Is my DMM Broken ???

[iso9001]

I'm trying to get to get the total current pull of my curcuit. It has a 12V regulator feeding a 5V pic.

I put the probes on the 12V and GND lines, set my MM to mA and got a reading of 23.2, with the unit off. I tried with the unit ON, and got the same reading.

I tried switching it to Amps and got the same reading. uA same again.

Do I just not know how to use a multimeter or is this thing broken ?

Voltage AC / DC, resistance, continuity, etc etc all work fine.

Ideas ?

 

[HarveyH42]

Need to put the ammeter in series, between power and load, not across (parallel)to the power source.

 

[iso9001]

Hmmm.... Thats going to be tricky.

How can I figure out the total current draw of my circuit if it goes POWER -> uC -> GND then ?

 

[audioguru]

You blew the fuse or wiring in the current-measuring part of your DMM because you shorted your supply with it.
As you were told, a current meter should be connected in series between the supply and its load.

How can a 12V regulator feed a 5V PIC without blowing-up the PIC?

 

[iso9001]

I meant the 12 INput end of the regulator. I'm trying to see how much load the entire cicuit is pulling including the regulator.

I don't know... I can't remember ammeter part ever working right.

So what I'm saying is I tested with one probe on 12V and the other on ground... thats in series to my board. How else could I have tested it ? I only have 2 power wires.

I should probably try another DMM then ?

 

[dknguyen]

You are going to have to break the circuit and put something series with the circuit to measure the current, whether it be ammeter or current sensing resistor. It's a dictated of physics and you can't get around it by buying a more expensive DMM...so don't try another DMM, nothing will be different. Use some wires and probe clips or something.

The ammeter never worked right because you were never using it right (probably what happened is you tried to measure current with it by connecting it in parallel with the load one time in the past, and then blew the fuse which was never replaced and prevented the ammeter from ever working properly since). There is nothing wrong with your DMM except that you blew the fuse because you used the ammeter in parallel instead of in series. Replace the fuse, break a part of the circuit and stick the DMM in ammeter mode in between.

The other way is to put a resistor in series and then measure the voltage across it and use V=IR to find the current. The problem is that you need an accurate DMM to measure the voltage because you the resistor must be very small to not have a large voltage drop across it. Either way, you still need to break the circuit. This method is only used for automated current sensing by a computer rather by someone testing the current draw of the circuit.

Well, there is another way... to use a hall-effect or magnetic field current sensor. It senses the current by sensing the magnetic field generated...but that's a project all in itself since you need another uC to interpret the output of the sensor. If you buy a meter that does that it will be very expensive and your currents are way too low for that method to work anyway.

Voltmeter- goes in parallel with the load
Ammeter- goes in series with the load
If you mix either configuration up with the other you will either blow the fuse (like what you did) or you no power can flow through the high-resistance of the voltmeter to the rest of the circuit and it will shut off.

 

[Papabravo]

Let 'em blow up another DMM. Serves 'em right. Maybe after the forth or fifth one a little light will go on...or maybe not.

 

[iso9001]

I found out that one fuse was blown. I wasn't the original owner so I have no idea when it happend.

I replaced it, checked in series and got the SAME READING. It doesn't change if its set to A, mA, uA.

But thanks for that last smart ass answer.

I do actually appreciate everyone else's advise.


Any recommendations on how to check a regulator's current draw ?

 

[ThermalRunaway]

I agree with your critisism of the last "smart arse" answer iso, no need for that from where I'm sitting. This is the "general" electronics section, meaning that any questions can be asked from the most basic to the most sophisticated.

If you want to find out the current consumption of a circuit, you MUST connect the meter in series with the supply line. In your case, you must cut the 12V positive supply line and put one end into the "mA" connection of your meter, and the other end into the "COM" connection of your meter. If you then switch to the D.C. mA scale of your multimeter, you should see a current reading - provided of course that you haven't managed to blow the fuse again?

I've done a very quick drawing to try and illustrate the point, take a look and see how it compares to the way you're currently using your meter. [Excuse the pun]

Let us know how you get on.

Brian

 

[dknguyen]

Any recommendations on how to check a regulator's current draw ?

You have to put break the circuit and put the DMM in series. THere is no way around it, something has to be put in series with the circuit for current draw. If it is still not giving a reading if it's in series with the circuit under ammeter mode with a new fuse, then perhaps your DMM really is broken.

Try sticking a very low resistance in series and measure the voltage drop acrosst that. Use V = iR to find the current (I assume the voltmeter part of the DMM still works fine). Remember about the tolerance in resistance, but your measurement probably doesn't have to be too accurate anyways.

 

[RadioRon]

To check the current draw of any load you should put an ammeter in series with the load. This means that you disconnect your circuit from +12V, then connect the + wire of the ammeter to the +12V, then the - wire of the ammeter goes to your circuit. You must choose the correct range on the meter switch, and on some meters you must also remove the + wire from the Volts/Ohms input and plug it into the Amps or MilliAmps input.

When you select AMPS or MilliAMPS function on your meter, you switch in a very low value resistor that is wired from +IN to -IN (or equivalent labels on your meter). As current flows through this resistor, the meter actually measures the voltage across it and then translates that voltage using ohm's law to display a current value to the user. So, the meter has a very low resistance when using the AMPS function. This is very different than when you are using the Volts (AC or DC) or Ohms function which make the meter look like a very high resistance. This resistor is tailored to the range being used. For example, if you are using a 10A range, the resistor is probably about 0.1 ohms, and may be capable of dissipating 1 Watt of heat. If you choose a lower range, like say 100 mA, then the resistor value is probably higher, maybe 1 ohm or higher and it can probably handle about 1 watt of heat again. Now, imagine what happens when you mistakenly put this resistance from your +12V supply rail to ground by toucing the + wire to +12V and the - wire to GND. Ohm's law says that if, say, you are on the 100 mA range, then perhaps the power supply tries to push 12 amps through that poor little meter's internal resistor. Of course, the power supply probably isn't capable of delivering 12 Amps, so it does what it can. Let's say it pushes 3 amps. The heat that poor little resistor has to handle is 3^2 x 1 = 9 watts! That's a lot of heat. And its only rated for 1 watt, so guess what happens. It burns out very quickly. That is, unless there is a fuse that blows before the resistor fries. Most of the meters I've had don't have a fuse in series on the current scales.

To avoid such problems, you must always be careful when using the Amps or Milliamps inputs or ranges on a meter. And most used meters that I have come across have blown internal resistors, all because previous owners did not understand or did not care how to use the meter.

I'm guessing your meter no longer works on the Amps or Milliamp scales. So the next easiest thing to do is to use an external resistor placed in series with your circuit (between the power supply and the regulator input) and then measure the voltage across the resistor. The likelihood that your meter's Volts range is still working is quite good. But again, think about ohm's law. Since you are putting a resistor in series, you have to worry about voltage drop and you need to avoid an excessive voltage drop. Usually in such cases, a voltage drop of 0.4 volts or less is considered harmless, so estimate the current draw that you expect (using IC data sheets and ohm's law), and choose your series resistor to give you about 0.4 volts of drop. Most people choose values like 1 ohm or less.

 

[iso9001]

THANK YOU:
ThermalRunaway, dknguyen, RadioRon, I really really really appreciate all the time you guys put into the posts, diagrams, and explainations!

The wiring going to this thing is very very difficult to get to but your posts gave me an idea. I'll test it tomorrow and report back here.

My calculation is 1.25mA in standby/sleep mode. Which for a new car is OK (according to the service manual), but I'm not sure if it sits well with me. I'de like it a lot lower. We'll see what real world comes out to.


Thanks agian audioguru, HarveyH42.

ps> My manual says these new cars have a standby of roughly 20 days in before the radio and engine controller lose ram, then a few more before they won't start. It says they have an ignition off draw of 35mA... I don't think someone did the math.

 

[Hero999]

It depends on the current you want to measure, if it's >10A or so it's better to use a current clamp which is a hall efect sensor that measures the magnetic field generated around the conductor.

 

[iso9001]

Eh. I hooked it up today and I think my DMM is broken or just plain sucks. Can't decide which.

First off. I set it to mA and, tested 12V to my 5V regualtor w/ the micro in sleep and rest of the board shut of via MOSFET. All pins set to output low. And I get a reading of 22mA (!!!). I have an led on the board I turned on (its a smd led w/ a 464ohm resistor to it, pic outputs high to turn it on) and I get a reading of 29mA. So this makes me think the DMM is 'working' but I think that LED should be drawing 10mA no? According to the meter its drawing 7mA. Also, I can not for the life of me figure out why my board is drawing 22mA at sleep, with all outputs low, all other IC's on the board shut off via p-ch mosfet. It should be 1mA.

What also makes me think some thing is wrong is that when I select the unit to A and test it gives me a reading of 2.00A.... Uh... shouldn't it read .02A ? In uA it just beeps at me (thats fine, I get it).

I decided to test a car to see what that read. I turned the car off, pulled the + battery terminal and tested there (no key in ignition). The A setting read 2.0A to me again (arg) and the mA setting just beeped at me. The service manual for the car says it has an ign off draw of 35mA. Obviously it must be higher since I can't read it w/ my meter (which must have a mystery limit on what it will read).

Ideas ? Is this thing broken ?

 

[dknguyen]

What do you measure when you just test a piece of wire?

 

[audioguru]

Test your current meter with something like a known voltage with a known resistance for its load.
We don't know the actual idle current of your regulator, we don't know the actual voltage of your LED and we don't know how high is the output voltage of the PIC when it has a load.

 

[iso9001]

I'll try the wire test and the known value... I think a resistor and a battery should work well for that.

I'm a little concerned b/c if its right I need to cut 22mA out of my sleeping/idle board. That sucks.

 

[Nigel Goodwin]

Ideas ? Is this thing broken ?

They way you've misused it, I would suspect it's got a very high chance of being!.

 

[ThermalRunaway]

If you have managed to destroy it, Ebay is quite good for multimeters. Make sure you know how to measure current properly before you use a new one though. Grab yourself an Electronics simulation package - that'll give you an idea of how to use one without the risk of blown up test equipment.

Brian

 

[iso9001]

Nah, Its working fine. Its my circuit thats having a problem.

I'm using a load switching mosfet to shut down half my board. Somehow even with the source (5V) unplugged I'm getting ~3-4V on my Drain pin. So the rest of the board is powering back up. I have no idea where its getting this power 'from' but its my goal for tomorrow.

I'm guessing one of the devices I have shut down is directing all loads placed on it (maybe the SPI pins) into VDD and thus back to my drain pin ? I don't know... Its late and its pissing me off.

If I figure it out I'll post.

DMM works great tho, thanks for fuse / general advice. Quiescent current of regualtor is .520mA, really close to what the datasheet said. Thats at least one good thing I've found.