Voltage to current converter

[mossy5000]

Hi Guys. I want to convert say a 5 volt input signal to 0-100mA output, adjustable via a pot. Also i would like
to convert another 5 volt input to 0-100mV ouptut signal. Any ideas on where to start? I can buy something
that does it for me from RS components (610-348) but as i need 3 of these that becomes quite expensive.

Many thanks for any help

Dean

 

[MikeMl]

Does the 100mA source share a common ground with the input signal?
What is the highest resistance value that you need to force 100mA through? (Compliance)

How much current does the 100mv signal have to provide?

 

[mossy5000]

Hi Mike, The 100mA source can share a common ground it is going into a piece of kit that compares the difference between two mA inputs.
The 100mV signal will not have to supply hardly any current as again is only doing a comparison.

 

[MikeMl]

Hi Mike, The 100mA source can share a common ground it is going into a piece of kit that compares the difference between two mA inputs.

Ok, but what resistance is the 100mA flowing through?

The 100mV signal will not have to supply hardly any current as again is only doing a comparison.

A simple two resistor voltage divider. 0.1 = 5*R2/(R1+R2)
Say R2 = 100Ω, then 0.1 = 5*100/(R1+100)
R1+100 = 5*100/0.1 = 5000
R1=5000-100 = 4900

Or a 5K 10turn trimpot.

 

[mossy5000]

Hi Mike, the device has a burden resistor of 310 ohms

 

[MikeMl]

Hi Mike, the device has a burden resistor of 310 ohms

So how about this? You have a 5V low-impedance (voltage source) input, right. If you connected it directly to the 310Ω burden resistor, the current that would flow is I = E/R = 5/310 = 16mA, far short of 100mA.

Let's turn the calculation around. How big does the input signal have to be to force 100mA into 310Ω? E=I*R= 0.1*310= 31V. So you will need an amplifier that makes a 31V signal out the 5V one.

I'm beginning to question the value of the burden resistor???

Can you post the schematic of the input stage of your device?

 

[schmitt trigger]

Google current to voltage converters.
But as MikeMi mentions, you do require to apply ohm's law to determine if the combination burden resistor and available supply voltage are physically possible.

Why do you need such a large current anyways? Most current loop transducers are 20 mA.

 

[mossy5000]

Hi Guys, I am developing a test for a DCP126 Impedance relay. These devices read input's from a 10KA circuit breaker, which has various outputs two being a 0-20mA which gives the state of the Breaker. What i want to do is simulate "faults" and be alble to vary the 100mA and 100mV inputs to the impedance relay. I have googled this and found some signal coverters that will do this for me, just wanted to do something a bit cheaper, as am looking to build a significant number of these.

 

[MikeMl]

A 0-31V adjustable power supply will cause 0-100mA to flow through 310Ω.