HEF4051B multiplexer protection - Page 1
Forum section: electronics / projects

author: edba
author_rank: Contributor
timestamp: March 23, 2019, 11:38:33 am
content: HiI am using the HEF4051B 8 channel analogue multiplexer as a switchable input to an ADC. The eight inputs goto a connector which then connects to the outside world. I was wondering about the protection you should provide for the HEF4051B? The HEF4051B does have internal protection circuit with clamping diodes. Is it ok to rely on this or should you provide additional protection?

author: aldi
author_rank: Contributor
timestamp: March 23, 2019, 12:09:02 pm
content: Protection against what? Surges? ESD? Faulty wiring by the user?The internal clamping diodes are rated for +/-10mA if I read the datasheet correctly, so a current limiting resistor is a good start. If you want to clamp higher currents you can add external diodes, with a better forward voltage spec (=lower) than the internal diodes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 23, 2019, 12:17:15 pm
content: If the usual suspects (noise, ESD), a series resistor, filter cap, clamp diodes, and another series resistor is a good idea.BAV99 is a good enough diode for clamping. You probably don't want to use schottky (BAT54S being the other common choice) for analog signals due to leakage current.Tim

author: edba
author_rank: Contributor
timestamp: March 23, 2019, 05:19:00 pm
content: Trying to protect mainly against ESD but these are user inputs so just wondering what other people do. I have initially designed in a series 100R resistor with 10nF cap to GND.On analogue input expansion cards just wondering what sort of protection they put on the inputs.

author: edavid
author_rank: Super Contributor
timestamp: March 23, 2019, 06:16:06 pm
content: Depending on your budget, you could use a fault protected multiplexer like the ADG528.

author: floobydust
author_rank: Super Contributor
timestamp: March 23, 2019, 08:06:33 pm
content: I would not use a 4051 with field-wiring. They always get zapped. You would need a zillion clamp diodes and resistors (32 parts) to protect them and it gets awkward.Consider 4851 which is a toughened 4051 usually for automotive applications. But 2-6V power. It has the built-in clamp diodes and some protective series resistance, injection current rated.http://www.ti.com/lit/ds/symlink/sn74hc4851.pdf

author: edba
author_rank: Contributor
timestamp: April 02, 2019, 12:14:00 pm
content: Hi floobydustDo you provide any extra protection when you use the 4851?

author: floobydust
author_rank: Super Contributor
timestamp: April 02, 2019, 08:59:49 pm
content: I always add series current-limiting resistance, and if there is a BCI or EMI requirement, add capacitance to GND. So an RC filter upstream of each mux. input for lowest cost.If the mux inputs can see voltages above Vcc, i.e. 12V or 24V if a sensor gets wired incorrectly/shorts, you don't want that flowing to your MCU rails. Protection against lightning needs another level of protection added.I've seen a lot of equipment with a mux to an A/D that can't take much ESD or surges because the designer did not include overvoltage protection for the mux.