Technical Amp gauge vs Volt gauge with generator

I know the difference between an amp gauge and a volt gauge. Generally I prefer a volt gauge, but with a generator instead of an alternator is there an advantage to using an amp gauge?

 

Most generators only put out 25 to 30 amps, alternators 50 to 150 amps. Generally you don't want as much power as an alternator puts out right in the dash, which is why newer cars use a volt meter. With a generator you will be fine with an amp gauge.

 

Another advantage is that most old cars that came with a generator, also came with an ammeter, so you can use the original gauge.

 

Well I'm building an A and putting aftermarket gauges, just deciding which to use. The amp gauge would be more traditional.

 

A volt gauge will tell you the charge state of the electrical system by just turning on the key, Amp gauge only works if it's running or how much current draw there is.

 

Because of the differences in how a generator works compared to an alternator, an ammeter will give a better picture of how a generator is working. For an alternator, use a voltmeter....

 

If you're planning on using an aftermarket, universal, wiring harness - make sure it's possible to easily use an amp meter with it. Don't ask me how I know.

 

In any case, main problem with an ammeter without a fusible link or other protection it can be exposed to full battery output if shit happens under the dash. Jack E/NJ

 

I suppose it would go in line from the starter solenoid to the fuse panel, where the fuse panel is picking up it's power from? Or would that only show discharge?

 

I think if all the fuse panel's loads are connected to the solenoid's battery side, and not the other ammeter terminal, it would only show charge. Jack E/NJ

 

ammeter connects between the battery, and everything else (except the starter)

 

If I was fitting new instruments with an alternator I would use a voltmeter. In my experience, alternators either work or they don't, and the voltmeter shows the state of the battery from the moment the ignition is turned on as said. An ignition warning light will tell you whether the alternators is working or not.

But where there is an generator era ammeter already installed along with a modern alternator, the maximum output of the alternator will almost certainly exceed the ammeter's ability to read current. If the battery is flat or near flat when you start the motor, the alternator will for a short while send its maximum output. When I bought my '50 Chevy truck -fitted with 68 amp alty by the PO- the ammeter was pegged right off the scale.



I managed to free the ammeter needle without doing any further damage. I did some experimentation which indicated that the Chevy ammeter goes full scale at just on 40A. So I made up this current shunt which is wired in parallel to the ammeter. The shunt array has 3 x 1 Ohm 100W resistors which are themselves wired in parallel. So the shunt presents 1/3 Ohm resistance and has 300W power capacity. The effect of the shunt is to roughly halve the current that goes through the ammeter, so now its FSD (Full Scale Deflection) is 80A


BTW, the chunk of PCB at the top of the Aluminium heat sink is doing something else on my truck -but power resistors do need to be mounted on a heat sink of some sort.

Here is a Current Shunt that I made up to work with an early Brit car ammeter which has a 20A max reading. The owner had installed 40A alternator. This one has 2 x 1.3 Ohm 100W resistors wired in parallel; this increases the FSD to about 45A. (Lucas moving coil style ammeter.)



There are four apparent connections on this one because the two connections are twinned. This allowed all the car's wiring connections to be made on the shunt. The heavy orange and brown wires disappearing to the left of the photo are the only ammeter connections.

 

Nice writeup Stueeee.

 

How are the volts and amps from a generator different from the volts and amps from an alternator?

 

"Where does the 150 amps from an alternator go if you aren't using an ammeter? If it doesn't go into the dash, where does it go?"
It goes to the battery.

 

In many harnesses the Alt Batt wire goes to the fuse panel and then there is another wire that goes from the fuse panel to the Batt terminal on the starter solenoid. In those cases, the Alt output goes into the dash area regardless if it uses an ammeter or a voltmeter.

 

That's why a fusible link or main breaker right at or near the battery hot terminal is advisable in all cases.. A fully charged battery can deliver far more amps than any alternator or generator if a wire connected to the hot terminal accidently hits a ground. Jack E/NJ

 

Ammeters connect between the battery and all the loads, along with the gen/alternator, EXCEPT the starter. That way it shows power coming out, or going into, the battery.

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Most of the fuse panels that the alternator output goes to on newer cars are in the engine compartment, not in the interior.

 

In a properly wired application, the Alternator and Load (car circuits) are on one side of the ammeter. The other side of the ammeter goes to the battery. The ammeter only sees current if the battery voltage is higher or lower than the Alternator and car Load circuits. That's why it reads zero, or neutral, when everything is normal. The Alternator (or generator) is providing all of the current for the car load. If the battery voltage is lower than the Alternator and Load, then the ammeter will show current flowing to the battery (Charging on the ammeter). If the Alternator and Load are lower voltage than the battery, the ammeter will show current flow to the Load side (Discharge on the ammeter). The ammeter indicates whether the battery is being charged or discharged.

A car ammeter does not measure the current being used in the car circuits. It's a 2 direction meter indicating the current flowing to or from the battery. Ammeters in other applications typically indicate the total amount of current flowing in a system. That's why those gauges have a 0-100 Amp or some such scale.

 

A generator when overextended will usually maintain voltage, but will fall short on current output; so a voltmeter can show adequate voltage but you won't know you have a problem until you run out of battery. An ammeter will show the reduced current output. An alternator will show reduced voltage under the same conditions, so a voltmeter is the better choice for those.

 

That's the first I've heard of this. Where did you get this information? Jack E/NJ

 

Generators are GENERALLY less than 50 amps.
Alternators are GENERALLY over 50amps.
The volts will be 14.8 VDC (volts DC) at the battery after passing through the regulators.
Most alternators have internal regulators.
All generators used external regulators unless a custom build.
A shunt is a good way to SAFELY us an AMP gauge.
However made sure you use 10 gauge wire minimum to feed the AMP
gauge because if the shunt or gauge short out a lighter gauge will
cause a FIRE FOR SURE.
You could always have the cluster rebuilt by Classic Gauges
To convert all of the gauges to 12 volts making the AMP gauge a
Voltmeter.



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I'm a retired electrician...

Without delving deeply into generator design/theory, the basic limiting factor for a generator is the 'conversion' of AC to DC via the commutator/brushes. Operating as basically a mechanical half-wave rectifier, you're both throwing away 1/2 of the generated power as well as 'clipping' the ends off the sine wave you do use. Switching current limitations through the commutator/brushes also limit available current. This 'clipping' also has the effect of increasing the 'average' voltage available as you're clipping off the lower part of the voltage sine wave.

 

What happens if an ammeter shorts out, not to ground, but across the 2 terminals on the back? What happens if you connected together the two wires on the back of a car's amp gauge? Would it cause a fire? I would say no. If it, or any wire in the whole car, shorts to ground, then there is a problem.



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Thanks Stueee, that is a neat way of doing it.
I was thinking the same thing, and you saved me getting out the calculator to work out the values! (This set-up works for both right & left hand drive electrons)

 

I noticed in the tables for the correct setpoint of a voltage regulator, a generator has a higher voltage, almost a full volt, at some temperatures, compared to an alternator. I wondered why that would be. I think that's it?

 

It's not so much that the voltage is 'higher' as it is the fact the gauge calibration shows it as higher. A DC generator (at least car units) is usually single-phase, so the voltage output isn't very smooth; lots of ripple in the output. Nearly all car alternators are three-phase, so they have much smoother output; although the ripple is still there, you don't need as high a peak value to arrive at a good 'average' value.

 

Does this hold true for measurement, analog meters aren't so "jumpy" as some DVMs. Or, will a digital voltmeter show a bit of an inaccurate reading with a generator? Input resistance? Sometimes the old test gear is better for measuring old gear.

 

A voltmeter without units sucks as totally worthless. I've a Ford truck with a worthless factory voltmeter. Sucks badly.

An ammeter without a knowledgeable operator sucks i.e. is totally worthless. Get an AAA card.

Whatever floats your boat (gives you information) is indispensible, next to a bilge pump of course.