do i really need a voltmeter?

tsttm · Post by **tsttm** » 22 Jul 2009, 12:34

ok, maybe a stupid question & i know it's probably not a big deal to buy one of these things...but i'm just wondering- the step where you cut up the usb hub & connect it to the camera usb...is there any harm to the camera if the wires are crossed? or will it just not fire?

i'm just wondering, because then it's just a 50/50 trial and error to get the cables right before soldering.

If that's the case, it saves me from having to go out & buy a voltmeter that i'll never use again.

thanks!

tsttm · Post by **tsttm** » 22 Jul 2009, 12:36

ps how do you edit a post? and how do you subscribe to a thread? can't seem to see it.

oh well, i'm replying so that i can subscribe to notifications...

you1 · Post by **you1** » 22 Jul 2009, 12:44

I inadvertently crossed my wires, and my cameras still operate just fine; but, that doesn't mean anything.

I got extended warranty when I bought my cameras; therefore, I have been a little more brave with regards to experimentation.

Post by **daniel_reetz** » 22 Jul 2009, 13:39

I have also accidentally connected the V+ to GND, and had no ill effect, but it was only for a second. It's really a cost-benefit question. A 5-10$ multimeter may save you a $100 expense. Here in Fargo we have a Harbor Freight Tools, and there I can get multimeters for $2. I must have ten of the damned things by now.

: meter.jpg (46.57 KiB) Viewed 11088 times

If you are logged in, you should see an edit button on your posts. I don't know how to subscribe, I just dig through these forums obsessively.

tsttm · Post by **tsttm** » 23 Jul 2009, 19:23

wow..that's cheap. how do you guys in the states find enough things to spend your money on!

I decided to bite the bullet & pick up a multimeter (it was $30 CAD! PLUS tax). Maybe i just don't know where to buy these things for cheap. Anyway, i was wondering if someone could be kind enough to explain how to use this thing. It didn't come with any instructions. Here's what my multimeter looks like:

If i wanted to test the +5V & ground on the usb hub, which 'dial' do i set it on? And when i'm testing stuff, am i using the red probe or the black probe or both? If both- how do you use to test the hub since you're testing each one at a time. Also, i understand that i'll know the positive pin when i get +5V. How will i know i've found the ground?

Maybe the first thing i should do is find out how to test that my multimeter works. I tried taking a AAA battery & touched the red probe to + & black to - & went through all the dial options & the needle never moved. The multimeter takes two AA batteries & i also see that it also has two fuses. How do i know if the multimeter is working?

Any help to those questions would be appreciated. thanks!!

Post by **daniel_reetz** » 23 Jul 2009, 22:04

You want to measure DC voltage, so turn the knob to the "DCV" part, which is white. You want to measure around 5V, so try the 5V setting, but if the needle goes all the way to the right, you have exceeded 5V and you will need to go up to 25V to "see" the voltage.

You need to make a complete circuit to test anything. That means that you need to use both probes. To measure a AAA battery, simply put the black lead on the "-" side, and put the red lead on the "+" side (the side that sticks out). If your multimeter is set to an appropriate range (2.5 or 5V DC), you will see the needle move to around 1.5v. This makes a complete circuit.

Your meter should have come with a manual that will help you make measurements. There are also lots of great videos online. This one from Make Magazine is not too bad. http://www.youtube.com/watch?v=KzjMIcER4EU . Personally, I would try to get a digital meter instead of an analog meter. They are a lot more flexible, and for $30, you should be able to find all kinds of options.

Post by **daniel_reetz** » 23 Jul 2009, 22:08

Where are you located in Canada? Maybe we can find you cheaper stuff by searching online.

tsttm · Post by **tsttm** » 23 Jul 2009, 22:21

yeah, i was trying to do that with the AAA batt...& tried it again. I think i have a defective one or the fuse is blown 'cause the needle isn't moving. I think this one is a return, which is probably why there isn't a manual.

I'll exchange it & try a different one tomorrow.

It's cool, i usually do buy stuff online when i can. But, it's alright, my scanner's coming together now..i was going to hunt for a few things online..but i'd rather just work on it than wait for stuff to arrive in the mail. thanks for the offer- i'll let you know if i change my mind.

So regarding testing the usb hub- where do you put the probes? red on the pin i'm testing i assume...where does the black one go while i'm doing this? Also, how do you test for ground? (positive i know i look for +5V..but ground?).

thanks!

tsttm · Post by **tsttm** » 23 Jul 2009, 22:47

oh, & thanks for the youtube link. that was helpful..i should have searched youtube before.

but, from what i understood, they referred to black/-ve as 'ground'. So, is it safe to say that to determine ground & positive of the hub, i simply find the combination for the black & red probe such that i get +5V reading? That would make sense to me...but please confirm.

ok, haven't done any soldering yet, but you gave me the idea to find some soldering tutorials on youtube. thanks!

Turtle · Post by **Turtle** » 24 Jul 2009, 04:23

So, is it safe to say that to determine ground & positive of the hub, i simply find the combination for the black & red probe such that i get +5V reading? That would make sense to me...but please confirm.

It's not really safe to probe all the wires in all sorts of combination until you get 5V using the voltmeter you recently bought or the low cost Harbor Freight one. They're great for measuring stuff that don't contain logic circuits or sensitive circuits. Because as you begin to learn to use it, you can fry some of these circuits.

But it is safe to probe all the wires in all sorts of combination using a high impedance multimeter. For newcomers to electronics, get a high impedance digital multimeter with auto-ranging. This is fool proof stuff with simple readout. I found one on Ebay shipped for $9 some time ago (brand was Sperry.)

I use to own an expensive Fluke multimeter and it was a fluke, it died after a year. Some will have better luck. I also own an EZ oscilloscope and the function generator part of it died doing what you were attempting to do, which is simply finding the combination in a circuit. Lesson learned, know your circuit before probing.

This image shows a split USB wire. The wires are color coded to keep things orderly. Stay away from the white and green wires to be safe.

Red is positve +
white is data
green is data
Black is ground -

: usb.jpg (98.75 KiB) Viewed 11064 times

(above image: creative commons image photographed by wabbit42 of http://www.flickr.com/photos/wabbit42)

To answer the intial question "do i really need a voltmeter?" The answer is no. Here's why:

1) You can use the color coded wire picture from wabbit as as a guide.
2) You can use a diode as a polarity checker. They're in almost every electronics we use. Get it there and they're virtually free. Use it to find the polarity of a power supply. What does a diode look like? In the center of this creative commons picture is a diode with a numer "8" on it.

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