Measuring current from an alternator - Page 1
Forum section: electronics / projects

author: bborisov567
author_rank: Regular Contributor
timestamp: June 16, 2021, 05:32:53 pm
content: I am designing a little device to measure some temps and the voltage in my car when it occurred to me that it wont be such a bad idea to also measure the charging current from the alternator. The device is based on Arduino Nano so i believe i have two options - shut resistor or hall effect based current sensor. The first one is out of considerationSo I started looking online and found the ACS758 current sensor which claims to measure up to 200 amp but still has to be wired in series. It seems kind of silly to tamper into the wiring of my car for that purpose. So is there a way to measure dc current without anything wired in series?

author: bdunham7
author_rank: Super Contributor
timestamp: June 16, 2021, 05:46:04 pm
content: There are external hall-effect transducers for this, but you probably won't like the price. I don't know of a cheap way to do this.https://www.lem.com/sites/default/files/products_datasheets/lf_210-s.pdf

author: Gregg
author_rank: Super Contributor
timestamp: June 16, 2021, 07:42:38 pm
content: The power output wire from the alternator is basically a shunt resistor. Depending on the vehicle wiring configuration; you could affix sense wires to both ends of that wire and calibrate the result in software using a hall effect meter like the Uni-T (discussed at great length on this forum) for reference. You may have to engineer some noise suppression into the setup.

author: Benta
author_rank: Super Contributor
timestamp: June 16, 2021, 07:45:54 pm
content: Quote from: Gregg on June 16, 2021, 07:42:38 pmYes, and as it is copper, its resistance is practically constant over temperature!/irony

author: Zero999
author_rank: Super Contributor
timestamp: June 16, 2021, 09:26:43 pm
content: Honewell also make them. I don't think it's too expensive. The downside is the sensitivity depends on the supply voltage, so it would require a voltage reference, or regulated power supply, to get an accurate reading, but you'll need that anyway, if you want to measure the voltage.https://www.digikey.co.uk/product-detail/en/honeywell-sensing-and-productivity-solutions/CSLA1DJ/480-4821-ND/3072993

author: bborisov567
author_rank: Regular Contributor
timestamp: June 16, 2021, 09:36:05 pm
content: Quote from: Zero999 on June 16, 2021, 09:26:43 pmBy looking at the picture isn't that just a ferrite bead with a hall sensor in the cutout part?

author: bdunham7
author_rank: Super Contributor
timestamp: June 16, 2021, 09:57:33 pm
content: Quote from: bborisov567 on June 16, 2021, 09:36:05 pmYes, that's an open-loop sensor, which is a lot less accurate--although maybe good enough. The one I linked is closed loop and has a similar core construction, but there is a toroidal coil around the core and the sensor electronics send current through that coil to cancel out the magnetic field so that the hall sensor is always at its null point. That coil current is also the output--so it is a current loop sensor. Either sensor would need some additional support electronics right at the sensor location.

author: bborisov567
author_rank: Regular Contributor
timestamp: June 16, 2021, 10:16:59 pm
content: I got a bit confused about the lem sensor. How do you sense magnetic field when it gets cancelled out? And how that coil acts as an output?

author: ejeffrey
author_rank: Super Contributor
timestamp: June 16, 2021, 10:33:02 pm
content: The measurement output is the amount of current that you need to exactly cancel the field from the signal. This avoids impact of the non-linearity of the ferrite and the hall sensor.

author: james_s
author_rank: Super Contributor
timestamp: June 16, 2021, 10:49:54 pm
content: What's wrong with using a current shunt? They exist for exactly this sort of purpose. The output from the alternator is a single wire in the cars I've worked on, it's trivial to disconnect it and insert a shunt or hall sensor.

author: fordem
author_rank: Regular Contributor
timestamp: June 17, 2021, 03:45:19 pm
content: Quote from: bborisov567 on June 16, 2021, 05:32:53 pmWhat is it you really want to measure? Is it the charge current or the current from the alternator (alternator output current)?The two currents are not the same, the alternator output feeds the car's electrical loads AND charges the battery.If it's charge current, the measuring device needs to be located between the positive battery terminal and the vehicles main power bus, if it's alternator output current, the measuring device needs to be located between the alternator and it's connection to the vehicle's main bus.There's very little reason to constantly monitor either current - the charge current is going to spike to 50~80% of the alternator capacity for the first minute or two after the engine is started and then gradually drop to a minimal current, the alternator output will constantly vary as the loads change - turn signals flashing will cause a variation, as will stepping on the brake - the variations will be so frequent and so minor that they would be a distraction whilst driving.

author: bborisov567
author_rank: Regular Contributor
timestamp: June 17, 2021, 11:06:03 pm
content: I want to measure the total current drawn from the alternator, not for any kind of a particular reason I think it will be interesting to see the energy impact of different accessories. The shunt resistor is not really an elegant solution, not only because of the losses but because of idea of messing with such an important system of the vehicle. And it is not much of a challenge either.Today i got some ferrite cores and a hall sensor so i will be conducting test and sharing results soon.

author: Zero999
author_rank: Super Contributor
timestamp: June 18, 2021, 08:06:35 am
content: Quote from: bborisov567 on June 17, 2021, 11:06:03 pmIf it's just for test purposes, rather than a permanent installation, use a clamp meter.https://uk.farnell.com/tenma/72-3500/dmm-clamp-auto-200a-3-5-digit/dp/2667179?gclid=EAIaIQobChMIgrHYzdyg8QIVCbLtCh2kxACuEAQYBiABEgJuRPD_BwE&mckv=s_dc|pcrid|459864482312|plid||kword||match||slid||product|2667179|pgrid|114464710984|ptaid|pla-307817500464|&CMP=KNC-GUK-SHOPPING-SMEC-Whoops-Newstructure-Test737-Desktop-Hi&gross_price=true

author: james_s
author_rank: Super Contributor
timestamp: June 18, 2021, 05:49:21 pm
content: I don't understand the issue with "messing with" an important system. It's one wire, shunt resistors are made specifically for this purpose, it is the widely accepted way of doing just what you're trying to do. The losses are negligible, the shunt is a small fraction of an ohm.

author: fordem
author_rank: Regular Contributor
timestamp: June 18, 2021, 07:25:36 pm
content: Quote from: Zero999 on June 18, 2021, 08:06:35 amI strongly suspect that that particular clamp meter will be of little use - it doesn't measure DC current.

author: Zero999
author_rank: Super Contributor
timestamp: June 19, 2021, 04:49:37 pm
content: Quote from: fordem on June 18, 2021, 07:25:36 pmSilly me. I should know better. Lots of clamp meters, just use a current transformer, which is much cheaper. Here's one with a Hall effect sensor, which can measure DC.https://uk.farnell.com/tenma/72-3545/fork-meter-auto-200a-3-75-digit/dp/2667187

author: bborisov567
author_rank: Regular Contributor
timestamp: June 24, 2021, 01:22:50 pm
content: Today i finally got to make some tests. I cut a ferrite bead and got an A3144 (datasheet:https://pdf1.alldatasheet.com/datasheet-pdf/view/55092/ALLEGRO/A3144.html) hall sensor. For a load i used a 12v bulb which draws around 4 amps. Everything was fine until i realised the output of the sensor is not linear with regard to the strength of the magnetic field and works like a switch with preset threshold. Now my question is what kind of a hall sensor should i be using?

author: ledtester
author_rank: Super Contributor
timestamp: June 24, 2021, 02:48:54 pm
content: Look for a "linear" Hall-effect sensor, such as a SS49E.