BS2 as digital amp meter.
justmatt83
Posts: 6
Hello I am looking for a way to read out the amount of amps being pulled by my trucks electronic system. I have a 4x20 LCD screen and would like to display the volts of the truck and the amps bing pulled from the battery. I was thinking about using a shunt at first but I think I would prefer something else. I was wondering if anyone had any ideas. I am looking at 11-15 volts and anywhere from 0 to 100 amps I have allot of extra electronics on my truck. Thanks for any help.
Comments
Put it in the negative lead, where the battery gets hooked to the truck chassis. In fact, if that existing cable is a foot or so in length, that will make a good shunt.
Then, build an op-amp circuit to read the voltage drop generated in that ground lead. You can adjust the gain of the op-amp to give you a signal you can feed into an a/d for the stamp to read. Don't forget that the battery current direction will change as the battery charges and discharges. I'd introduce an offset in the opamp so the output is about 2.5 volts with zero current flowing. Then, zero volts would be when the battery is discharging and 5 volts would be when the battery is taking a full charge.
Any college level book on microelectronics will show several examples, or pick up a Forrest Mims book which will show some good working examples.
Cheers,
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Tom Sisk
http://www.siskconsult.com
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It's a device that produces a voltage between 0 and 5 volts that is proportional to the current passing though the test leads. You just run that output into an A/D converter and you get a perfectly linear binary number that represents current. No messing with a shunt resistor.
(BTW, whatever method you use, if I were you I would wire it so that the starter current does NOT go through your current meter. Technically the 200 amp version of the device I linked to would not blow out even if you ran the starter through it, but I still wouldn't consider it a good idea.)
I have messed up with current sensing for my sail boat.......
I started as Stamptrol suggested by using the negative connection of the battery, but there are many drawbacks with this approach, the high copper temperature coefficient makes it difficult to have the same reading at different temperatures, (and I think in a truck the temperature range is very higher than in boats), the second brawback i meet with was the fact than being in the negative branch, all the electronics and other services are somehow lifted from ground (and that inserts noise in all the circuit).
I then tried a proper shunt (+/- 200A = +/-200mV) connected in the negative leg of the circuit, the problem due to the temperature coefficient was solved, not the problem of the ground being lifted, not to talk of the problem of the amplification of the signal, always prone to noise altering the measure, then i moved the shunt in the positive leg, the measure was very complicated due to the fact that it as to be done by a differential op-amp configuration (similar to that used in the medical instruments = very messy and difficult to tune properly).
I finally tested the allegromicro current sensor, the 200 Amps and the 50Amps models.
I agree with Dennis (I fried one sensor with a 2.2 kw starter motor of my boat's engine) the nominal current was around 180A but my opinion is that It was not the surge current, it was instead the countercurrent spike due to the windings of the starter motor (something similar to what appen with a relay) to fry the chip.
Another drawback i see is that it's not easy to make a proper connection from the huge cables of the starter motor circuit to that tiny chip, I had to invent a sort of clamp with brass to connect the chip in the primary circuit.
So I decided not to measure the current of the starter motor, but only of the services circuit, for that i used the 50A chip (0754-50), and that made thing easier, the linearity and accuracy are extremely good (confronted to that of traditional shunts), and there is no more noise in the circuit.
Hope this helps you to clear plus and cons of any choice You will take.
For now I wish You many miles enjoing with (toys) in Your truck as I am enjoing playing around with the thecnicals of my boat.
Take care
Stefano
Do you mind posting a picture and/or a more detailed description of the clamp that you made? I am considering a similar project and have been following this thread. I took a look at the datasheets for the Allegro Microsystems chips when they were recommended. I saw that they have fancy board layout patterns and used 4oz. copper traces for their connections. Do you think your homemade clamp is likely to work as well as Allegro’s $20 evaluation board?
- Sparks
Sorry for answering so late unfortunately now I'm abroad for work and not have any picture, the current sensor is mounted on my boat wich is far from me.
I saw the system they suggested, but apart from the fact that for me in Italy it's not easy to find as an hobbyist the 4oz copper pcb, i do believe that any small section in the wiring of a low voltage wiring, causes a drop in the voltage that sometimes is not desired, expecially with high surging actuators, an example, the fridge compressor while nominal 5 amp, will in certain situations surge up to 30A for a small amount of time and if the voltage drops to 8 volts or less, it wont work.
So huge sections are a must, for my boat i choose 1 square mm for nominal amp.
I can try to describe it:
There is an insulating material that acts as stand-off, I used teflon, on it i fixed with screws and nuts two bar of 4mm copper wide rougly as the lenght of the primary terminals of the sensor, I made the first prototype soldering the terminals to the copper, but this metod is brutal, and may be shortening the life of the chip (overheating!!!!).
The second try i did was a sort of clamp made with two bar of copper with a slot between wich accommodate the terminals, just before the terminals there is screw and nut trough the copper to keep everything together, this late sistem makes easy to remove the sensor.
If I have time i'll try to make a drawing to let it more clear, at the moment i'm in an internet caf
TC
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We all make mistakes when we are young………That’s why paste is edible!