Voltmeters - what kind do I get?

[KDOG3]

Any good inexpensive voltmeters that would let me measure current draw from the tailcap on my lights?

 

[clintb]

Define "inexpensive".

I've seen the Harbor Freight cheapie meters mentioned as having very good accuracy compared with very expensive units. The HF, seen here, can be had for around $3 on sale. I have one and it's good, just doesn't have much in the way of resolution (number of digits). Don't let that deter you though.

If $139 would be acceptable, you should check out the Protek D620, seen here. It has an optically isolated RS-232 output, logging, all kinds of good features and goes to five digits.

 

[Mr Happy]

Any good inexpensive voltmeters that would let me measure current draw from the tailcap on my lights?

If you want to measure current draw you need to use the 10 A range on a meter so that the voltage burden is minimized. And the accuracy on the 10 A range is inherently somewhat less than the best accuracy available on a voltage range. So one answer is that any meter will do. Simply buy the cheapest one you can find with a 10 A range, including one of those $3-$10 meters from Harbor Freight.

However, another complexity is that with regulated LED lights the current draw is not pure DC but has an AC component due to the switching circuitry in the buck or boost module. To measure that accurately you will need a very expensive meter like certain Fluke models, which is not probably a good answer to your question.

I have an idea that analog meters are better at DC+AC measurements than simple digital meters -- maybe someone with an EE background can chime in here?

It still comes down to the same answer though. Buy the cheapest meter you see that has a 10 A range, and then after using it for a bit you might want to make an informed decision about an upgrade. Consider your first meter as a toe in the water and don't agonize over it too much or spend too much.

 

[uk_caver]

Would there be an easy way to check the response of a meter to fluctuating DC currents (or voltages (ie where the current/voltage drops at most to zero, and doesn't reverse direction)?

Presumably, if a meter reading was stable, that would mean it wasn't taking snapshots at different parts of a cycle, but that it was doing some kind of averaging.
Is there any kind of cheap averaging/smoothing that would give a result that wasn't an average of the input signal, and if so, would it tend towards over or under-reading?

 

[Justin Case]

Average of a pure sine wave is about 64% of the peak. RMS of a pure sine wave is about 71% of the peak. Thus, averaging will underestimate the AC reading.

 

[uk_caver]

With flashlight electronics, we're not really dealing with AC, but [potentially] varying DC, generally something like a square wave.
RMS doesn't see to be a meaningful concept with flashlight drivers, since it relates to 'equivalent' DC voltage in the case of resistive loads, and with electronics and LEDs, we don't have anything like resistive loads.

Measuring average current drawn from a cell, a straight average over time is presumably what we want, so that if there's a 2A load on a 25% duty cycle, we'd be hoping for a meter reading of 0.5A?

 

[IMSabbel]

Average of a pure sine wave is about 64% of the peak. RMS of a pure sine wave is about 71% of the peak. Thus, averaging will underestimate the AC reading.

Well, averaging a pure sine wave is just 0....

 

[Justin Case]

Very clever, but we are talking about measuring AC current and the definition of the average of a pure sine wave AC current is the average for just the positive or rectified part of the wave.

 

[Mr Happy]

Well, really?

If we are considering a sine wave imposed on a DC current like this, then isn't the average current going to be where the dotted line is?

If we want the RMS average current that won't quite be true of course, but if the AC magnitude is much less than the DC magnitude it will be very close. Which gratifyingly means the reading from any ordinary current meter is not likely to be wildly off base when doing tail cap measurements.

 

[mudman cj]

Getting back on topic, I bought one of the HF cheapies and was disappointed for two reasons. First, I noticed right away that the sockets for the banana plug leads were crap. They were so loose that the leads fell right out and sometimes I even had to jiggle them to be sure I was getting a connection. Needless to say, I never used that meter for measuring resistances or high currents. Have you noticed that I have been using the past tense here? That is because after a few months of use, the voltage readings started drifting on me, and even with a brand new battery the thing was still so far off that I pitched it. The upside is that I bought a 'good' meter.

 

[Mr Happy]

Yes, my own experience of the HF meters is that they are far from high quality. However, as a $5 item to learn with I'm sure you get more than $5 worth of education from them...

 

[Justin Case]

Well, really?

If we are considering a sine wave imposed on a DC current like this, then isn't the average current going to be where the dotted line is?

You have your AC component and your DC component. The DC offset is a DC component.

 

[greenLED]

KDOG, get yourself one of the DMM's sold at Sears (Craftsman brand).

I'm not a rocket scientist, and the one I have (paid $9 for it, day after Thanksgiving sale) has done everything I need it to, and then some more.

 

[Mr Happy]

You have your AC component and your DC component. The DC offset is a DC component.

Right. You have a DC component with an AC component superimposed upon it. And I believe that

average(DC+AC) = average(DC) + average(AC) = DC + average(AC)

which is where the dotted line is?

Another way of saying this is that the AC ripple on a smoothed DC supply becomes less and less significant as the ripple becomes a smaller component of the DC base.