Simple circuit to test current direction? - Page 1
Forum section: electronics / beginners

author: polemon
author_rank: Contributor
timestamp: August 12, 2016, 11:06:36 pm
content: Hello, I have a battery charger for a car lead-acid battery (it charges from a solar panel). I'd like to use simple means of display, for instance a single LED to check whether the battery is charging.How should I do this?The idea I came up with, is using two large diodes on the (+) terminal one in each direction and kinda putting a led in parallel with one of these large diodes. But I'm really not sure.Thanks in advance~--polemon

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2016, 11:21:25 pm
content: The easiest way to do this is to connect the solar panel to the battery via a diode and connect and LED (with a suitable series resistor) on the solar panel side of the diode. The LED will illuminate when the solar cell is generating a voltage.The disadvantage with the above solution is the solar panel may be outputting a high enough voltage to turn on an LED but not enough to charge the battery. Another solution is to use a transistor to turn on the LED when the voltage across a sense resistor exceeds 0.6V but it's late and I can't be bothered to draw it at the moment. I'll draw it tomorrow, if no one else has.

author: polemon
author_rank: Contributor
timestamp: August 13, 2016, 04:23:03 am
content: Quote from: Hero999 on August 12, 2016, 11:21:25 pmNot really an option. The panels is connected to a power management unit, which manages charging the battery and load balancing when a load draws enough current, so that it draws power from both the solar panel and the battery.I want specifically the "current direction LED" on the battery side.

author: kwass
author_rank: Frequent Contributor
timestamp: August 13, 2016, 04:43:48 am
content: Quote from: polemon on August 13, 2016, 04:23:03 amThe LED you want sounds like something the power management unit should have had built in. You can't be the only person looking for this, have you searched for hacks to this unit? Maybe post the model number of the unit and see if anyone here has done so or can supply a schematic for it.

author: Jeroen3
author_rank: Super Contributor
timestamp: August 13, 2016, 07:30:32 am
content: 1 opamp a shunt and two leds can indicate charge discharge, I think.Positive current, opamp clips V+, led green.Negative current, opamp clips V-, led red.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 13, 2016, 03:30:13 pm
content: maybe try this... adjust R1 value so that voltage across it 0.7V at desired current flow. R2 is led current limit, you may need it you may not. ymmv.

author: Zero999
author_rank: Super Contributor
timestamp: August 13, 2016, 05:39:13 pm
content: Quote from: Mechatrommer on August 13, 2016, 03:30:13 pmYes, that's what I had in mind but beware that the BJT won't be able to handle much base current and will be fried if the current exceeds 50mA or so.

author: Brumby
author_rank: Unknown
timestamp: August 14, 2016, 02:10:53 am
content: You want simple....?How about a length of copper bar as a shunt with a centre-zero galvanometer/microammeter across it?

author: polemon
author_rank: Contributor
timestamp: August 14, 2016, 02:17:13 am
content: Quote from: kwass on August 13, 2016, 04:43:48 amIndeed it has a "charging LED", but it doesn't make sense. Even when current is drawn from both, the battery and the solar panel, the LED is lit (I've tested this with a multimeter).It seems to me that the LED is always lit, when the solar panel generates enough voltage to charge the battery, but when the load is large enough it stays lit, even though current is drawn from both, the panel and the battery.Quote from: evb149 on August 13, 2016, 06:28:21 pmWith series current sensing resistor, do you mean shunt resistor?QuoteWell, it's a standard car battery, so 10V to 15V is the basic operational range.QuoteYeah, I thought a max. of 100mA into the battery should trigger the LED on. Actually, anything >0A into the battery should actually turn it on, etc. but well, that's up to testing, etc.QuoteThanks, man! Nudging me into the right direction is a load of help already! I'll look into the part you linked and see if I can find more information about current sensing, etc.Feel free to add to this thread if new ideas pop up!

author: Assafl
author_rank: Frequent Contributor
timestamp: August 14, 2016, 04:57:47 am
content: Quote from: Brumby on August 14, 2016, 02:10:53 amThat is actually a good idea. Get one of those center zero NOS things on eBay (some are cheap!). At a glance, takes very little energy, and you are actually measuring a physical entity.

author: jeroen79
author_rank: Frequent Contributor
timestamp: August 14, 2016, 10:37:08 am
content: What about using one of the battery wires for the shunt?

author: dannyf
author_rank: Super Contributor
timestamp: August 14, 2016, 01:09:38 pm
content: a resistor + two back-to-back leds will do.

author: Brumby
author_rank: Unknown
timestamp: August 14, 2016, 01:23:04 pm
content: Quote from: jeroen79 on August 14, 2016, 10:37:08 amWorth investigating.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: August 14, 2016, 01:33:15 pm
content: KISS - keep it simple stupid .http://www.stuttgartperformanceengineering.com/inductiveammater.htmlDepending on the current range , low - high tape it to wire , the needle will show the direction and the amount of current

author: Cerebus
author_rank: Super Contributor
timestamp: August 16, 2016, 12:18:53 am
content: Quote from: dannyf on August 14, 2016, 01:09:38 pmThink for a second. Big lead-acid battery, it's going to be 60 Ah at a minimum, so a 0.1C charge current of 6A dropped through the 1.6V that a LED is going to want to see is going to be the best part of ten watts or potentially much, much more. You've just cut the efficiency of the system by something on the close order of 15%, both during charging and discharging. Moreover, what's the charge management system doing to think of an extra 1.6V drop?

author: Cerebus
author_rank: Super Contributor
timestamp: August 16, 2016, 12:22:56 am
content: Quote from: Assafl on August 14, 2016, 04:57:47 amPlus proper meters are cool. I recently picked up an old HP 6002A power supply and it has two big proper analogue meters on the front - even SHMBO approves of it.Now all you need is a Jacob's Ladder or two and you're good to go.

author: C
author_rank: Super Contributor
timestamp: August 16, 2016, 12:59:22 am
content: car amp meterThis type uses no power setting thereeditQuote from: Brumby on August 14, 2016, 02:10:53 amThis is what a car amp meter is.

author: Brumby
author_rank: Unknown
timestamp: August 16, 2016, 12:59:22 am
content: Quote from: Cerebus on August 16, 2016, 12:18:53 amMy response was more...

author: SL4P
author_rank: Super Contributor
timestamp: August 16, 2016, 04:06:55 am
content: Quote from: dannyf on August 14, 2016, 01:09:38 pmI'm glad someone else said this - I was just about to post!!Obviously Not as the current carrying element!!Shunt it - and watch the side path current drive the LEDs

author: Brumby
author_rank: Unknown
timestamp: August 16, 2016, 05:11:50 am
content: ... that would work in principle, but the voltage drop required to light up the LEDs would make it a poor solution.

author: Brutte
author_rank: Frequent Contributor
timestamp: August 16, 2016, 08:09:54 am
content: I second the battery wiring concept.Plus two thin sensing wires.It is a comparator circuit so the positive hysteresis is needed. And filtering.The 100mA sensitivity should not be a problem here as long as LED indicates:(-inf:0mA] always off(0mA:100mA) unknown state (could be either on or off)[100mA:inf) always onSo basically the challenge is: "offset voltage temperature drift" + same TCO of resistors used.For 5mm2 Cu wire of 1m length we get:R=3.3e-3 ohmsSo at 0.1A that gives 330uVYour comparator must not have the offset voltage drift higher than ~330uV/2 in whole operating range and the circuit is doable if you adjust zero to +50mA.