Amp meter install shunt size

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Peter Poulikakos
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Amp meter install shunt size

#1 Post by Peter Poulikakos » Thu Jul 11, 2019 8:15 pm

Hi all
I would like to install a amp gauge in my 6 volt 356.
I have a 60amp VDO gauge that requires a shunt
.
Do I need a 50amp shunt as the generator is rated 50amp?
Can I connect directly off the BAT terminal of the regulator?
Any information wouldbe appreciated.

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Ron LaDow
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Re: Amp meter install shunt size

#2 Post by Ron LaDow » Fri Jul 12, 2019 12:30 am

Regardless of the shunt (and it may well be built in to a VDO ammeter), have you considered the routing and wire size required to put the meter anywhere other than the engine bay?
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Re: Amp meter install shunt size

#3 Post by Peter Poulikakos » Fri Jul 12, 2019 1:13 am

No I have not. One step at a time.
Do you have any ideas where I should connect the gauge?

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Re: Amp meter install shunt size

#4 Post by Eric Lenius » Fri Jul 12, 2019 9:46 am

Shunts are normally designed to "shunt" off most of the current so that the meter does not carry the current (since most meters reach full scale in a small number of milli-amps). So, a 60 Amp gage would use a corresponding 60 amp shunt which will cause the meter to read full scale when the shunt and meter, paralleled, are carrying 60 Amps (the meter current is negligible in comparison to the shunted current). The idea here is that you want your gage to have a range greater than the capacity of the system. Typical designs often have meters read mid scale at normal operating currents for most accuracy. I had never thought about how these gages are normally installed. A smart thing to do would be to install the shunt in the engine compartment and reduce the need to add gigantic cables bridging the distance between the engine compartment and the dash and run reasonably small wires from the shunt to the gage.

Now is the time for a VDO gage expert to chime in...…...

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Re: Amp meter install shunt size

#5 Post by Steve Douglas » Fri Jul 12, 2019 11:06 am

You don't need a shunt as the output of the generator/regulator should never exceed the 60amp capacity of the meter. I have had mine hooked up for several years, a cheapo RAC meter, hooked to the bat terminal with a 6" piece of #6 wire with crimped and soldered terminals. The wire is stiff enough to support the meter.
Works only on the + charge reading unless the regulator stick and back feeding to the generator, (a bad thing).
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Re: Amp meter install shunt size

#6 Post by Greg Bryan » Fri Jul 12, 2019 11:41 am

You could install a voltmeter instead. Much simpler installation and gives you the information that your charging system is working.
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Re: Amp meter install shunt size

#7 Post by Richard Shilling » Fri Jul 12, 2019 12:46 pm

I agree with everything written above. An ammeter has to have wires the same or larger diameter than the wire on the voltage regulator to read correctly. A voltmeter is much simpler to install, you can easily get voltage from the ignition switch, and it's easier to read. When you start the car and rev the engine above 3,000 RPM the voltage should rise to 7.2 volts. If it doesn't, something is wrong.
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Re: Amp meter install shunt size

#8 Post by Steve Douglas » Fri Jul 12, 2019 1:13 pm

I agree that the volt meter is easier to install, I have one that is connected to the fuse box and mounts under the dash with velcro, tried digital and settled on a small analog. The reason I use an ammeter is that I had a problem with a regulator that would cut back the amperage when it got warm, I'd get 7.3 volts as long as the load was less than 10-15 amps, nothing higher. It also helps see the battery charge state, if the running unloaded output is like 20 amps then it needs to go on the charger for a little while, saving the generator. At 6.2 volts (without engine running) the draw is about 26 amps with everything on, with it running and 7.3-7.4 volts about 22amps. Voltage is very important too, that's why I have both. I have an A generator/regulator rated at 160watts, so pretty well maxed out. Considering upgrading to a 200watt C generator and a Delco Big black box Regulator (1953 Caterpillar Tractor) that will allow 27-30 amps at 7.2 volts.

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Re: Amp meter install shunt size

#9 Post by Eric Lenius » Fri Jul 12, 2019 1:32 pm

The meter shown in the engine compartment has an internal shunt. The shunt matches the low current meter movement to what you want for maximum scale. If the VDO meter has an internal shunt (this I don't know), you don't need a shunt but do need a large wire installed to and from the dash. As Steve says, regarding the Volt meter, it does provide good information but not the whole picture. All of the electrical components involved in charging, in reality, can handle far more voltage than the system voltage. The controlling factor for all of these same components (and wires) is the amount of current being drawn by the system. So, an ammeter, in addition to being super cool, is the most direct indicator of how things are going back there. I'd love to have both though. I need to think about the charge/discharge function of the ammeter.....

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Re: Amp meter install shunt size

#10 Post by Eric Lenius » Fri Jul 12, 2019 5:41 pm

Does anyone have original installation instructions for VDO Ammeters? I looked at the information on VDO ammeter operation under the resources tab. It shows schematics with the meter installed at the dash with no shunt and an installation with a shunt in the engine compartment. The information states that these are derived from VDO installation details. These can't be the same meters. Without a shunt, the meter will peg and burn out the meter coil in a matter of seconds. So, they either intended for the meter and shunt to be installed at the dash or there are both internal and external shunt versions of this ammeter......

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Re: Amp meter install shunt size

#11 Post by PaulLima » Fri Jul 12, 2019 7:18 pm

Good afternoon Peter. I'll try to summarize the interesting data above. If you know a lot about electricity and are convinced you need an ammeter, there are still some questions to answer. If you are interested in the charging current from the generator, the connections need to be between the generator and the voltage regulator as shown in the picture above. If, also, you want that gauge in the cabin, you'll need to run two fat wires (probably 10 ga or even 8 gauge from the voltage regulator to the gauge in the cabin. This is a very nasty job, and, if not done VERY carefully, is quite dangerous. An alternative measurement would be the current draw through the ignition switch (i.e. all the normal loads other than the starter). The lower currents here mean shorter wire run to you gauge (still needs to be heavy wire). Also, this really doesn't tell you much. SO, as stated by many above, a much more useful and far simpler solution is a voltage gauge. It will tell you clearly if your battery is in good shape before starting, it will tell you at what RPM the generator is providing a charge to the battery, and, when driving with multiple loads (e.g. H4 headlamps, radio, windshield wipers, etc.) whether or not your battery is charging/holding its own, or, discharging.

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Re: Amp meter install shunt size

#12 Post by Ron LaDow » Fri Jul 12, 2019 9:29 pm

The things are also dangerous.
Many years ago, working under the dash of a TR3, and wearing a metal watch band, I found out how quickly that watch band heated up while acting as a shunt. YOW!
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Re: Amp meter install shunt size

#13 Post by Bill Oldham » Fri Jul 12, 2019 11:36 pm

Lots of ammeters do not have a built-in shunt for obvious reasons (dealing with the large current). A common standard is a 50mV meter (ie a voltmeter which is 50mV full scale.) For example a 50mV meter with full scale marked at 60 A requires a .050/60 =.00083 ohms shunt. Its expensive to buy such shunts, but you can fashion them out of ordinary wire. For example in this case you could use about 25 inches of #6 wire (google says it has a resistance of 1.3mohms/meter at room temp)_. Look up the resistance of other wire sizes and see what length would be required to get .83 mohms. Very small wires can lead to the actual meter... it draws little current.
You need to consider if the shunt gets hot. At 25A ( about the maximum you can safely draw continuously from your ancient electrical system) the power dissipated in the shunt would be about half a watt.... not a problem for a big sized resistor like this. So if you really have a 60A meter wanting an external shunt (and it is a standard 50mV meter) then you can use a coiled up piece of wire up near the battery hooked between the system wire (small wire to battery) and the battery. The voltage across this shunt is delivered to the meter with say a lamp-cord wire (well insulated cheap flexible wire). Your meter should be a zero-center meter and it will read current to and from the battery. Turn off the motor and you can see what the accessories draw and with the motor running you can see how well the generator/regulator keeps the battery charging.
Having said all this, I once set it up and abandoned it; I just didn't like the looks. BTW when I hook anything directly to the battery (or the unfused connections to the battery) I always fuse whatever I connect. In fact on my cars I fuse the system wire (50A), so that the only unfused wiring in my car is the huge starter cable. I have no idea what Porsche was thinking when they ran all the unfused circuits around the car.... maybe the intention of selling replacement wiring harnesses. (In my MG group they call the result of a short "letting the smoke out of the harness")
Bill

Peter Poulikakos
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Re: Amp meter install shunt size

#14 Post by Peter Poulikakos » Sat Jul 13, 2019 6:12 am

I have ordered a 60 amp vdo gauge with an internal shunt. I will use 6awg cable that should cope with the current.
I have also found a usb volt meter on ebay that I will plug directly into the lighrer socket with a usb adapter.

https://www.ebay.com.au/sch/i.html?_fro ... r&_sacat=0

I would like to place the gauge inside the vehicle, Im not sure as yet how to route the cable.
Any ideas?

I will keep you all updated.

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Re: Amp meter install shunt size

#15 Post by Al Zim » Sat Jul 13, 2019 5:23 pm

Greg Bryan is correct! You should install a volt meter. No heavy wires to run. It can be plugged into the back of the ignition switch for the power in and the ground will be through the connection to the dash by the bracket that holds it in place. I have a 60 amp VDO Ammeter which is the same size as the clock brand new 1973 manufacture. Cheap al zim 800.356.2964
 

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