# Problem with testing current using a multimeter

Discussion in 'The Projects Forum' started by alexx, Jun 22, 2011.

1. ### alexx Thread Starter New Member

May 29, 2011
29
0
hello!

Im trying to find how much current is going to a viewfinder i have:

I detached the viewfinder from the bulk of the camera and then i found the + and - wires going to the viewfinder. then i measured different values from + to - using a multimeter:

direct voltage= 5.12 V
alternating voltage= 10.4 V

so that was good i think, then i measured the resistance and i got a bunch of different values (can someone please explain this):

Ω at - 200 = -61 on multimeter
Ω at - 2 K = -1.583 on multimeter
Ω at - 20 K = -7.05 on multimeter
Ω at - 200 K = -191.7 on multimeter
Ω at - 2M = -0.601 on multimeter

note* the values showed negative because when i measured with the red multimeter tester touching the + source of power (from the bulk of the camera/camera battery) it did not work for some reason. so as an alternative i measured with the red tester touching the - source of power from the camera battery

Now about the current measuring, i have the bulk of my camera powered on and the view finder attached, the view finder is on and working because i can seethe blue screen, but as soon as i try to measure current the entire camera powers and the current falls to 0 quickly. my camera makes a weird sound when i do this

CAN SOMEBODY PLEASE HELP ME MEASURE ALL THESE VALUES?!

the reason i need to do this is because i want to power the viewfinder using a smaller battery source like 4 aa's, so i need to match up all the values so i dont fry my expensive viewfinder

THANKS !!!!!!!!!!

2. ### alexx Thread Starter New Member

May 29, 2011
29
0
just to clarify some typos:

DC voltage= 5.12 V
AC voltage= 10.4 V

Ω at --> 200
Ω at --> 2 K
Ω at --> 20 K
Ω at --> 200 K
Ω at --> 2M

3. ### someonesdad Senior Member

Jul 7, 2009
1,585
141
Do you know how to connect an ammeter to make measurements? If you connect it wrong (e.g., connect it across a voltage source), you could blow a fuse or damage the source. It would be best to make sure you know what you're doing -- read the information on ammeters here.

4. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,223
101
You are short circuiting the camera!! You can't measure current or resistance the way you're doing it. Resistance must be measured on un-powered circuits and amperage must be measured in series with the load.

I hope you haven't damaged anything yet but if you continue as you've described you eventually will.

5. ### electro_yas New Member

Dec 20, 2010
15
0
In some multimeters there is only + and - terminal for both voltage and current measuring specially in digital multimeters, but most of the multimeters there is a seperate terminal for current measuring . You will need to change the red wire only to the current terminal leave the balck (common) as it is if it is in the common.

6. ### alexx Thread Starter New Member

May 29, 2011
29
0
someonesdad: I think I know how but I will read this link and confirm, Thanks for the response!

CDRIVE: Oh that explains why the camera was getting hot! You said “Resistance must be measured on un-powered circuits and amperage must be measured in series with the load”. I don’t understand this because how can you measure resistance if the circuit has no power running through it? And okay so I have to measure the amperage with a load, but my viewfinder was still attached so isnt that the load? If not could you please help me with what i can do to make a load so I can measure the current going through the viewfinder. Also, when I am matching these values from a separate & smaller battery source , should I put 5.12 V to the viewfinder (see my first post), or 10.4 V (also first post). I don’t really understand where the 10.4 V came from but it said that on the multimeter screen. Thanks for the response!

electro yas:That’s exactly what the multimeter im using is, I left the black terminal in the common hole, and the red terminal had two holes I could choose from, 1 said it measured milliamps and the other said it measured amps. I tried both but, which one should I use for this type of power source? Its and older sony camera, one of the big black ones. Thanks for the response!

7. ### Adjuster Well-Known Member

Dec 26, 2010
2,147
301
Resistance meters supply a small amount of current from their own terminals as part of the measurement process. Any external power source may invalidate the measurement or even cause damage.

Current meters require to be placed in SERIES with whatever constitutes the load, in your case the viewfinder. You would do well to get these ideas really clear in your mind before going any further, or you may destroy something - depending on the power source of your camera (Lithium cells )you might even injure yourself or someone else

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8. ### pistnbroke Member

May 9, 2011
32
1
you would be better to measure the current drawn from the main battery with and without the viewfinder and deduce the difference is what the viewfinder draws ....

9. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,223
101
You can measure the current like this. Measure first with the range finder unplugged, then measure again with it connected, then subtract the difference.

Note! Since you said that you attempted measurement on both the mA scale and A scale, it's a very good chance that the mA scale now has a blown fuse.

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10. ### alexx Thread Starter New Member

May 29, 2011
29
0
okay, appreciate the replys, so what Ive gathered so far is to measure values according to this picture i drew (there is also a pic of my multimeter):

http://www.youtube.com/watch?v=oA_rMmwu_4g

So, please correct me if im wrong:

Set up 1 will give me the voltage being supplied to the view finder only. should i measure this voltage with the V -- on my multimeter or the V ~? in the manuel it says V ~ is AC voltage and V -- is DC voltage

set up 2 will give me the current going to the viewfinder? and when i measure the current do i use 2m, 20m/10A, or 200m (see the multimeter in my link) and which terminal do i put the red prong in?

set up 3 will also give me the current going to the view finder if i first measure the current with the view finder then subtract the current value measured without the viewfinder.and when i measure the current do i use 2m, 20m/10A, or 200m (see the multimeter in my link) and which terminal do i put the red prong in?

Note in my diagram i have my goal. once i measure the voltage and current going to the viewfinder can i match the voltage with small battery's (aa's) and use a resistor to match the measured current? will this work?

Thanks a lot for your time!!!

11. ### wayneh Expert

Sep 9, 2010
13,627
4,420
Probably not. A resistor only works when the current is constant, which may not be likely. Light bulb, yes, viewfinder, I think not. But if the power supply to the viewfinder is actually a regulated steady voltage, which makes sense, that's all you need. The viewfinder will take whatever current it needs, at the regulated voltage.

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12. ### alexx Thread Starter New Member

May 29, 2011
29
0

are you saying that if i measure the voltage using set up 1 in my drawing and the multimeter says 5.14 V, i can supply 5.14 volts from a different battery straight to the view finder and i dont need to worry about a large amount of current frying the viewfinder?

13. ### Pencil Active Member

Dec 8, 2009
271
38
I think I remember your "viewfinder" thread from a while back.

Let me try and help you with your meter questions.

First open the back or battery compartment and check for
a fuse. Check fuse to see if it is blown. The fuse you are
looking for is probably 200 or 250 mA (milliamps) (.2A or .25A). Replace fuse
if necessary.

To measure an unknown current:
1. Move red lead to 10A connection.
2. Set dial on meter to 20m/10A
2. Connect meter in series with load. (Like your set-up 3 or 2)
3. Look at value on meter.

If reading is above 200mA (.2A) This is your value. You are done.

If reading is between 20mA (.02A) and 200ma (.2A):
1. Disconnect meter from circuit
2. Change red lead to mA connection
3. Set dial to 200m
4. Connect meter in series with load. (Like your set-up 3 or 2)
5. Look at value on meter. If value is above 20mA this is yor value
you are done.

If reading is between 2mA (.002A) and 20ma (.02A):
1. Disconnect meter from circuit
2. Change red lead to mA connection
3. Set dial to 20m
4. Connect meter in series with load. (Like your set-up 3 or 2)
5. Look at value on meter. If value is above 2mA this is yor value.
You are done.

If reading is below 2mA (.002A):
1. Disconnect meter from circuit
2. Change red lead to mA connection
3. Set dial to 2m
4. Connect meter in series with load. (Like your set-up 3 or 2)
5. Look at value on meter. This is yor value. You are done.

What is the purpose of doing this?

Post a description of what you are trying to do and a link to
the devices involved. Maybe there are other ideas.

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14. ### alexx Thread Starter New Member

May 29, 2011
29
0
wow Pencil that is actually so helpful thanks!!! i appreciate it. i will measure the voltage and measure the current using the technique you posted and ill reply with the results to confirm that their reasonable. i will also post a link of the overall project goal and hopefully i can also get some help on how to match the measured voltage and current using aa batteries /// basically just a way to power the viewfinder without frying it

15. ### Pencil Active Member

Dec 8, 2009
271
38
I understand your concerns about applying too much current, but
you must consider this fact.

The device in question probably draws varying amounts of current
depending on what functions it is performing. An example would
have the device drawing very little current when just sitting
there doing nothing, but as soon as activity begins the current
needs will go up. If you limit current with a resistor based on data
gathered when device was "sitting idle", you could find that as soon
as you tried to "make the device work" it would run out of steam,
no more current would be allowed through the resistor, the device
probably would not work properly, if at all.

This is what wayneh was trying to tell you.

16. ### Pencil Active Member

Dec 8, 2009
271
38
Weren't you trying to connect this viewfinder to
a webcam or something? If so did you find out if
the viewfinder output was compatible with the input
requirements of the other device?

On the basic operation of your multimeter:
Don't be afraid to ask questions. Most test equipment,
even the most basic has instructions that assume you
have some prior knowledge, which may or not be true.

Even instructions for a toaster tells what the knobs do but,
if you didn't know what toast is you'd be lost.

17. ### alexx Thread Starter New Member

May 29, 2011
29
0
Oh okay that makes sense, so the viewfinder most likely has a maximum current that it draws and a minimum current it draws. After I measure the voltage & current and obtain/post the values, and after I figure out how to match these values with regular batteries, if I supply the matched values to the viewfinder and it doesn’t work properly can I alter the new power source (the batteries) to allow a little more current flow (to match the maximum current being pulled, and constantly supply this constant maximum current)? Or is this risky

and yea my original plan was to use a webcam, but to many problems came up with compatibility like you suspected, so I am going to use a pinhole camera. A pinhole camera has a positive and negative terminal that connects straight to a 9 V battery (the cameras designed to run off 9 V), and it has one video terminal (which will connect to the viewfinder video terminal). So it shoulddd be compatible with the viewfinder, not to mention I read that people have done this before. Any thoughts on that?

18. ### wayneh Expert

Sep 9, 2010
13,627
4,420
I'm saying that's very likely, however you'll note I said "probably" in the previous post. That's because it's impossible to know for sure. It would be very normal the viewfinder to be powered by a voltage-regulated 5v supply. It's hard to imagine it NOT being designed around a fixed voltage supply. But my inability to imagine doesn't make it a fact. There's always some risk in repurposing a circuit, in assuming you "know" something.

19. ### WellGrounded Member

Jun 19, 2011
32
2
alexx,

The "AC" voltage measurement that you got may be because you were picking up a signal of Pulsed Width Modulator (PWM).

An easy way to check is to get an analog meter and set it to "AC" in the range you want, or one range higher. Test the voltage and then reverse the leads and test again. If you get only one reading then it is a pulsed DC voltage.

An analog meter has a built in diode in the circuit to the coil for testing AC and it is only measuring 1/2 of the AC cycle. When you reverse the leads the internal diode may now act as a blocking diode to the DC voltage and no current goes to the coil if the voltage is DC or pulsed DC whether it is a square wave or sine wave.

Another way to check the voltage type is to put the analog meter on DC and put the scale higher than it should be and check with normal polarity. Increase the scale until the needle moves slightly only. Reverse the leads and test again. If you get a small reverse movement of the needle then you have a DC or pulsing DC circuit.

On a full wave AC voltage if you use the DC setting the needle won't move since the fast reversing magnetic field in the coil neutralizes itself and coil doesn't have time to respond. When testing for AC with the DC setting use a much higher range setting than what you are testing for because AC current is still going through the coil.

Danny

20. ### Pencil Active Member

Dec 8, 2009
271
38
So far it sounds like you need a regulated 5V power supply.
This may or not be true depending on further testing.
If you do need 5V.

Are you interested in building one quickly, easily, with readily available
parts?

7805
.22uF Capacitor
.1uF Capacitor
Board

Total cost≈\$8

Buy these items, assemble using solder and some smallwire if necessary.
Schematic for connections are in datasheet attached. Page 1 upper left.
Check package for pin locations on actual part.
Get any battery (or battery pack) anywhere from 7.2V to 30V.
(Hint: Rechargeable tools have good, detachable battery packs.)
Hook battery to input + and - respectively.
Test output for 5.0v (may be slightly higher or lower (4.8-5.2))
Hook device to output + and - respectively.

This is not the most efficient way of regulating voltage, but is good
for experiments.

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