Help with level shifting circuit

[George.F]

Hi.

I have an analog input DC-signal from 0-45V that I want to measure. But I am onlu interested in measuring between approx. 33V to 45V, as I really don't care if the signal goes below 33V. The interval between 33v - 45V I then wan't to scale to 0-5V so it can be read by my Arduino ADC port.

My first approach was to design some kind of op-amp level shifter, but I really can't get it right. Anyone here that can help me with some pointers or suggestions on how to do this!?

Thanks in advance,
G.F

 

[FvM]

Look for https://en.wikipedia.org/wiki/Operational_amplifier_applications#Differential_amplifier

 

[George.F]

Hmm, thanks ...

So a diff setup with an opamp, setting a reference voltage of 33V to cut the voltage down, and then a ratio of about 0,42 at the resistor would get me pretty close.

I'm a little worried thoug if my voltage reference of 33V drops for some reason I'll get higher voltage than 5V on my output, and that might harm my electronics? Or will this automatically be fixed if I supply my OP-amp with maximum 5V?

Ghee, sry for my noobish in op-amp, I'm feeling really rusty!

 

[pplus]

concept

 

[FvM]

If the resistors are calculated correctly, thevoltage at OP inputs must be below 5V supply voltage. Higher voltages are clamped by the substrate diodes. The resistors should assure that currents into the OPinput can't be higher than e.g. 1 mA.

 

[erikl]

I have an analog input DC-signal from 0-45V that I want to measure. But I am onlu interested in measuring between approx. 33V to 45V, as I really don't care if the signal goes below 33V. The interval between 33v - 45V I then wan't to scale to 0-5V so it can be read by my Arduino ADC port.

I've calculated the difference amplifier for your level shifting pb. up to the advices from this link, which seems to work well, see the PDF (generated from an Excel sheet): View attachment Diff_amp1.pdf.

I used a ref. voltage of 5V at the inv. input resistor. In order to get the full output voltage range of 0..5V I'd suggest to supply the amp with a slightly larger supply voltage, at least -1/+6V .

 

[ashugtiwari]

use zener as voltage reference, it will also protect ur circuit from surges. I dont know how to use it in op-amp for level shifting.
You can surely go for voltage divider circuit at one input of op-amp with capacitor at its input terminal to protect any surges and configure op-amp as voltage follower. If voltage goes above 48V i.e. the input of op-amp goes above 5V, u can clamp the output to maintain it to 5V.