Voltage to current convertion - Page 1
Forum section: electronics / beginners

author: TheBrick
author_rank: Regular Contributor
timestamp: November 06, 2012, 08:44:11 pm
content: Hello all.Another stupid question, I'm afraid.I was considering the problem of voltage to current conversion. Looking online I came upon the common solution show below.Analysing the circuit however I used the initial assumptions that due to the no current flows into a op amp rule that1. sum_j Ij= 0.Also because the op am will try and do what it can to make the inverting and non inverting inputs to be equal that,2. VA= Vin.This implies to me a requirement thatVin< GND ,where GND is the ground reference point and further more thatVS-< OR = Vin< GND.However intuitively this all seems wrong, I feel like there is an error in some of my initial assumptions that has lead me down this path. Can someone please correct me. I'm sure its something smacking me in the face.EDIT:Remove Latex formatting.

author: TheBrick
author_rank: Regular Contributor
timestamp: November 06, 2012, 08:45:12 pm
content: After that post I'd like to put in a request of Latex support for the forum!

author: IanB
author_rank: Super Contributor
timestamp: November 06, 2012, 09:08:34 pm
content: By "voltage to current conversion", do you mean you want an output current proportional to an input voltage?

author: TheBrick
author_rank: Regular Contributor
timestamp: November 06, 2012, 09:21:28 pm
content: Yep.E.g for the sake of examples and simple numbers4-20 Volts in 4-20mA out.R1would represent the resistance of the circuit we are trying to push the required current I_1 through.

author: Kremmen
author_rank: Super Contributor
timestamp: November 06, 2012, 09:23:43 pm
content: Quote from: TheBrick on November 06, 2012, 08:44:11 pmThis circuit will generate a current through the resistors proportional to the input voltage.The amp is in the non-inverting configuration. + input will float at Vin, causing the amp output to slew in the positive direction. This in turn will drive increasing current through the resistors into ground, until the node connected to amp - input is raised to the same voltage as the + input. At that point the amp is in balance and the output current is defined by the resistance connected to the output. All voltages are positive (or negative for that matter, but same polarity).

author: Kremmen
author_rank: Super Contributor
timestamp: November 06, 2012, 09:30:15 pm
content: Quote from: TheBrick on November 06, 2012, 09:21:28 pmNote that in this circuit I1 = I2 because the op amp input impedance is essentially infinite.R2 defines the current to voltage ratio. 1 kohm will result in 20V voltage at 20 mA current. The swing of the amp output limits the maximum value of R1. A typical max supply voltage of 30V would mean R1 can be max 500 ohms and you still can drive 20 mA through the resistors with 30 V max output.

author: TheBrick
author_rank: Regular Contributor
timestamp: November 06, 2012, 09:58:25 pm
content: Thanks Kremmen. That is how I understand the the system worksQuoteYep that is where I got mysum I_j =0I've just realised my mistake. I was taking I1 = I2 to be equal in magnitude but NOT in direction. I having the node A to have zero current flow at A in my head because of the op amp very high input impedance but for some reason had convince myself that I1 and I 2 where equal in magnitude but not in direction. This is of course is rubbish as it would require the op amp to sink 2*I_1 which violated my initial assumption of no current flow into the op amp. I knew I was a simple sign error or something. Grrr. I'm an Idiot. Its always the stupid stuff that trips you up!Thanks.