Current sense amplifier protection  - Page 1
Forum section: electronics / projects

author: tdarlic
author_rank: Contributor
timestamp: April 07, 2018, 09:44:43 am
content: Hi guysI have problem protecting the current sense amplifier and the circuitry. The circuit below is used to monitor the operation of the pump. It is an array of DC pumps with some control circuitry drawing max 200mA. The pumps are drawing anywhere from 1A to 5A when operating. The current monitor is detecting the start of the pump and pulls down a 3.3V line which is detected by a microcontroller and then sent by a radio link to the control station. Current setup is OK when connected to the 12V pumps but when connected to the 24V pumps fails when technician reverse connects it by accident. At least that's my theory as that is the only way i managed to damage them in testing. The devices are fully potted after assembly as they are in a damp environment. It's hard to get to this devices so any replacement is expensive.R1 is shunt resistor and I did not put the rectifier diode as a reverse voltage protection as the pumps are safety critical and rules do not allow anything but shunt connected in series with the pump. I'm preparing the next batch and was thinking of protecting them better when connected in reverse. The monitor needs just to detect when is the pump on (draws more than approx 500mA) so precision is not essential here.I tested putting the BAT54A instead of P-CH mosfet and 10k instead of 10R input resistors and it seems to work fine on my bench. Do you have any better ideas? I appreciate any input.Please note that the VCC is fully protected from the reverse connection and microcontroller will now detect and report that state also so I would know this is the case.Original device:With BAT54A:

author: nctnico
author_rank: Super Contributor
timestamp: April 07, 2018, 09:49:05 am
content: I'd skip the 0.1% resistorsand use 1% instead because the leakage current in the TVS diodes will cause a larger error. The BAT54A is not going to do anything.The typical trick is to put anti-parallel diodes in parallel with the shunt. I probably would put a big TVS onto the pump's supply line to catch the really nasty spikes. Putting too much circuitry in the sense lines is going to kill accuracy.

author: tdarlic
author_rank: Contributor
timestamp: April 07, 2018, 10:00:18 am
content: Thanks. I actually use 1% resistors in final assembly.I thought that bat54 will quickly lift the SENSE_L and SENSE_H lines and avoid reverse current passing trough the amplifier.With the original setup accuracy was within 50mA and that's way better than what i need.

author: Marco
author_rank: Super Contributor
timestamp: April 07, 2018, 12:21:53 pm
content: With reversed 24V, did you blow the MOSFETs or did you blow the INA168? The INA168 can take 10 mA of current on its protection diodes, if -24 is all you expect I don't see how it's going to pull more than 10 mA through the internal 5K resistors.

author: tdarlic
author_rank: Contributor
timestamp: April 07, 2018, 02:09:38 pm
content: Thanks for answering. I'm not sure as the one that failed is potted and I destroyed it trying to remove the epoxy. I noticed that ones that failed were 24V. They are hard to get and guys replacing them reported that they were wrongly connected. Will retest them now in both configurations.