# Multimeter selection, what to look for?

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1. Oct 18, 2014

### sur4j

I am trying to build a laser diode driver circuit which which should give a required output of around 1.25v and 200-250mA, I have been having problems with this and want to start inspecting it with a multimeter. The only problem is, I dont have one and really dont know how to select the correct one for my application. The laser diode driver circuit I am working on is the following:

http://2.bp.blogspot.com/-22qdMZWVALM/UtsZbSFpTPI/AAAAAAAAB3g/DZEglREGiro/s1600/DRIVER FOR 250ma OUT.jpg

Please note, this also includes an NPN transistor within the application. I would also like to use this multimeter for analyzing future projects which include large stepper motors such as the Nema 23 (270 oz) stepper motor.

The multimeter that I have been looking at is the following:http://www.ebay.co.uk/itm/VC99-6999..._Measurement_Equipment_ET&hash=item1c3095aa23

Would this be "fit for purpose" for my requirements?

Last edited: Oct 18, 2014
2. Oct 18, 2014

### nsaspook

I would save my money and buy a decent meter (Fluke) but the V99 should be OK.

Last edited: Oct 18, 2014
3. Oct 20, 2014

### vk6kro

A multimeter that isn't auto ranging is a lot easier to use.

Auto ranging meters can switch ranges very quickly and you have to keep checking to see if they have done this.

You can read 50 volts then 60 mV and think the voltage has increased.

To avoid this, you can lock the range on an auto range meter, but this gets annoying when you have to keep doing it.

Meters that let you leave the leads in the same sockets for most functions are more convenient than those where you have to move the leads for voltage and current.

Meters are available that give you extra functions, like capacitance, inductance, frequency, diode test and transistor gain testing. You may as well get any functions you might use in the future.

You can get meters with large numbers and tilting displays.

You can get meters that show the function being measured on the display along with the maximum value of that function.

That current generator is adjustable, so you should adjust it for minimum current before you connect the laser diode, by connecting the multimeter as a current meter across the output.

Then gradually increase the current while measuring the current with a multimeter in series with the input of the current regulator.
If you don't do this, you might damage the diode before you get the current adjusted.

4. Oct 21, 2014

### jim hardy

In addition to vk6's always sound advice, there's this other practical consideration:

I use an el-cheapo meter like this for 99% of my work.
http://www.walmart.com/ip/14521542

and keep a nice digital Fluke with true RMS in safe storage under the bed for those rare occasions needing precision.

If you are just starting out, it might be wise to begin with a $10 meter which, when it gets blown up by accident, isn't a great loss. One of these cheapies you can keep in the car trunk . They're quite handy for aiding stranded motorists. I do believe that little$11 meter would do your job on the circuit you linked. It has a 2.5 volt scale that's handy for transistor circuits.
A word of caution though - on any meter, form the habit of starting on a high range and switching down.
And on meters that have current scales - never rotate the range switch through a current range while connected to a circuit. You cannot turn the switch fast enough to avoid blowing the internal fuse(which means a trip to the hardware store).

old jim