Voltage-to-Current (V-I) converter module

Thread Starter

RicAlSSilva

Joined Nov 8, 2023
5
Hi!

I am currently in search for a Voltage-to-Current (V-I) converter module that can provide an output current ranging from 0 to at least 4 A, controlled by a voltage between 0 and 10 V or something like that.

If it has galvanic isolation between the input and output would be perfect, so I could bias the output. But it is not mandatory, I could pass the control voltage in a optocoupler before.

The idea was to have this in some kind of module to integrate in a controller box for an instrument.

Does anyone know if there is anything like this on the market? I've been searching but I've only found something like this with a maximum current output of 1A. Thanks in advance!
 

Jerry-Hat-Trick

Joined Aug 31, 2022
494
If the current can be made to flow through a low value resistor to ground then the voltage across it can be fed back to the -ve input of an op amp with the control voltage into the +ve input. Output of the op amp into the base of an NPN transistor will control the current so that the voltage into both op amp inputs are the same. Better to use a MOSFET as there is (virtually) no current into the gate whereas a bipolar transistor has some base to emitter current related to it's gain.

The resistor needs to dissipate power so an array of 0.5W resistors in parallel to make say 0.5R would need to dissipate a total of 8W. The voltage at 4A will be 2V so the control voltage needs to be dropped with a potential divider. Use a rail to rail op amp.
 

Thread Starter

RicAlSSilva

Joined Nov 8, 2023
5
If the current can be made to flow through a low value resistor to ground then the voltage across it can be fed back to the -ve input of an op amp with the control voltage into the +ve input. Output of the op amp into the base of an NPN transistor will control the current so that the voltage into both op amp inputs are the same. Better to use a MOSFET as there is (virtually) no current into the gate whereas a bipolar transistor has some base to emitter current related to it's gain.

The resistor needs to dissipate power so an array of 0.5W resistors in parallel to make say 0.5R would need to dissipate a total of 8W. The voltage at 4A will be 2V so the control voltage needs to be dropped with a potential divider. Use a rail to rail op amp.
Thank you for your answer. If necessary I'll build one, yes. But I'm wondering if there isn't something similar on the market already.
 

sghioto

Joined Dec 31, 2017
5,369
What is the load and what is the supply voltage this 0 to 10 volt control voltage will be applied to?
Is this for dimming LEDs?
 

Thread Starter

RicAlSSilva

Joined Nov 8, 2023
5
It will be to turn on a filament in vacuum. So less than 1 ohm load that will increase when the filament becomes bright. But I expect 5 or 6 W maximum. Anyway, it would need to have a small power supply.
 

Jerry-Hat-Trick

Joined Aug 31, 2022
494
What voltage power supply ?
Good question. 4A into 1R means 16W which is way more than "6W maximum". Even if the supply voltage is just enough to supply the desired current the transistor will be dissipating a lot of heat. In practice, if controlling brightness is the real objective then some kind of PWM switching of the transistor, maybe with a 555 controlled by an LDR, would be do the trick.
 

Thread Starter

RicAlSSilva

Joined Nov 8, 2023
5
Ok. I will give more information about the project. A 0.15 mm diameter filament of thoriated tungsten with a length of 12 mm should dissipate a power of about 5 W when approaching 2.8 A of current. This is what I hope to consume. When I talked about the 4 A, it was to have a margin to be comfortable. In fact, I will need a considerable transformer to provide the necessary power, but this is an unavoidable problem. And I will also need a large heatsink, as you suggested.
The purpose of the filament is not to produce radiation, but rather to extract a current of electrons through an electric field (in a thermionic emission). Sparing the details, what I will basically control is the temperature and I will have a feedback circuit defining the control voltage of the current source.

Returning to my question, what I was really interested in was knowing if there is any commercial solution available. I don't need the power supply, I don't need the feedback to control, I just need something that receives a voltage control and gives a controled current as the output.

Thank you for you comments and advices:)
 

Jerry-Hat-Trick

Joined Aug 31, 2022
494
Ok. I will give more information about the project. A 0.15 mm diameter filament of thoriated tungsten with a length of 12 mm should dissipate a power of about 5 W when approaching 2.8 A of current. This is what I hope to consume. When I talked about the 4 A, it was to have a margin to be comfortable. In fact, I will need a considerable transformer to provide the necessary power, but this is an unavoidable problem. And I will also need a large heatsink, as you suggested.
The purpose of the filament is not to produce radiation, but rather to extract a current of electrons through an electric field (in a thermionic emission). Sparing the details, what I will basically control is the temperature and I will have a feedback circuit defining the control voltage of the current source.

Returning to my question, what I was really interested in was knowing if there is any commercial solution available. I don't need the power supply, I don't need the feedback to control, I just need something that receives a voltage control and gives a controled current as the output.

Thank you for you comments and advices:)
Thanks for sharing some detail. My thinking is that the original idea I suggested with the op amp controlling current should work fine. If you start with a 5V 3A power supply like https://www.amazon.co.uk/Multibao-3...nt-Black/dp/B0BGGV97KT/ref=asc_df_B0BGGV97KT/ you should be okay - enough voltage headroom, a higher current 5V supply would be nice if you can find one.

A voltage supply higher than 5V would mean losing more power in the transistor, hence a bigger heatsink. The beauty of these switch mode power supplies is that they are really efficient, losing minumum power. Using a darlington pair power transistor https://uk.farnell.com/multicomp/tip120/darlington-trans-npn-60v-5a-to/dp/9294210 with a gain of 1,000 would require the op amp output to rise above the voltage across the control resistor by around just 1.2V - I'd choose this rather than a MOSFET.
 
Top