Voltage sensing with a voltage regulator? - Page 1
Forum section: electronics / beginners

author: MichaelW
author_rank: Contributor
timestamp: August 28, 2017, 11:05:53 am
content: So, I have this friend who is working on a project for use in an automobile that involves sensing when the turn signals are engaged, and which one is on. This is sensed with a microcontroller (ARM M0 dev board). I would have suggested to him to use a voltage divider to drop the 12v down to something the GPIO is comfortable with (in this case max 5v), and use a zener diode with current limiter to take care of any transient spikes, at the divider tap. However, he is using a couple of 5v voltage regulators, one on each of the wires that go to the turn signal lights. Other than using more power, which shouldn't be a problem, and higher cost for the components, I can't think of a reason that this is a bad idea. I'd rather go the divider/zener route though but I can't explain why other than cost/power consumption. Any thoughts?

author: Zero999
author_rank: Super Contributor
timestamp: August 28, 2017, 11:41:26 am
content: Other than extra cost, there's no problem,I'd probably use a single, high value series resistor and let the internal ESD protection diodes clamp the input.Does the vehicle have CAN? Another possibility is the turning signals are not directly controlled by the switches near the steering wheel but by CAN bus.

author: MichaelW
author_rank: Contributor
timestamp: August 28, 2017, 12:14:33 pm
content: I was thinking that the voltage spikes can't be TOO high, maybe 15-16v max? So the overvoltage protection shouldn't have to be to elaborate.It does have CAN, and I thought about suggesting that but he's pretty well into his current solution. I'll keep that one around just in case.

author: Kalvin
author_rank: Super Contributor
timestamp: August 28, 2017, 12:37:25 pm
content: Better design for a load dump and other automotive transient events, so make sure that the regulator is designed for automotive use.http://www.electronicdesign.com/power/eliminate-those-automotive-load-dump-circuit-protection-headaches

author: digsys
author_rank: Unknown
timestamp: August 28, 2017, 01:49:33 pm
content: IMO, the simplest and safest method of connecting ANYTHING direct into a micro, in a vehicle environment is - opto-couplers !! Both In and Out.Isolate the micro at all cost. The input to the optos can be a very simple resistor / zener, and add a filter cap for debounce. I even use linear optos to isolate analogue inputs.

author: fcb
author_rank: Super Contributor
timestamp: August 28, 2017, 02:04:41 pm
content: Opto's would be unnecessarily expensive & will decrease the MTBF.A simple resistor divider with a zener clamp would be enough, but do the calculations based on load-dump.

author: Benta
author_rank: Super Contributor
timestamp: August 28, 2017, 02:21:16 pm
content: Automotive ICs are usually designed to withstand a 60 V load dump, both positive and negative. If your friend is using automotive vregs, it should have been taken care of by the manufacturer.

author: digsys
author_rank: Unknown
timestamp: August 28, 2017, 02:24:27 pm
content: Quote from: fcbDon't know where you obtain your figures from, but I've been doing vehicle interfaces for 40+ yrs (mainly trucks), and haven't had any failures in "normal" conditions.I only exclude accidents and physical wiring damage. Costs are not that much more, I doubt ~$1 will kill the viability, and I've seen NO actual evidence of lower MTBF.YMMV

author: Kleinstein
author_rank: Super Contributor
timestamp: August 28, 2017, 03:45:39 pm
content: One problem with the voltage regulator version is that many semiconductors fail with a short. So if the regulator fails it might cause a short. So extra protection (e.g. a fuse, PTC) against overheating may be needed. The simple divider is less likely to fail with a short. Even if the zener for voltage limiting would fail with a short, the turn signal would be still OK. So the simple divider is expected to be more reliable.

author: MichaelW
author_rank: Contributor
timestamp: August 28, 2017, 06:41:43 pm
content: Quote from: Kleinstein on August 28, 2017, 03:45:39 pmThats a very good point, thanks!And thanks to everyone else for the information as well, lots of stuff for me to research and think about.