Parasitic draw amp meter

[brazil716]

I am trying to build this tool, what is confusing me is the ground that comes off of the negative side from the 0.5A circuit braker, just below D4. Since this is suppose to be an AMP meter I don't understand what this is grounded to. Also the picture of the tool only shows two wires coming out of the tool , a red and black wire. Can someone please give me some guidance.
Thank you

 

[Harald Kapp]

This ground is internal to the circuit only. It is the commoen center of the two batteries so you can have -9V and plus 9V on pins 4 and seven of the operational amplifier. Do not connect this ground to the car's electric system. Current measurement is only through the two input wires, the 1Ohm resistor and the circuit breaker.

Note that you have to open the electric circuit of your car at the battery's + pole. Modern car equipment (car stereo, car computer etc.) often require that you enter a code after re-connecting the battery (that's for theft protection), so have your codes ready.

 

[brazil716]

Parasitic amp meter

Mr. Kemp , I appreciate your response. I am still having difficulty understanding this ground. Your saying that it is an internal ground part of the circuit, this is a plastic case and nothing else is grounded internally. What would I attach this ground too? Could you please elaborate a little further. Thank you

 

[(*steve*)]

ground is simply a name given to that part of the circuit from which most voltages are measured. This may be connected to the chassis or to an earthed connection, or it maybe floating.

In your case it sounds like it is floating (not connected to anything external).

Part of the problem is that we use the word "ground" to refer to several things.

 

[Six_Shooter]

Wow, most of the "info" in that video is bunk.

In my 20+ years of working on automotive electrical I have used nothing else but my off the shelf DMM(s) to find parasitic current draws. I have not had a problem.

I'm curious how they expect someone to find the offending circuit without removing fuses to disable circuits, it is never explained in the video.The only way to find the offending circuit IS to break the connection to it, I.E. start pulling fuses, one at a time, until the draw reduces to the level it should be at, THEN reference a wiring schematic of what is in that circuit, and disconnect each component, one at a time again, until you find the offending device. This is the exact technique I used back in teh summer to find the amplifier that was staying powered up in my Yukon, though I didn't have to refer to a wiring schematic since the fuse was labeled in a way that pointed directly at the amplifier being the problem.

Ok, I watched the "How to use the parasitic current draw meter" video. They make one huge mistaken assumption, and that is that the circuit that has a problem will be on the first fuse that shows a large drop in current when pulled. This is not always the case. Sometimes there is a large current draw because the control module is staying powered on that draws low current, but the device it has signaled to stay on is the larger current draw. This is why you need to remove and replace one fuse at a time. Yes you may have to wait for the module on that circuit to go into sleep mode again, but it's the best way I have found to find faulty devices. I find the fuse that shows the largest drop in current.

I'd also like a good explanation of how current would flow back into the battery with the engine not running or a battery charger attached to some point other than the battery, as in "negative current". "Loss of ground" is not a good enough explanation since it really doesn't mean much.

The only points that I agree with are that some vehicles can take quite a while to allow everything to go to a sleep mode, some as much as 6 hours for certain modules. IIRC there are some newer GMs that take this long. *rolleyes* And that 20 mA is enough to cause a battery to drain quickly.

That being said I will likely build something similar, since it's very similar to another device I plan to build called the uCurrent, for measuring very small currents with an off the shelf DMM. I do like the idea of not needing to leave my DMM attached to a vehicle when I have other vehicles to work on, so that would be handy to have a self contained low current ammeter.

The node that the ground symbol is attached to does not connect to anything else, there is no extra connection to be made there. In this case it is showing where you would use as a ground reference if you have to take measurements of the circuit itself, to diagnose an issue with the circuit.

 

[(*steve*)]

Wow, most of the "info" in that video is bunk.

I have to agree,

I wrote my reply whilst on the train on the way home. I didn't watch the video, leaving that for later.

Oh, I wish I had.

I lost count of the errors.

 

[brazil716]

In reply to six shooters reply, I have to say that you are right in many aspects. I have been a mechanic for over 30 years. In the days of old, we use to hook up an amp meter to the batter negative and start pulling fuses till the draw dissipated. Today's cars are a lot more technical. For instance, I was looking for a current draw in a Mercedes, when I set my meter to milli amps , my meter never goes to Zero, it fluctuates well into the 20 milli amp rage. That is one reason I am trying to build this device. Second, if you pull a fuse and replace it , you wake up that module. Lets say that the amp draw lands on the 10th fuse you pull. Well, if you pull fuses and replace them , modules will be waking up and some going to sleep which will skew your readings. So pulling one fuse out at a time until your meter goes to an acceptable reading is the best way to check for a draw. What I like about this meter is that it can be zeroed out before being hooked up. This way , I know that everything I am seeing on this meter is an actual draw. My problem is with this ground , the case is plastic and no other device in the circuit is attached to this ground. So I am confused with this ground and what purpose it really has .

 

[Six_Shooter]

I have found that every time I tried pull a fuse and not replace, I got the wrong circuit, or wrong master feed.

I ALWAYS replace the fuse and wait for the current to settle again, this is the only way to know that another module or circuit is not keeping the offending system alive or being a contributing circuit to the draw. With how integrated the systems in newer cars are, simply removing fuses to find the draw will not always get you the correct circuit.

You will never see zero mA current draw on any vehicle newer than about 1982, I would actually say older than that. What you need to look for is the lowest possible draw. Generally once you get down to between 2 and 6 mA this will be the lowest standby current draw. A battery in good condition should last a couple weeks at this level of draw, depending on ambient temperature.

I'm not a mechanic, tried it for about 3 months and was bored out of my tree, but I have worked on automotive electrical systems professionally for more than 20 years, specializing in electronic accessories and diagnosing electrical issues.

As I said before you don't need to worry about that ground symbol in the schematic, it's a reference point, not a connection.

 

[(*steve*)]

You don't connect it anywhere.

 

[KrisBlueNZ]

I agree there's a lot of rubbish in that video, and I don't see any reason to make a special device to do what a multimeter does.

When you're measuring current in a fused circuit, set the multimeter to current range and connect it across the fuseholder before you pull the fuse. It will keep the circuit going and you'll avoid resetting all the modules. Multimeter probes with alligator clips are good for this. Don't disconnect the battery negative.

You have to be very careful to ensure that the clips are firmly attached and will not short onto other fuses or pop off when you unclip the fuse. And of course use a plastic tool to pop the fuse out.

 

[Six_Shooter]

The problem with that is that in many cars it is impossible to access the fuse holder contacts, to keep the item powered up. The other issue is that you may not find the overall current draw.

To find parasitic draw the measurement point, to at least find the offending circuit, is at the battery. Once the circuit is found, if it's a main feed to other fused devices the DMM can be moved to the main fuse location, or simply remove one fuse at a time in the down stream fuses, with the DMM still at the battery.

While it may take a while, each fuse needs to be re-installed if it is not the offending circuit. There are so many control paths, and possible current sink paths through other devices that each component needs its power re-applied to protect those devices, and get an accurate picture of what is happening. This also removes the possibility of placing the wrong fuse back in the wrong circuit.

I've been looking at the circuit, and think I can simplify it, eliminating 1 battery, and using a meter that only swings in one direction, or replacing it with something else. This would also eliminate the "zeroing" potentiometer, which seems odd to me anyway. It seems like a "show me what I want to see" deal not a "show me what is actually happening" scenario.

 

[KrisBlueNZ]

What advantage(s) do you think that project would have over a digital multimeter?

 

[Six_Shooter]

There are two reasons I'm going to build one (or a couple) in the future:

1) Practice with electronics and modifying circuits for my own knowledge.
2) I can use my DMM on other cars while performing long term current draw tests.

The OP seems to think that zeroing out the current draw is an advantage.

 

[KrisBlueNZ]

Well, you don't need any active circuitry to do that. This is all you need for a unipolar automotive current meter with a display range of 0~50 mA.

F1 is a Polyswitch resettable fuse. It's rated for a hold current of 0.4A. Current through it causes it to heat up, and at a certain temperature it quickly transitions to a nearly open circuit.

The time to trip depends on the ratio of current to rated current. The RXEF040's initial resistance is about 0.7 ohms; with a 12V system, this limits the current to about 20A. At 10A or more, the RXEF040 will trip in less than 30 ms.

D1 and D2 are 3A diodes. They will clamp the voltage across them to about 1.0V (positive or negative). They can withstand short overloads much greater than 3A.

R1 is the current shunt. A resistance of 5.6 ohms will drop 0.28V at 50 mA, which is the maximum design current of the circuit. At 0.28V the leakage current in D1 and D2 ought to be a lot less than 50 mA, but the graph in the data sheet only goes down to 1A. If anyone knows what the typical forward and reverse leakage current will be for a 1N5400 at 0.28V, please let me know!

R2 drops this voltage to a value suitable for M1, and provides the calibration adjustment. M1 is an electromechanical (pointer-type) panel meter with a full scale deflection current of 50 µA. The coil resistance is 3000 ohms; I got this value from the specifications for a 50 µA panel meter that's available from Jaycar (an Australian and New Zealand electronics supplier), catalogue number QP5012 (see http://www.jaycar.com/productView.asp?ID=QP5012). A 3000 ohm resistance at 50 µA has 0.15V across it; R2 drops the 0.28V down to 0.15V.

Simpson Electric (http://www.simpsonelectric.com) in Wisconsin also make 50 µA panel meters but theirs have a coil resistance of 1800 ohms. A value of 5k for R2 will work with these meters too.

Simpson Electric also make centre-zero analogue meters. A Simpson Electric 25-0-25 µA meter should drop in to that circuit if you want a bipolar indication.

The small components cost under $2. The main cost is the meter movement, the enclosure, and the probes.

The overall circuit has a resistance of about 6 ohms from one probe to the other, and can withstand a direct short across the battery. The polyswitch fuse can take a little while to cool down after an overload though.