Ammeter

[ron wilson]

How do you suggest I go about hooking up an ammeter to my 74B

 

[jlaird]

Is just in line on any hot wire as I remember.

 

[danmas]

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How do you suggest I go about hooking up an ammeter to my 74B

[/ QUOTE ]

You really don't want to. Not that there's anything wrong with using anammeter, putting one in a 74B is not an easy task, as the attached picture shows. You have to remove all the brown wires from the starter solenoid and connect them to one side of the ammeter, and connect the other side of the ammeter to the solenoid where the brown wires came from. Much easier said than done from a physical standpoint. Electrically, it is not a problem, but getting the wires extended to the back of the ammeter, etc, will be a real bear to do.

 

[DrEntropy]

Yup. An alternative to that is a voltmeter: You ~CAN~ connect that to any switched hot lead, and it will tell you what's going on with the electrical/charging system nearly as well. Much easier to install.

 

[jlaird]

Woops, steped on it did I.

 

[Scott_Hower]

Dan offers sage advice. As nice as it would be to know how much current is flowing around, you pretty much need to route the entire drain through the ammeter. I'm not wild about having that kind of current behind the dash.

Connecting a voltmeter to any hot will tell you the voltage.

 

[DrEntropy]

For me it's not so much the current behind the dash as accomodating the rewiring task... a voltmeter will be sooo much easier and still give a fair "picture" of what's happening with the electrics: key on, 12~12.5V, running @ 2K RPM 13.5~14.5V and all's right with the world! Simple. /ubbthreads/images/graemlins/grin.gif

 

[JamesWilson]

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For me it's not so much the current behind the dash as accomodating the rewiring task...

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I'm considering using an ammeter with an external shunt resistor. That will make the wiring simple and also keep the heavy current away from under the dash.

But its hard finding these for automotive applications. Does anyone know of a company that makes this type of ammeter? Or do I get to perform some surgery on one and move its guts about?

 

[ron wilson]

Again Thanks for the advice re ammeter vs voltmeter. A volt meter sounds like the way to go.

 

[David64MGB]

There are ammeters that sense the current flowing through the wire rather than being in-line. Creates a current preporational to the current in the main wire. Probably not cheap. Definitely not normal automotive equipment.

 

[Deleted member 3577]

For a dynamo equipped vehicle it is a lot easier.

Just run the big brown wire from the starter relay/switch to the ammeter & back to the "A" terminal on the control box.

I think starter solenoids and alternators complicate things a bit too much.

So far as trusting all that amperage behind the dash....
EVERYONE should have a battery disconnect that they can get to in a hurry.....And an extinguisher!

 

[Limey]

[ QUOTE ]
[ QUOTE ]
For me it's not so much the current behind the dash as accomodating the rewiring task...

[/ QUOTE ]

I'm considering using an ammeter with an external shunt resistor. That will make the wiring simple and also keep the heavy current away from under the dash.

But its hard finding these for automotive applications. Does anyone know of a company that makes this type of ammeter? Or do I get to perform some surgery on one and move its guts about?

[/ QUOTE ]

An ammeter that uses a shunt resistor is not really an ammeter, it is a voltmeter that measures the volt drop over a low resistance, high current, resistor - typically 1 ohm or less. Then the voltage drop across the resitor is proportional to the current through it (V=IxR).

You can buy shunt resistors in various sizes from most electronic supply houses, then you just need to find a suitable voltmeter, but you will probably need to make a faceplate to read in the correct units.

 

[danmas]

[ QUOTE ]


An ammeter that uses a shunt resistor is not really an ammeter, it is a voltmeter that measures the volt drop over a low resistance, high current, resistor - typically 1 ohm or less. Then the voltage drop across the resitor is proportional to the current through it (V=IxR).

[/ QUOTE ]

This points out why a shunt type ammeter isn't used. With a 1 ohm resister and a mild current draw for the car's electricals of a modest 15 amps, there will be a voltage drop of 15 volts! Out of the 14 volts available, that doesn't leave much for the car /ubbthreads/images/graemlins/smile.gif Dropping the resister down to a 0.1 ohm resister would give a voltage drop of 1.5 volts, still too much to live with, what with dim lights, sluggish fans, etc. Droping down to a 0.01 ohm resister gets a little bit better, with a 0.15 volt drop. However, with this resister/voltage drop, you will need a voltmeter with a zero center and a +- range of 0 - 0.3 volts (allowing for a current draw of 30 amps under maximum alternator output). These are hard to find!

The ammeters typically used in an automobile are quite diferent. If you open up, for example, the ammeter from a TR6, you'll find nothing between the two terminals but a short piece of wire, approximately 12 gauge, curled into a single loop. The needle is completely isolated from this loop of wire, using only magnetic coupling to swing the needle. Trying to convert one of these into a shunt type ammeter is not a viable option. The only option would be to completely redesign the meter, totally replacing the guts with a new design, in effect, making it into the -0.3 - + 0.3 voltmeter described above.