# Measuring voltage drop across a 100 ohm resistor from an isolated power supply

I want to measure the voltage drop across the resistor RS1. The op-amp is powered by a 5 V supply rail. The 50 VDC is coming from the same supply source but it's isolated using a step-up transformer and a rectifier circuit.

Where should the ground pin of the op-amp be connected? Is it safe to connect it to the ground of the +50 VDC rail or can I just connect to the battery ground (5 V power rail source) which is isolated from the 50 VDC rail?

• You need to show your op-amp power rails as part of your schematic. Also, be clear about the op-amp pin names. Jul 7, 2020 at 15:36
• 1. If you keep the 50 V ground isolated from the low-V ground, this circuit won't work. 2. If you connect the 50 V and low-V grounds, then you won't have galvanic isolation any more (actually, this circuit already breaks galvanic isolation). You'll need to say more about why you want isolation, and how you are using this circuit for us to be able to give you a useful answer. Jul 7, 2020 at 16:21
• I am using this circuit to measure the voltage drop across the 100 ohm resistor. There is not need for galvanic isolation. I just had to mention that because i am confused where the ground pin of the op-amp would go.The opamp is powered by a 5v from a regulator and the question is with the ground. How do i make my opamp to successfully buffer the voltage to an adc? Jul 8, 2020 at 6:02
• Simply connect your hand held meter across the resistor that will give you the voltage. If get a - reading and do not like it reverse the leads.
– Gil
Feb 23 at 21:36

It is OK to use an isolated transformer to boost a signal to 50V and measure its effective output dependant on the load.

This Load regulation of source voltage depends on the impedance ratio as your step-up voltage, impedance also steps up by the ratio squared.

However, if you know the range of loads to this secondary voltage that you expect, you can choose high resistance values and attenuate x0.1 to the 0 to 5V range while increasing the input range with gain and offset for sensitivity. e.g. If you want to measure the output with more gain using a range from say 45 to 50 you can use a 4.5V reference instead of the 0V for inverting gain but use the non-inverting input so that the gain and attenuation match to give overall unity gain.

simulate this circuit – Schematic created using CircuitLab

Then the gain of Vo/Vi= (50-45)/(5-0)= 1 but the apparent offset is the average of 45 to 50V.

Then using this hypothetical example you get this clever example. You may choose something different for design specs for gain, offset and isolation or leakage current. Depending on your application use of ground currents you may want something to reduce common mode rejection ratio or CM gain.

• This is a good insight. The voltage in the 50V power rail is not always 50V. I just used it as an example, it can go as low as 1V. I will spare you the background, its a complicated bit where I am changing the tapping of the transformer (like a autotransformer), different stages of voltage multiplier etc. so, the common mode range must be from 0v to 5.5V this way it can detect the drop even when the current flowing through the resistor is low. Jul 8, 2020 at 6:08
• Yes good answer but vague question Jul 8, 2020 at 6:11
• So, it is totally fine to connect the ground of the 50V power rail and opamp when the opamp is powered by the 5v power rail? :x i am sorry, i am just learning, this is a question that has bothered me. Jul 8, 2020 at 6:38
• Nothing wrong with just a common ground. What else are you not telling? Jul 8, 2020 at 6:39
• That's about it :p hahaha, I am just experimenting with circuits and came across this problem. So, you are telling me, that, a supply from secondary side of a transformer that is connected to voltage doubler circuit can share the same ground with a 5V power rail? that's awesome. Any disadvantages of this kind of connection? Jul 8, 2020 at 7:03