# Ramping voltage to current converter?

#### 24Volts

Mar 21, 2010
164
hello,

I have a ramping voltage signal of 0.4VDC to 1VDC commingbout of an op amp.

I need that signal to control linearly an IR
led’s current that can vary from 0-1A.

Therefore I want to convert a
0,4VDC-1VDC to 0A -1A

Therefore the output of my
op amp needs to go to a device
that can drive the led.

I was thinking of using a transistor but
not sure it would work!

What would be the best device to do this with?

Thanks for any help

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Compare your ramped voltage to the voltage across a resistor in series with the device. The op amp will cause the current to be proportional to the input voltage, and the choice of resistor allows you to determine what the ratio is.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,766
Google "voltage controlled current source". Here's just one example.

#### 24Volts

Mar 21, 2010
164
Compare your ramped voltage to the voltage across a resistor in series with the device. The op amp will cause the current to be proportional to the input voltage, and the choice of resistor allows you to determine what the ratio is.

Hi Steve,

It’s 5 years ago I haven’t touched this circuit so I’m a littl out of touch....

So you mean connect one end of the resistor to the output of my op amp and the other end to of the resistor to the positive of my Led and the other side of my led to ground?

If that’s not it can you draw a schematic .
This would really help.

Thanks Steve!

Last edited:

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Harald's suggestion will point you in the right direction.

You have 2 things to do.

One is to create a variable current source that will carry between 0 and 1A for a voltage varying between 0V and 0.6V (a 0.6Ω current sense resistor may be useful here).

The other is to scale the input voltage to subtract 0.4V from it. A summing amplifier with a negative voltage on one input is one method.

#### 24Volts

Mar 21, 2010
164
Hello fellows,

Ok I need to review all my transistor and op amp theory notes cause I don’t remember
a lot of stuff. Also I realized I never really built a voltage controlled current source circuit so I’ve got a lot of work on my plate here :-(

So I googled “voltage controlled current source” and found a video tutorial.

So for starters is this video close to what I want to do:

The only thing that différés from what I need to do is that how do I connect my ramping voltage from the output of my op amp to the circuits in the video?

Also the circuits in the video has less( or slightly different components than Steve’s circuit)?

Is it possible to obtain a full circuit schematic example that’s adapted to voltage signals? I don’t mean to seem like I just want the answers but a circuit example would allow me to post relevant questions to the circuit as I review my notes.

Thanks for your help.... guys much much appreciated !
24v

Last edited:

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
The circuit you show is not a voltage to current circuit, and operates open loop so it's not very accurate or stable over temperature, load, and line variations.

Look here. The sense resistor in this case is 50Ω which will have 1V across it at 20mA. In your case you want 1A at 0.6V, so change the resistor appropriately. You will also need a transistor which can handle the higher current. I would pick a Darlington or maybe a mosfet. If you use a mosfet, a 50Ω gate resistor would be a good idea.

Last edited:

#### 24Volts

Mar 21, 2010
164
The circuit you show is not a voltage to current circuit, and operates open loop so it's not very accurate or stable over temperature, load, and line variations.

Look here. The sense resistor in this case is 50Ω which will have 1V across it at 20mA. In your case you want 1A at 0.6V, so change the resistor appropriately. You will also need a transistor which can handle the higher current. I would pick a Darlington or maybe a mosfet. If you use a mosfet, a 50Ω gate resistor would be a good idea.

Ok so where does my output of my op amp connect to you circuit
Between R5 and the power source?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Vin is shown as being connected to R5 and being with respect to ground.

What you need between your op amp that produces a voltage varying between 0.4V and 1V is a voltage adder . This must add -0.4V so the voltage swing will be from 0V to 0.6V.

Google non inverting summing amplifier and you'll get something like this.

#### 24Volts

Mar 21, 2010
164
Vin is shown as being connected to R5 and being with respect to ground.

What you need between your op amp that produces a voltage varying between 0.4V and 1V is a voltage adder . This must add -0.4V so the voltage swing will be from 0V to 0.6V.

Google non inverting summing amplifier and you'll get something like this.
Ok steve thanks for your help...

I’ll review some electronics and get back to you.

I think today we all try to do too much, lol

Between doing company administration, MCU programming, CPLD and RF development studio softwares and PCB manufacturing softwares .... where is one on top of all that going to get time to review electronics .... lol

Ill get back to you on this post in a couple of weeks until I get some intelligent questions to ask you....

Talk soon

Thanks

#### BobK

Jan 5, 2010
7,682
For the voltage range converter, it would be easier to use a difference amplifier with the inverting input connected to a 0.4V reference. Then, using a rail-to-rail op amp, you would not need a negative supply. The rest of the circuit is that same, a voltage controlled current source that puts out 1A at 0.6V.

Bob

#### 24Volts

Mar 21, 2010
164
Hi Bob,

A lm324 ... is it a rail to rail op amp?
Cause that’s the only one I have currently

Thanks

#### BobK

Jan 5, 2010
7,682
It is not rail to rail but it is single supply. It can go to near zero, but not all the way up to the supply voltage. That would be okay for your application.

Bob

#### 24Volts

Mar 21, 2010
164
Ok thank you Bob.....

It’s very appreciated!

24V

#### 24Volts

Mar 21, 2010
164
Hi Bob,

After looking at the LM324 it seems like it’s
Single or dual supply.... I could be wrong
but it says +/-16VDC

Anyways I don’t have the data sheet in front of me but I do remember seeing that it can work
As single supply ground to +32VDV
Or
+16VDC to -16VDC

24v

#### BobK

Jan 5, 2010
7,682
Yes, it is single supply.

Dual supply op amps cannot have output a voltage near either rail. The only way you can get an output of zero is to have a positive and a negative supply.

Single supply op amps can have an output near zero (but not exactly, it will typically be limited to a few 10s of mV). They cannot output near the upper rail.

Rail-to-rail op amps can output near either rail. You can run them on a single supply and have a range of outputs that nearly goes from 0 to the full supply voltage.

Bob

Mar 21, 2010
164
Thanks Bob

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