I saw a request for a write up on how to use a multi-meter in the electrical forum. I did the basics for the DMM, then on the second page, I started adding some Ohm's law and wire size charts. Since some people ask about what size wire here, I though it might be handy to go to this page, http://sites.google.com/site/reax222...attredirects=0 It's a rough draft, and I am still trying to think of interesting info to add. If you have comments or questions please let me know.

 

Looks pretty good and I'm sure lots of folks will find this useful! I'll keep this in the back of my mind over the next couple days and see if I can think of anything else useful that would fit in that writeup.

PS.. I see you tamed down the resistive color code

 

ReAx,
Good write up but wanted to point out a couple of added notes.

1) Using continuity test you may still here a beep under low resistance measurements so even when the beep is heard it may not always exibit that it's the same wire or conductor just maybe a low resistance circuiit. Typically 100 ohms or less.

2)Power cable table should be identified by " Insulated Cunductors" not cable. Conductors also have a derating in higher temperatures.

I know this is a basic chart for most users but wanted to clarify some of the details.
Old school ohms law used to be Rape Our Young Girls. (red,orange yellow, green) guess they seemed it was inappropriate in this day and age.

 

I took out good before the connection bit in the continuity. It's close enough for anyone needing a primer on meters.

I also changed the power cable table to power wire (insulated) table. For the purpose of this chart it's close enough.

 

Pretty cool, nice of you to make this effort.

I would recommend re-evaluating this statement:

"Connecting a battery as above while in amperage mode will blow the fuse in the meter."

It won't. The only way you can blow the fuse in the meter shown is if the leads are connected between the "A" terminal and the "COM" terminal, as they would need to be for any amperage measurement with this particular meter.

Some DMMs use the regular voltage terminals to measure current on the lower ranges, and you have to move a lead to use the 10A range. I don't believe that is the case with this meter, though. Looks like you have to use the "A" terminal for all current measurements.

Other than that, looks great!

 

By mode, I had hoped it implied both switched to Amperage and the leads in the correct pins on the meter. I really wanted to state it that way because the other meters I have used (cheap ones) had only one pin for each lead and on that style if you switched to Amps and metered a battery, you'd be replacing a fuse in a couple of seconds. I was trying to generalize to help with any multi-meter.

The Fluke I have now is more sophisticated than I need and probably more meter than most people have sitting around that don't use them professionally. I have a feeling that most people who can use this, have a meter like my back up that I couldn't find last night. A real simple $20 DMM, basic functions and just a red and black pin.

 

That's what I meant by "tamed down" LOL

ReAX -- this might be a good place to put a little description about LEDs and get into why some of them don't work with a normal flasher, without the load resistor, etc etc.

Also, another gotcha that you run into sometimes with a digital meter is if you're looking for a fairly fast signal (turn signals comes to mind) the DMM may not react fast enough to really see it. That sometimes leads to false test results, so you'd rather use an analog meter. Not sure if this is worthy enough for this article but it happens.

And, if you get into analog meters, be sure to warn to not reverse the leads.

That's all that came to mind. Good work! I tried to rep ya for this but it says I need to spread some around first. I thought I'd been spreading it around pretty good... oh well...

 

I've never used a VOM aside from just checking that I have voltage (not for the actual voltage). So I am not knowledgeable enough to write about they now.

Have you had the turn signal issue with meters? My auto range acts a little funny, but I can tell it is adjusting and therefore has voltage. I usually either just switch to manual range or use the hold modes.

On LEDs I don't know enough about them to write a short thing on them. I can calculate them, but I can't make a simple explanation between them and Ohms law that wouldn't take another half a page. Care to write something out?


Actually, I don't mind making another that covers LEDs, Ohms Law and a basic cheat sheet. I might work on that a little later.

 

No, just that a digital meter works the way you describe. When the voltage is there for a short time, you just see the display flicker and begin autoranging. Some folks get confused by this since they don't see it read 12V. Its not a big problem though usually.

I guess the main thing about LEDs other than the calculations and such is how some of them (alone) don't work with a normal flasher that uses a bi-metal switch. A fairly common question is that someone installed new fangled LED bulbs for their turn signals, but now they don't flash, they just stay on solid. Here's a revised version of something I wrote up on a Bronco forum back in 2005. Maybe its helpful.. maybe not, I don't know what kind of flashers we have on our trucks these days. Maybe its not relevant anymore...?

"One of the deals with the flashing lights is that LEDs draw much less current than a regular bulb does, so the bi-metal part of the flasher circuit doesn't work properly. So, you'll need to get the load resistor to connect across the LED to simulate a regular bulb. This will allow the circuit to draw 2 amps or so, as opposed to the 10-20 milliamps(mA) that the LEDs alone will draw.

The flashing action is due to a make & break of the circuit, because of the flasher. Inside the flasher can is a bi-metal switch that is normally closed (to turn the light on) and as current flows through it, it heats up and the bi-metal action breaks the connection (turns the light off). Now that the circuit is open, no current is flowing, so the bi-metal part cools down and reconnects to its original position (and turns light back on) and the cycle repeats. 10-20mA is not enough current to get the bi-metal action to work"

 

The bimetal cans are slowly being replaced with electronic flashers, but I think the 99-04 still had them.

Try this for a cheat sheet. http://sites.google.com/site/reax222...attredirects=0

 

Looks good

 

I am reworking a page just for LED wiring. I love this formula I just found. It is so much easier than having to work ohms law back and forth to get the resistance and amperage right.

I have 1 and 3 LEDs diagrammed in series. I am going to put up an example or two for series Parallel too. Anything else I should add? Now I can't remember how to do parallel/series.

 

LED's (Light Emiting diode) exhibit specific semi-conductor functions and shouldn't be confused with or considered exhibiting the same or similar characteristics as a light bulb. A diode or LED has several different functions. Of coarse the "Lighted diode" (LED) has a more specific function as it is being used for indication. It is a common electrical device that restricts flow of an electrical current. It also requires flow in a particular direction. It contains a Anode and Cathode.

From a basic standpoint the anode of a LED (or any device containing such) is the terminal where current flows in. The cathode of a device is the terminal where current flows out.
LED's have different voltage ratings, typically very low (around 5 volts or less) therefore needing a limmiting resistor to minimze excess current flow. LEDs live and die on current, the resistor helps protect them from getting too much.

For example a 3.4V, 100ma (milliamperes) LED with a 4 volt power source will need a resistor for each LED in parallel, and each will want 100 mA for itself. 10 LEDs will need at least 1 amp availiable.

Now, if you had a higher voltage supply, you can wire several LED's in series. for a 12 volt/ 1 amp supply, you could run 30 LED's, only 10 resistors (3 LEDs in series + one resistor).

 

I understand the theory, it's the math I'm sucking at. I think I rethought it enough though. I am using 3.1v 20mA LEDs in my examples. I have two strings of 3 LED. Each LED is 155ohm, so each string is 456. The two strings in parallel have a resistance of 232.5ohm. I need 0.040A to power this circuit, meaning I need a total resistance of 300 ohms. So I use one 67.5 ohm resistor after the two strings? Does this work?

 

I put page up to now. I would really like some feed back on the parallel formula on the bottom.