Input protection for current sensing and clamping diode to avoid undoing gain. - Page 1
Forum section: electronics / projects

author: i509VCB
author_rank: Contributor
timestamp: January 09, 2025, 09:54:23 pm
content: I've been working on a current measurement circuit for a larger project. The main thing to note is that the measurement range tops out at 2A.Firstly I need to prevent the output of the circuit from exceeding 3.3V. This would obviously be the application for a voltage divider. However since I apply a gain of 10 at the AD8421, dividing it down from +15V at a theoretical maximum to 3.3V would effectively undo half of the gain I just applied to the input signal (G = 1-100 -> 0.2->20)I will generally only be dealing with small voltage drops across the sense resistor (I set 100mV as the maximum drop I can measure, but want to handle cases where it goes above). I think a clamping zener diode (with a Vz of 3.0V) could do the job? (shown below). The resistor value might be too high, just a temporary value. Is this a reasonable idea?Secondly am looking for suggestions for input protection for the circuit. The AD8421 at the front of the circuit should protect me up to +-40V per the datasheet (I only plan to use this up to +12V and supply the amp with +15V for extra margin).The concern I have is preventing the sense resistors from melting away. With a 2A current draw and a drop across the 50mOhm resistor, my speced part with a 1W current rating should be no issue for a 100mV drop. I was initially thinking of putting a resettable fuse at DUT+ and DUT- that would trip at 3A, but from searching for parts most resetable fuses have a relatively high resistance (250mOhm for example).

author: moffy
author_rank: Super Contributor
timestamp: January 10, 2025, 03:38:31 am
content: The AD8421 has restrictions on input voltage and output voltage ranges, you have the -ve supply connected to ground, that will affect the input common mode voltage range (-Vs+2.3V to +Vs-1.8V) as well as the output voltage swing, the minimum -ve voltage is -Vs + 1.2V which in your case would be +1.2V.

author: i509VCB
author_rank: Contributor
timestamp: January 10, 2025, 06:14:12 am
content: Quote from: moffy on January 10, 2025, 03:38:31 amWell I guess I need a negative rail now.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: January 10, 2025, 09:20:24 am
content: I don't see the need for the zener diode, but a 10k resistor at the input of the TVL9361 is necessary to protect the internal diode from excessive current when the voltage limit is exceeded.

author: youngda9
author_rank: Regular Contributor
timestamp: January 10, 2025, 01:44:55 pm
content: Put a resistor and zener at the input of the first amplifier to protect the inputs.The first amplifier can drive the second input above it's 3.3v input voltage range. So put a voltage divider (10.2K/3.4K) between the two amplifiers so that the second amplifier can't be over-driven. You can then delete the zener clamp circuit between the amplifiers.

author: mtwieg
author_rank: Frequent Contributor
timestamp: January 11, 2025, 03:02:25 pm
content: Quote from: i509VCB on January 10, 2025, 06:14:12 amOr you could just use a different sense amplifier, like the INA240/INA241. Use the same 3.3V single supply as the next amplifier stage op amp, eliminating any concern with overvoltage.