the signal to oscillate between 2-10. Going by the logic in backyard brains I want Vc to be at the midpoint of my Vcc, so 6. My current will be 10 mA because I have no idea what else it would be or why. This will set my Rc to 600.

Now, since I want a gain of 2, my Re will be half Rc? so 300.

For the Arduino output, I need a base of 0.7v to start the process. This means my “DC offset” will be at 2.85, with an amplitude of 2.15 v.

As you can see in the attachments, I have a weird bump on the lower voltages. Why is this happening?

I have a weird bump on the lower voltages. Why is this happening?

Because the gain is too high for the signal. Either back off the gain or cut down the signal. Or try and control the offset level so when it overloads it does it symmetrically.

Is this simulated? If not what exactly is your 1KHz source?

However:-

I have a modulating valve I need to control that takes a 4-20 mA signal, or 2-10 vDc.

You can't say that. A 4-20 mA signal is a current loop control, you can't just replace that with a 2 - 10V DC signal. Current loops compensate for the length of cable by always keeping the current constant, something you can't do with voltage alone.

The base voltage is reduced by 0.7V (Ube) and occurs on R2 (Re). 5V will give (5-0.7)/300 = 14mA. Ub below 0.7V will result in almost 0mA.

Adjust Re for the maximum 20mA current @5V Ub, and program Ub minimum for 4mA.

Omit Rc when connecting the external device. You can replace it by a current meter, and insert the external load in series with or instead of the meter.

Hi,
Welcome to the forum.

Please read the first post in any forum entitled how to use this forum.
http://forum.arduino.cc/index.php/topic,148850.0.html .

Ops circuit and graph.

Can you please post a link to the specs/data of the modulating valve please?

Thanks.. Tom.. :slight_smile:

DrDiettrich:
Omit Rc when connecting the external device. You can replace it by a current meter, and insert the external load in series with or instead of the meter.

I am not sure I follow that, can you draw it out?

See OP's circuit, replace Rc by the meter and/or current loop load.

But then you just end up with a voltage follower on the emitter.

The circuit (also) is a tiypical constant current source.

The base voltage forces the corresponding current through the resistor, with that current (-Ib) entering at the collector. See #2.