Multimeter question

[AlessandroShinoda]

Hi everybody,

I would like to ask two questions:

Do we need a multimeter that measures uA for building guitar pedals ( fuzz, overdrive, big muff) or mA is already enough?

Second question: do we need a multimeter that measures HFE (transistor testing)? If not, how do you test your transistors?

Thanks!

[bluebunny]

Welcome, Alessandro!

I would say mA is plenty for pedals, unless you're doing something esoteric (can't think what that might be).  Indeed, in all the years making pedals, I've never once measured current - ever.  Now voltages on the other hand...  As for h_FE, you'll probably find it useful when making fuzzes and similar.  A lot of multimeters seem to offer it these days.  Or else check out the ubiquitous Chinese multi-tester gadgets on eBay that are great for testing components (leaving your meter for testing your circuit).

[AlessandroShinoda]

Thanks, Bluebunny.

Sorry for this newbie question, but how come you never measure current?

Instead of having the HFE function on the DMM, would this method work as well? It seemed to work...and btw in this case he did measure uA.

https://www.youtube.com/watch?v=nD7fYhdISGI

Thanks.

[DIY Bass]

I have measured current exactly once.  EQ board in a bass amp that was drawing no current and passing no audio, so I figured there was a good chance that all the op amps were dead, as they should draw something even idling.  Replaced them all and that was about 20 years ago and it's been fine ever since.

[antonis]

Sorry for this newbie question, but how come you never measure current?

It's not Marc's bad habbit.. 
(NOBODY can measure current - not even DMMs..)

Current is calculated by the voltage drop across a known value resistor (Ohm's Law)

About h_FE measurement: It's only useful for low gain / high leakage depended circuits (like Ge fuzzes) 'cause (IMHO) a well designed circuit should be independed of actual BJT h_FE (and its variations..)
(BJT switches excluded, of course..)

[EBK]

(NOBODY can measure current - not even DMMs..)

You lost me there.  I can totally measure current with my DMM.  I just don't because it is often inconvenient to do so (need to place the meter in series), and voltage measurements give me the information I am looking for.

[anotherjim]

The point made is that the meters we have are actually always only measuring voltage, even on resistance ranges. To measure current, you need to insert a known value resistor (known as a "shunt") in the meter IN SERIES with the feed you want to find the amperage of and measure the voltage drop across the shunt resistor. To reduce the effect on the circuit of inserting extra resistance, the shunt is a very low value like 0.1ohm. To do small current measurements, the shunt doesn't need to be particularly low ohmage so meters with mA ranges usually have a separate socket and a higher value shunt resistance.
If the shunt was 1ohm then all the dmm need do is measure voltage across the shunt and display A or mA instead of V, since as said, ohms law tells us 1V across 1R means 1A flowing through that 1R. For a 0.1R shunt, it just displays the decimal one place left.

The big issue, and why we don't habitually measure current with the meter is one of convenience.
Most DMM force you to plug the + probe into a socket in the meter reserved for current measurement.
With cheap DMM, you won't get told to do this, and more importantly, won't be told to move the probe back when you have finished.
Using the current jack in the meter means that the internal shunt resistor is now connected across the meter probes. If you test voltage in that condition, that low value shunt is as good as a short circuit between the probes.
The meter current socket is protected by a fuse. Measuring a power supply voltage with that will probably blow that fuse. The DMM then won't work on current until you replace the fuse.
More expensive DMM have switching in the jacks to detect the probe positions. If you select a voltage range while you have a probe in a current jack, it will display a warning "probes!" or something similar.
A major inconvenience is that you must break the circuit under test in order to insert the meter probes in series with it.

Anyway, sooner or later, you will get tired of changing the current jack fuses and find current another way. Besides, fuses have fairly uncertain resistance, as does the fuse holder contacts  - this is a major concern for calibration accuracy. In a professional test environment, a meter with a blown fuse should go right back to the calibration lab before it can be trusted to measure current. This is also why the calibration lab often puts its sticky label over an access screw.

So, do it with voltage...

If a Wha pedal has a power supply input filter consisting of a 1k5 resistor with a 220uF capacitor, then ohms law gets us the current if we measure the voltage across the 1k5 and divide it by 1500.

If a tube triode has a 100k plate resistor then measure voltage across that 100k and divide by 100,000 tells you the idle current of that stage. Now, I can't imagine why anyone would want to disconnect the 100k to clip a meter in series to get the current when you can do with voltage and not disturb anything.

[bluebunny]

Sorry for this newbie question, but how come you never measure current?

A bit of a blunt answer, I'm afraid, and not to devalue your question: but why would I need to measure current?  I can't think of a reason.    Unless you think you're within the last one or two mA that your chosen power supply will deliver and you've added yet another pedal to your board and you won't or can't buy an additional PSU...

Aside: I have had occasion to calculate current, but that was a valve amp build where these things are important in working out power dissipation, etc.  And the method was as mentioned above: you measure the voltage drop across a known resistance and do a little arithmetic.

[duck_arse]

and to your second question, I would say yes, we do need a meter to test hFE's. it is the easiest thing in the world to poke a transistor into the meter before you solder it into circuit to make sure you do actually have what you think you have, ie transistor polarity correct and emitter lead [and base, and collector] identified correct. seeing as the meter is laying there on the bench anyway, why not use it?

and welcome.

[PRR]

The needle-meter, standard for decades, *directly* reads current. Put a wire in a magnetic field. The force is proportional to current. Add a spring and a needle, you read current.

Yes, DMMs are really a 200mV Volt Meter plus a front-end to read other voltages or currents with multipliers or shunts. But it is still "a measurement" even if I mark the height of the porch on a stick, walk it off with a CD, and go measure the CD on a ruler. In transfer measurement you have to be aware of added inaccuracy, but many-many measurements happen this way and that's fine.

IMHO, current is not an essential function in most electronics. You have to break the circuit which is often inconvenient. A current meter is a LOW (ideally zero) impedance, and in a world full of Voltage Sources which can supply high current to low impedance, it invites blow-up. As Jim says.

Even if I want to read current in a battery lead which is already broken for repair, I really would rather throw in a low value resistor and then read the voltage across it. Voltage ranges are hard to kill. Resistors are cheap.

[EBK]

The needle-meter, standard for decades, *directly* reads current. Put a wire in a magnetic field. The force is proportional to current. Add a spring and a needle, you read current.

You received my psychic page!
I honestly thought, "I know Paul could set things right here!" (Although any sort of jokes and sarcasm could be expected here, I VERY non-jokingly thought you'd have something worthwhile to add, and I'm really quite glad you're chipping in here.
  )

[amptramp]

One of the current measurements I have made in the past was dark current in a car where the battery had gone down.  After recharging it to a sufficient voltage to actually drive normal current you have to use a DMM to measure the current (which is usually between 15mA and 20 mA).  I tried making this measurement and promptly killed the fuse.  The inrush current was far above the 200 mA fuse limit.  The way to do it properly is to attach the battery cable to the battery, attach the leads then separate the cable from the battery.  That way, the inrush current is taken before the measurement is made.

You often face the same thing in power supplies.

[AlessandroShinoda]

Thank you all for your replies.

So, basically if I need/want to measure current I only need to use Ohm's Law?

[bluebunny]

So, basically if I need/want to measure current I only need to use Ohm's Law?

Yep. 

[merlinb]

Do we need a multimeter that measures uA for building guitar pedals ( fuzz, overdrive, big muff) or mA is already enough?

No. The current range that most meters come with will be fine, if you ever need it, which you rarely will.

Second question: do we need a multimeter that measures HFE (transistor testing)?

Not really these days. It's better to get a cheap component analyzer. They do more and are easier to plug the legs into!

[AlessandroShinoda]

Second question: do we need a multimeter that measures HFE (transistor testing)?
[/quote]
Not really these days. It's better to get a cheap component analyzer. They do more and are easier to plug the legs into!
[/quote]


and what component analyzer would it be?

Thanks!

[merlinb]

and what component analyzer would it be?

Search for 'component tester' on eBay

[antonis]

(NOBODY can measure current - not even DMMs..)

You lost me there.  I can totally measure current with my DMM.  I just don't because it is often inconvenient to do so (need to place the meter in series), and voltage measurements give me the information I am looking for.

I might have to make discrimination between direct & indirect measurement..

P.S.
I always use a 0.1R/15W wirewound resistor for heavy currents "measurement" even if estimated current value is much lower than DMM's fuse rating..

[amptramp]

When rebuilding a tube amplifier, I usually put a 10 ohm resistor in series with the output tube cathode so I can get a scale of 1 volt = 100 mA for setting the bias so that the cathode current is correct for the output stage.  I leave it in just to facilitate testing.  Sometimes the output current is stated as a plate current so you have to be careful that there is going to be a screen current as well.  Not too bad with beam power tubes with aligned grids and the screen current is low but not so good with pentodes like a 6CA7 where the screen grid is wound in the opposite helical direction from the control grid.

[cnspedalbuilder]

I agree with all that it is useful to have a component tester separate from a DMM. It's much easier to use for checking all component values, and it doesn't take up much space. They are cheap too.

I definitely recommend shelling out a few extra bucks to get one with a plastic shell or case, because the wiring that holds the LCD is delicate and the LCD glass moves around a lot if not enclosed. I replaced my enclosure-less component tester with one that included an enclosure kit one for $9 from the unfortunately named site banggood.com