How to measure voltage and amp with pictures link?
- Thread starter bunbut
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broadgage
Enlightened
To measure voltage you need a voltmeter (or a multimeter set to the volts range)
To measure the voltage of a battery, connect one lead to each terminal of the battery, red lead to postive and black lead to negative.
The meter will display the battery voltage.
Ideally this should be done with the battery "on load" i.e. connected to the lamp or other load it is to power.
With most flashlights etc, the on load voltage cant be readily measured, because the battery terminals cant be reached when the light is on.
To measure current you need an ammeter (or a multimeter set to the amp range)
Connect the meter in series with the battery, lamp or other article, such that the current passes through the meter.
For example to measure the current in a common cylinder style flashlight, such as a maglight, turn the light on and remove the tailcap. Connect the black meter lead to the exposed negative end of the batteries, and connect the red lead to the exposed screw thread that the tail cap screws into. The light should come on, and the meter display the current used (generally between 0.3amps and 0.9amps for regular flashlight bulbs, though much more for some specials disscussed on these forums)
If dont have one already, purchase a cheap multimeter, it will come with illustrated instructions.
Until you have more experience, dont attempt to measure voltage or current on mains electricity supplies since mistakes could kill.
To measure the voltage of a battery, connect one lead to each terminal of the battery, red lead to postive and black lead to negative.
The meter will display the battery voltage.
Ideally this should be done with the battery "on load" i.e. connected to the lamp or other load it is to power.
With most flashlights etc, the on load voltage cant be readily measured, because the battery terminals cant be reached when the light is on.
To measure current you need an ammeter (or a multimeter set to the amp range)
Connect the meter in series with the battery, lamp or other article, such that the current passes through the meter.
For example to measure the current in a common cylinder style flashlight, such as a maglight, turn the light on and remove the tailcap. Connect the black meter lead to the exposed negative end of the batteries, and connect the red lead to the exposed screw thread that the tail cap screws into. The light should come on, and the meter display the current used (generally between 0.3amps and 0.9amps for regular flashlight bulbs, though much more for some specials disscussed on these forums)
If dont have one already, purchase a cheap multimeter, it will come with illustrated instructions.
Until you have more experience, dont attempt to measure voltage or current on mains electricity supplies since mistakes could kill.
Mr Happy
Flashlight Enthusiast
Be very careful when switching between voltage and current readings on a multimeter.
To measure current you need to use the 10 A or 20 A range, and this has a different socket to plug the red test lead into. This special 10 A socket is a direct short circuit between the leads! Always remember to put the red test lead back into the normal volt-ohm-amp socket when you have finished any current measurements. Never leave the test lead plugged into the 10 A socket.
If you try to measure a battery voltage with the red test lead in the wrong socket you might at worst blow a fuse or damage the meter if it is a really powerful battery. If you try to measure a mains voltage with the lead in the wrong socket (heaven forbid), something much more nasty might happen.
To measure current you need to use the 10 A or 20 A range, and this has a different socket to plug the red test lead into. This special 10 A socket is a direct short circuit between the leads! Always remember to put the red test lead back into the normal volt-ohm-amp socket when you have finished any current measurements. Never leave the test lead plugged into the 10 A socket.
If you try to measure a battery voltage with the red test lead in the wrong socket you might at worst blow a fuse or damage the meter if it is a really powerful battery. If you try to measure a mains voltage with the lead in the wrong socket (heaven forbid), something much more nasty might happen.
Bonky
Banned
Repeat a bazillion times!
At the very least this advice will save you money.
To bunbut - unless you are freakishly mistake proof , you will make this mistake sooner or later if you are going to start measuring volts and current.
I'd suggest implementing some kind of procedure now, while you're starting.
You could get a spare set of multimeter leads and mark both ends of one of the spare positive leads - use that for 10A range only. Alternatively , use some kind of plug to cover the 10A socket, so you have to remove it before using - not bullet proof but adds a step that reminds you danger lurks.
I'd rate myself above average for these kind of mistakes (not making them!), but I've destroyed 2 meters , blown several (expensive) fuses, destroyed the 10A function on 2 meters, I can't remember details of all incidents so unable to list damaged electronics etc. I could probably buy 3 Surefire M6s with the money wasted with those mistakes.
So think about it twice before you take any measurement :twothumbs
hopkins
Enlightened
I've made that mistake and burned out a sm resistor inside my digital dmm
- BK Tool Kit 2704B - followed the burned smell to the bad component on the
dmm circuit board. Replaced it and was very surprised the meter was fully functional again. A good lesson to pay attention.
- BK Tool Kit 2704B - followed the burned smell to the bad component on the
dmm circuit board. Replaced it and was very surprised the meter was fully functional again. A good lesson to pay attention.
Marduke
Flashaholic
Ooh what??
I've seen a meter blow right out of a guy's hand when it was on the wrong setting.
hopkins
Enlightened
people do seem to make mistakes with electricity a lot. Remember the MythBuster show where a .22 cartridge was used as a fuse inside a
pickup truck?
Does the OP really want pics of someone holding probes to a mag light?
measuring current I think? Youtube has many clips that'll give an idea.
pickup truck?
Does the OP really want pics of someone holding probes to a mag light?
measuring current I think? Youtube has many clips that'll give an idea.
TigerhawkT3
Flashlight Enthusiast
I have an Extech 411 that emits a continuous beep if you have a probe plugged into the amp jack while the unit is set to measure voltage. Maybe you guys should think about getting a DMM with a similar safety/warning feature. :shrug:
Bonky
Banned
wait why does it mess up the meter if it's plugged into the 10A jack while the dial is on voltage?
I mean, circuitly, why does this happen?
I mean, circuitly, why does this happen?
bluecrow76
Enlightened
If you're in the US, go to Harbor Freight and pickup a couple of their $5 Centech volt meters. They work great and give have given me the same readings as my BK Precision bench meter. I keep one setup for voltage and one for current on the 10A. Sometimes they're even cheaper than $5... lowest I've gotten them for is $3 each.
I did not know they existed. I was going to say manufacturers should do something - buzzer / flashing light etc.
Let's say you want to take a tailcap measurement , to measure currrent.
On a Mag , you remove tailcap , select 10A range on meter , with leads in COM and 10A, one probe on body , one on -ve of last cell - no problems you get your reading. You turn meter off , all is happy in the world.
Few days later you want to check the open circuit voltage of some LiON cells so you pick meter up , don't check what sockets you are connected to and proceed to take to take a voltage reading of the cell. You are now dead shorting the cell. Hopefully it is a protected cell and the protection circuit kicks in.
If you are measuring a mutli cell NiMh (say a "Torch" pack) probably a 100 or so amps will flow instantenously for a short period. Hopefully all that will happen is the fuse in the meter will blow. Worst case scenario can be very bad.
If you get a car battery and place a crow bar across the terminals , you know that's a bad thing right ? Massive current will flow, if the crow bar was held or jammed in place it would become red hot. Placing your meter leads , one on each terminal , using the 10A range is doing the same thing. If you remember to connect the leads to the correct sockets on the meter, you are simulating connecting a light bulb that is so low in power that you would need pitch black to be able to detect any light. ie extremely small amount of current will flow and nothing will melt.
It really is best to do some reading / reseach on these basics , if not for financial reasons , for safety. Despite the trail of destroyed meters I have left , I claim to know what I'm doing but no claims to being a good teacher.
This is the metalworking equivalent of leaving the chuck key in (done it!)
naked2
Enlightened
Great advice from all! :twothumbs
TedTheLed
Flashlight Enthusiast
what have you got against fuses?
cute little things, use more than their rated amps and pft..
my Sears meter has two. My Beckman meter too.
cute little things, use more than their rated amps and pft..
my Sears meter has two. My Beckman meter too.
Bonky
Banned
why don't the 10A jacks have fuses?
hopkins
Enlightened
If dmm's came supplied with a third probe - colored yellow with black stripes for easy
identification- that was always plugged into the current jacks, you'd know to never use it for voltage measurements by mistake
identification- that was always plugged into the current jacks, you'd know to never use it for voltage measurements by mistake
2xTrinity
Flashlight Enthusiast
Ideally, a current meter will be zero resistance, that way, you will not affect the circuit you are trying to measure. Fuses and fuse holders however have some resistance to them. This resistance is small, and usually not significant for currents on the mA scale (as voltge dropped across them will be proporional to the current), but it me be very significant to change the behavior of some circuit you're measuring on the 10A scale.
One of my mistakes - meter had a fuse, which did blow. Several tracks on the circuit board were blown apart though.
If you make "the mistake" with a CR123 cell, it will probably top out around 4-5Amps , not blowing the fuse , but causing a dangerous situation - ie cell explosion. More dangerous if testing a pack of series cells.
An unprotected 18650 would probably not blow the fuse, but again , you'd be in a dangerous situation.
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