Ammeter shunt install help
- Thread starter Stevesnj
- Start date
This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.
- Joined
- Sep 17, 2003
- Threads
- 69
- Messages
- 4,601
- Location
- Sunnyvale, CA
- Website
- www.george4wd.taskled.com
I'm betting it won't work as pinhead describes. This unit is a low side sense - it is sensing the voltage between the yellow and black. That voltage is the effect of V = I * R (where I is the current flowing through the shunt and R is the resistance of the shunt).
As posted before, for a unit to be able to sense low or high side it needs FOUR wires (+ve power, -ve ground and TWO sense wires that go across the shunt). This unit only has 3 wires and is a LOW side sense device.
You need a high side sense for an alternator since the 'case' of the alternator is connected to block/chassis/ground.
cheers,
george.
As posted before, for a unit to be able to sense low or high side it needs FOUR wires (+ve power, -ve ground and TWO sense wires that go across the shunt). This unit only has 3 wires and is a LOW side sense device.
You need a high side sense for an alternator since the 'case' of the alternator is connected to block/chassis/ground.
cheers,
george.
- Thread starter
- #23
Should I just buy a positive side shifter shunt?
- Joined
- Sep 17, 2003
- Threads
- 69
- Messages
- 4,601
- Location
- Sunnyvale, CA
- Website
- www.george4wd.taskled.com
You need to find a unit that is either HIGH side current sensing OR a unit that has 2 sense wires that go to the shunt and state that the sensed voltage can be anything between GND and POSITIVE input voltage.
A unit with 2 sense wires is better since it only cares about the differential voltage across the shunt and thus is immune to voltage drops in the +ve or GND feeds to the unit. Note that you are sensing a pretty small voltage so noise pickup can be a problem. Shielded sense leads and don't run them near 'noisy' electrical/electronic wiring etc.
Consider: Say the shunt is capable of handling 100A. Assume they want to keep losses to less than 5W in the shunt, that means (P = V * I) 5W/100A = 0.05V full scale. Shunt resistance is (P = I*I*R) => 5W/100/100 = 0.0005 ohms. Now say you have a 10A current flowing, that means there's only (V = I *R) => 10A x 0.0005 ohms = 0.005V (5 millivolts) across the shunt that has to be sensed (pretty small voltage in an electrically noisy vehicle). All depends on what the shunt resistance actually is (obviously needs to be the right value for the gauge you have).
Do also note that the shunt will introduce a small voltage drop between the alternator and your battery - again it depends on the resistance of the shunt and how much current is flowing through it.
Personally it's more information that I don't need to see every day and as a somewhat obsessed EE I've never felt the need to closely monitor the output current of my alternator over the decades of owning 4wds. And adds yet another electrical connection for Murphy to play with...
cheers,
george.
A unit with 2 sense wires is better since it only cares about the differential voltage across the shunt and thus is immune to voltage drops in the +ve or GND feeds to the unit. Note that you are sensing a pretty small voltage so noise pickup can be a problem. Shielded sense leads and don't run them near 'noisy' electrical/electronic wiring etc.
Consider: Say the shunt is capable of handling 100A. Assume they want to keep losses to less than 5W in the shunt, that means (P = V * I) 5W/100A = 0.05V full scale. Shunt resistance is (P = I*I*R) => 5W/100/100 = 0.0005 ohms. Now say you have a 10A current flowing, that means there's only (V = I *R) => 10A x 0.0005 ohms = 0.005V (5 millivolts) across the shunt that has to be sensed (pretty small voltage in an electrically noisy vehicle). All depends on what the shunt resistance actually is (obviously needs to be the right value for the gauge you have).
Do also note that the shunt will introduce a small voltage drop between the alternator and your battery - again it depends on the resistance of the shunt and how much current is flowing through it.
Personally it's more information that I don't need to see every day and as a somewhat obsessed EE I've never felt the need to closely monitor the output current of my alternator over the decades of owning 4wds. And adds yet another electrical connection for Murphy to play with...
cheers,
george.