# exceeding an analog ammeter printed scale

I have a ~$10 DC ammeter I mail ordered however long ago, on the display is printed Dagatron 7203 and on the lower right side of display is an upside down U followed by an upside down T then 2.0 It is a ~2" square meter, with an analog display of 0 to 1 amp , with major markings every 0.2 amp with a minor marking of 0.01 amp. I used this gauge playing around charging lead-acid 12v automotive batteries. I now want to use it to measure parasitic drain on an installed car battery. My question is: what happens if DC current draw exceeds the 1 amp marking of the ammeter? I expect current to be well below 1 amp when I have this gauge connected, but in the event I open the car door and interior lights go on which is maybe around a 5 amp draw what would happen to the ammeter? Will it survive undamaged if it sees current greater than 1 amp? When exceeding the ammeter display how much over and for how long is reasonable to expect no damage? • You can measure the Voltage at 1A and verify the shunt R. Then put a smaller R across it to get more Amps FS. Null adjust may go off it he needle bangs the endstop. The overvoltage is often diode protected for moderate over voltage. – Tony Stewart EE75 Dec 23 '19 at 5:24 • @jsotola Open the car door, replace the fuse. Adjust the rear view mirrors, replace the fuse. I like this solution! With enough time and a large enough box of fuses, this could really work! – jonk Dec 23 '19 at 6:59 • ron, if you want, you can easily set things up to dump anything over an amp to bypass the meter without changing the current meter range and without pinning the meter. But of course... you'd need a circuit for that. (The parasitic drain on my Sienna (I just had to verify this) is a max of $200\:\text{mA}$ and, just after installing a fresh lead acid battery in the car it will sit right at about $200\:\text{mA}$ for about 30 seconds, drop to about $100\:\text{mA}$ for a little while longer, and then arrive at about a constant $50\:\text{mA}$.) Where would you install your meter? – jonk Dec 23 '19 at 7:01 • @jsotola I was thinking that the OP wanted a permanent installation. This allows me to instantly detect problems in the car that have as a side effect an increased drain. (Since it was what I wanted to do some time ago, perhaps I'm just projecting. But I also already know there are lots of products available to do this test if it is just a matter of disconnecting the positive terminal for a moment.) – jonk Dec 23 '19 at 7:25 • @jonk, that functionality makes very good sense – jsotola Dec 23 '19 at 7:29 ## 2 Answers You should expect that exceeding the rated current of 1A by even a small amount will instantly damage the meter. There is no other safe assumption given the limited information you have. So, you must limit the current flowing through the meter. There are two fairly easy ways to do this. You can add a shunt resistor in parallel with the meter so that most of the current will flow through the shunt. For example, if you add a shunt resistor equal to one-fourth of the meter's resistance then you will actually have a full-scale reading of 5A while only 1A goes through the meter. The other approach is to add a diode clamp across the meter so that if the voltage across the meter becomes too high then most of the current will pass through the diode and not the meter. However, this only works if the voltage across the meter happens to be as large as the forward voltage of a diode, about 0.3V for a Schottky diode or about 0.7V for a silicon diode. In either case, be sure to consider the total current and power ratings for the shunt element. • the ideal ammeter has zero internal resistance, so as to drop as little voltage as possible as current flows through it. I know my$10 ammeter is not ideal but it measures accurately and matches my digital meter for 500mA. I'm guessing ammeter resistance is less than 1 ohm? Confused as how to do anything like you mention and not affect accuracy of the meter or cause a voltage drop such that the body control module then sees too far less than 12.65 volts. – ron Dec 23 '19 at 18:08
• i was hoping an analog ammeter such as this when needle goes far right, would internally shunt and allow excess current through whatever little connection that's made? And the amount of excess current would be dependent on the size of that connection and heat generated in the gauge? – ron Dec 23 '19 at 18:10
• i also have the ammeter connected on the negative side, in series between negative battery cable and negative battery post – ron Dec 23 '19 at 18:17
• @ron - analog meter movements get their sensitivity from having many turns of wire on the internal coil which interacts with a magnetic field. In order to have many turns, the wire needs to be very fine. Very fine wire overheats/burns up very easily. So no, there is no internal shunt for overcurrents. You may be able to handle current spikes - and you may not. As for diode protection, put a 1 amp current through the meter. Then, use your digital meter to measure the voltage across it. This will let you know if a diode will help. – WhatRoughBeast Dec 24 '19 at 0:58

## Put an NC pushbutton across the meter

It shorts the meter at all times, except when pushed. Pushing the button un-shorts the meter, making the meter as the only current path, and causing it to give a meaningful reading.