 # LTspice

Hi,

what should be the Gain resistor Rg value and the sense resistor …in order to get output voltage of 2.5 for 200mA current. Can you suggest me.

Thanks.

Do you mean 20mA?

No its 200mA .. but the ckt shows 20mA

Vout - VREF = G(Vin+ - Vin-)

Where G = (49.4k/RG) + 1

RG = 49.4K/(G-1)

Vout = 2.5V

(Vin+ - Vin-) = 2.5V = 0.02A * 125

So RG = 0 Ohms

Let's say 2,5 are Volts . . .

Let's say Rg is R2 . . .

Ohm's law: Rg= 2.5 V / 0.2 A = 12.5 Ohm.

Regards

Vout - VREF = G(Vin+ - Vin-)

Where G = (49.4k/RG) + 1

RG = 49.4K/(G-1)

Vout = 2.5V

(Vin+ - Vin-) = 2.5V = 0.02A * 125

So RG = 0 Ohms

:o :o :fearful: :fearful:

Where do all those values come from?

OK. OK. I see them. :-X

But when i simulate it in lt spice the Vout pin shows around -600mV

I think OP is trying to make a current sense amplifier with a 200mA range.

forum.arduino.cc/index.php?topic=343696

In that case 125ohm is totally wrong.

Please tell us WHAT you want to measure/control. Maybe something like this is a better solution. Leo..

BillHo:
Vout - VREF = G(Vin+ - Vin-)

Where G = (49.4k/RG) + 1

RG = 49.4K/(G-1)

Vout = 2.5V

(Vin+ - Vin-) = 2.5V = 0.02A * 125

So RG = 0 Ohms

I think you’ve made an error.
G = 1
Rg = 1/0 Ohms => Rg is infinite.

You’ve wired it as inverting, then inverted again by the LT1413 following amp.

BillHo: Vout - VREF = G(Vin+ - Vin-)

Where G = (49.4k/RG) + 1

RG = 49.4K/(G-1)

Vout = 2.5V

(Vin+ - Vin-) = 2.5V = 0.02A * 125

So RG = 0 Ohms

Simulation results Doesn't show the 2.5V output .. rather it shows around 650mV

Wawa: I think OP is trying to make a current sense amplifier with a 200mA range.

forum.arduino.cc/index.php?topic=343696

In that case 125ohm is totally wrong.

Please tell us WHAT you want to measure/control. Maybe something like this is a better solution. Leo..

I want to measure the current in range +-200mA ... Can you tell how the circuit diagram look .. that is used in the site you mentioned

Adafruit board can be set to +/-400mA with 0.1mA resolution. 12-bit A/D onboard. digital output of voltage and current.

If I change the input current to 0.1… the output remains the as it was for 0.2

somavaram: If I change the input current to 0.1.. the output remains the as it was for 0.2

0.1 amps through 125Ω is 12.5 volts. Your circuit is powered with +5V and -5V.

Archibald: 0.1 amps through 125Ω is 12.5 volts. Your circuit is powered with +5V and -5V.

I made the resistor value to 12.5ohm .. but its showing the same

Try something like the attachment.

R2 is sized assuming +/- 200 mA I1 to get voltage in the range of +/- 1.64 V, that is, within the range of the Arduino A/D

Rg is absent (infinite) so U1 is unity gain, that is, the output is +/- 1.64 V

R4 and R5 set Vref to 2 volts so the output of U1 will be 2 V +/- 1.64 V, that is, 0.36 to 3.64 V. You can try to change R2 to 0.05 Ohms, VRFE connected to Gnd and RG = 198.4 Ohms.
you have to invert your current source (arrow down).

Don't forget a 4k7 or 10k resistor between opamp and Arduino pin. Leo..