Hi, not sure if anyone can help me but I am just starting out in electronics. I have bought a multimeter so I can go through a book I bought 'Starting Electronics' by Keith Brindley. I am trying to measure amps on it but cannot see where I can do it. I didn't buy the cheapest multimeter and thought it would do all of the basic functions but cannot for the life of me find how to do it unless I measure the resistance and voltage seperately. Here is a link to the multimeter I bought.
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any help would be appreciated, even if it is just to tell me I bought a duff multimeter :(
I wouldn't have thought it possible, but sure enough that meter won't measure current. Sorry to be the bearer of bad news. Here's a couple of cheap ones that will do it. It's nice to have more than one meter anyway. :-)
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If you can send the other one back and exchange it, you might like something like this, it has 4000 counts, better accuracy and a PC interface (if you care ;-)
It looks like a decent meter, but it has no current ranges. It looks like the usual current functions were traded off for the capacitance meter and frequency counter functions. You have a 200 mV range, so you could insert a .1-ohm precision resistor into the circuit and measure the voltage across it, reading .199V as 1.99 amps.
Reading DVM specs reminds me of the Fluke 8050A I abandoned to hurricane Katrina. I was too busy piling the computers, the insurance policies and the cats into the car to think of it.
Thanks for that Stephen, wish I understood what you was going on about hehe. I'm gonna bookmark this thread then read this in a few months time when I have learn't a bit more and will probably understand what you mean then hehe. :P
Be very careful when measuring current as you will be passing that current thru the meter. Never touch the probes to a voltage source when the meter is in current measuring mode, or you will immediately blow the internal fuse since nothing will there to limit the current to a safe level. Be sure to read the documentation for the meter. Get some extra fuses, you will need them. ;-)
I've had various meters for 35 years, and the reality is that as a hobbyist I've never really used the current range. I remember doing it, for curiosity sake, with that first VOM (an analog meter) circa 1972, and that was likely the time I pinned the needle so hard against the stop it bent. There was a period when I only had VTVM (vacuum tube voltmeter) and it didn't have the ability to directly measure current, and I never suffered.
And another side note about that is to always make sure you use the current jack instead of the voltage one or you will blow the fuse in almost al cases. Its a good idea to get into the habit of making sure you always look at which one your using before you try to measure anything. Sometimes I forget and end up blowing a fuse... and unfortunately the fuses my amp uses are not stock.
Like yourself I occasionally like an excuse to trot down to Maplins, so I rarely visit their website, IIRC they signed me up for the newsletter at the counter, you can also ask for it on their customer service number.
Make it clear that you are asking for this, a while back I signed up for the Mailing Preference Service to stop all the junk mail I was getting - then the Maplin price crash letter stopped as well! This usually contains about £7 worth of vouchers so I had to phone them and get them to start sending it again.
Circa 4 May 2007 03:11:26 -0700 recorded as looks like Noodle sounds like:
In your first post, it seemed clear that you know how to find amp values by combining voltage and resistance:
"...cannot for the life of me find how to do it unless I measure the resistance and voltage separately."
What Stephen is saying is the same thing, and many high-power circuits that have a need to know current conditions real-time, use these small, precision resistors. They are called shunt resistors.
The value is chosen small enough so not to interfere in more than a negligible way with the current in the circuit. So, you build a circuit, get it wired up, and oh by the way put a 0.1 ohm resistor in the common path. Don't forget to put test terminals on each side. Get the thing running, and measure the voltage across the resistor. Multiply the voltage by ten (which is the same as division by 0.1: I = E / R), and you have the current.
You can use this technique as a way to measure current in any circuit. Interrupt the output path, and insert the precision resistor in the path (if you put it in the return/common/ground path, exposure to high voltage will be avoided). Reconnect everything, and voltage measurements across the resistor will allow you to simply convert the measurement to amps. This is precisely how ammeters work, when you buy one that has the function. The big giveaway hint is that in addition to AMPS range on the meter selector, it will have a separate input jack for measuring amps.
It looks like this meter has banana jacks set at the "standard" 3/4" spacing. You can get dual banana plugs with a built in set of holes where you can install a sense resistor of 0.1 ohms (for up to 10 amps), 1 ohm for
1 amp, and 10 ohms for 100 mA. Being very obvious on the meter, it is not so easy to flip the range switch or forget you're plugged into the current hole when you go to read a voltage. You can get good enough resistors (at least 1 watt) for less than the multimeter fuse. However, without a fuse, an exposed resistor could explode if you apply mains voltage or a 12 volt battery directly, so *be careful*. Always wear safety glasses, and if you are really prone to wrong connections, don a welder's outfit and a flak jacket :)
It's probably a good thing that the meter doesn't have a current range. Newbies (and even old hands) are know to pop the fuse inside the meter
--or worse, smoke the resistors-- by leaving the meter set to a current range.
An external shunt resistor (if it's right at the meter) is also a reminder to reconfigure the measuring setup. If the shunt is part of your circuit, things may be even easier. . . BTW, it's OK to strip out much of the old text from your post when the blockquotes get to be quite long. All you need is a *reminder* of what it is you are responding to.
Then I (an electronics novice and wannabe competent) use my DMMs often to check the actual battery charging current from my old, rather indeterminate power supply with analogue meters. I don't need to check any more on my whizz-bang, digital-display, power supply, once I assured myself that it was near enough. [Interesting to see the actual voltage drop from my PS to the larger batteries at the end of 15' of figure 8.] jack
Back when I started out meters were a bit expensive, so it didn't take long to get in the habit of switching to the highest voltage range as soon as I'd completed a measurement, in fact this was one of the first things drummed into every student at the college I attended.
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