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Shunt Location
By SparWeb, Section Controls Posted on Tue Jul 15th, 2008 at 05:05:09 AM MST
Measuring Current with a Shunt

Sort-of a question, but maybe also a topic of debate:
Where's the best place to put a shunt for measuring current?
I personally can see it two ways. I made an extremely simplified diagram to illustrate the point, with as few elements as possible:

In the simple "generator charges a battery model", I can see a shunt can be placed either between the + connections of the generator device and the battery, or between the - connections.
I can see advantages and disadvantages in both. One is easier to measure, one is safer. I'm leaning toward the "high-side shunt", for lack of a better term, because in that system the ground is less ambiguous. A lightning strike is a real possibility in my area, and having even 0.1 ohm between the strike and the ground means lots of heat, smoke and fire.
The flipside is that measuring the voltage difference must be done differently with high-side or low-side shunts. Sure, it's possible to do it either way, but it seems much easier with a low-side shunt. In a nutshell, if everything knows what "0 volts" is, then everything can be measured against it, including the voltage drop across the shunt. But in a high-side shunt circuit, the battery floats around 13V plus or minus a volt, and the electronics making the measurement have to be below 5V or so, then when you try to measure the voltage drop across the shunt you have to take the 8V (and varying) difference between the power supplies into accound before getting a useful number.
I built a data logging device a few months ago, but I was aware that it required a low-side shunt to work. So I let it run until summer came. Now storm season is on, and I've taken the shunt out. I have an alternative circuit for the data logger sketched out, so I'm not stuck, but I bet some memberss out there have some great ideas on how they do this stuff.
Any suggestions? Have I missed something? If it helps, I could attach more diagrams of the stuff, but you can see the "Piclog" on: www.thebackshed.com and my system schematic looks like
this:

Thanks!
Steven

Shunt Location | 7 comments (7 topical, 0 editorial)

Re: Shunt Location (3.00 / 0) (#1)
by bob g on Tue Jul 15th, 2008 at 12:03:50 AM MST
(User Info)

Stephen:

i don't really think it is going to make a bit of difference if it is hi or low side
on the shunt,, current is theoretically equal throughout the circuit?

as for a lightening strike, i have no idea how you design around that concern,, if the lighteing gets downline past the arresters you are screwed where ever you put the shunt, or at least that is how it would appear to me.

knowing my luck
after a lighting strike if i were able to save the case/chassis i would be thrilled!

bob g

Re: Shunt Location (3.00 / 0) (#2)
by SparWeb (sparweb at ANTISPAM_hotmail_com) on Tue Jul 15th, 2008 at 01:20:26 PM MST
(User Info)

Probably so.

The exposure to high voltage from a lightning strike can mean some very high amps through the shunt. Burning out the data logger is not the concern in this situation. A momentary 2000 Amp current from the generator to ground will cause a 200 volt potential across the shunt!
That 200 Volt potential on the ground side could bias the battery backwards by 187 Volts. Boom.

The generator's negative would have to be at earth ground, and the battery's negative terminal will be "above" that ground level, in order to keep the current to ground from going through the shunt (maybe).

Steven Fahey
[ Parent ]

Re: Shunt Location (3.00 / 0) (#7)
by BigBreaker on Tue Jul 22nd, 2008 at 09:29:31 AM MST
(User Info)

I would't be worried about your logger or your shunt during a lightning strike. If the logger dies due to shunt voltages, your stator and transmission line are probably toast as well. If you are really nervous you can parallel the shunt with a spark gap that will conduct around the shunt or to ground at suitable voltages. A direct lightning strike will probably kill your system regardless of any protection you put in place. I won't worry about it.

[ Parent ]

Re: Shunt Location (3.00 / 0) (#6)
by la7qz on Wed Jul 16th, 2008 at 07:33:29 AM MST
(User Info) http://home.no.net/naomij

FWIW, it is more or less standard to have the shunt in the negative side in the battery monitoring systems that are installed in boats. (The Xantrex link series etc.)

This makes things a lot easier when there is more than one battery bank (engine cranking battery, generator cranking battery, house battery, electronics battery), particularly if the monitor is being retrofitted to an existing system which is often the case. There normally is (or should be) a common ground point for all the batteries and consumers and this is an easy place to install the needed shunts. Installations vary. Some monitor all batteries, others only monitor the house battery.

Owen
If you're not living on the edge, you're taking up too much space.
[ Parent ]

Re: Shunt Location (3.00 / 0) (#3)
by tecker on Tue Jul 15th, 2008 at 01:31:44 PM MST
(User Info)

I think you need something a little smaller unless your working with the heat .

Re: Shunt Location (3.00 / 0) (#4)
by SparWeb (sparweb at ANTISPAM_hotmail_com) on Tue Jul 15th, 2008 at 10:35:48 PM MST
(User Info)

...I think you need something a little smaller...

I was told: Lots is good, more is better, and too much is just enough!

Uh, joking aside I don't think I know what you mean, unless you mean the actual resistance of the shunt: and there I would agree, but I was just picking a convenient number out of the air. There's also the shunt I was using in the system before, with 0.086 ohms resistance, and that would get pretty warm, too.

I just finished making a 0.02 Ohm shunt. The datalogger lost a lot of sensitivity with it, so I'll probably make another one, maybe about 0.04 ohm.

This would be a lot easier if it was just an analog needle, but the electronics are so concerned with the "reference" voltage to measure the shunt voltage.

After looking at my little diagram again, I drew on the earth grounds, and it made a few things more clear. The "problem" with the low side shunt just goes away when I see the ground at the negative terminal of the generator. I'd somehow thought I would put it on the battery's negative terminal, but that's not necessary.

Thanks
Steven Fahey
[ Parent ]

Re: Shunt Location (3.00 / 0) (#5)
by tecker on Wed Jul 16th, 2008 at 06:18:47 AM MST
(User Info)

I see .If the data logger needs a larger swing you might consider a current transformer on each phase. I was thinking of trying solenoid coils in a few places I bought a bunch of 24 volt solenoids they're still in the box. I 'll break one out and see if the variance is good .They will fit tighter on the line than the cts I've scrounged. You could watch all three phases if you have the inputs available . Some times one phase is more dominant you might be able to balance them if that's ever a factor.

[ Parent ]

Shunt Location | 7 comments (7 topical, 0 editorial)

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