Another Scout Project

[Monkeh]

Because I doubt he's driving milliamps through LEDs with a nominal specified Vf of 3.6V. Even at milliamps, the burden voltage of a normal meter will be seriously problematic.

It's difficult (at least for me!) to be sure what he's dealing with on the basis of the 'data sheet' in the eBay listing. The 3.6V is specified as the 'maximum Vf' (seemingly at 30mA), but it's not just an LED (must have some flasher electronics) - so that figure may just be the 'maximum supply voltage', rather than Vf of the LED.

I do wonder, however, if he really does only have "μA" flowing. IME, anything less than about 1mA through an LED results in light output which is not necessarily easy to see! However, you may be right.

Kind Regards, John

A modern red LED is quite visibly lit at 200uA, and those are cheap ones.

But yes, you have a point, 3.6V is likely the maximum supply voltage for the electronics, the LEDs will be pretty conventional and possibly even constant-current driven by the flasher circuit.

 

[JohnW2]

Even at milliamps, the burden voltage of a normal meter will be seriously problematic.

I may be wrong, but I think you may be overplaying the extent of the problem posed by the voltage drop across the meter (what you're calling 'voltage burden'). ... I've just done some experiments with the cheapest and nastiest DMM I could find on my bench (well under £10 IIRC).

On the 20mA range (which reads down to 10μA), it has an insertion resistance of ~10Ω - i.e. when measuring a current of, say, 200μA, it results in a (measured) voltage drop (burden) of about 2mV; when measuring a current of 1mA, it results in a (measured) voltage drop of about 10mV etc. Particularly if you were right about the OP's LEDs only drawing "μA", this meter really ought to have been perfectly usable, shouldn't it?

Even on its 2mA range, the insertion resistance is only about 100Ω, hence voltage drops/burdens ten times the above figures.

Kind Regards, John

 

[Monkeh]

Even at milliamps, the burden voltage of a normal meter will be seriously problematic.

I may be wrong, but I think you may be overplaying the extent of the problem posed by the voltage drop across the meter (what you're calling 'voltage burden'). ... I've just done some experiments with the cheapest and nastiest DMM I could find on my bench (well under £10 IIRC).

On the 20mA range (which reads down to 10μA), it has an insertion resistance of ~10Ω - i.e. when measuring a current of, say, 200μA, it results in a (measured) voltage drop (burden) of about 2mV; when measuring a current of 1mA, it results in a (measured) voltage drop of about 10mV etc. If you were right about the OP's LEDs only drawing "μA", this meter really ought to have been perfectly usable, shouldn't it?

Even on its 2mA range, the insertion resistance is only about 100Ω, hence voltage drops/burdens ten times the above figures.

Kind Regards, John

My experience is that most meters are an order of magnitude greater than that, which leads to significant problems at these voltages.

Also probably worth noting that I'm generally in the mindset of getting usefully accurate readings, not 'ballpark' figures.

 

[securespark]

Whether it be insertion resistance (ooh err missus!) or burden voltage, or even a bit of both, my semi-posh DMM don't wanna know!!

 

[Monkeh]

Whether it be insertion resistance (ooh err missus!) or burden voltage, or even a bit of both, my semi-posh DMM don't wanna know!!

Let's see this semi-posh DMM of yours.

 

[JohnW2]

Whether it be insertion resistance (ooh err missus!) or burden voltage, or even a bit of both, my semi-posh DMM don't wanna know!!

What do you mean - that it gives no current reading at all? Does inserting the meter into the circuit make the LEDs go out? What range did you have it on?

Kind Regards, John

 

[JohnW2]

My experience is that most meters are an order of magnitude greater than that, which leads to significant problems at these voltages.

Well, as I said, they don't come much cheaper or nastier than the one I played with. I didn't lie

Also probably worth noting that I'm generally in the mindset of getting usefully accurate readings, not 'ballpark' figures.

Very laudable, I have to admit, but for the exercise in question 'ballparks' were really all we needed. Having said that, how 'usefully accurate' do you want? - a 2mV insertion loss when measuring 200μA is surely not going to impact significantly on 'useful accuracy', even with the small voltages we're talking about, is it?

Kind Regards, John

 

[Monkeh]

My experience is that most meters are an order of magnitude greater than that, which leads to significant problems at these voltages.

Well, as I said, they don't come much cheaper or nastier than the one I played with. I didn't lie

The cheaper of my two meters is also the better on burden voltage. On the other hand, it has vastly less resolution and accuracy.

Also probably worth noting that I'm generally in the mindset of getting usefully accurate readings, not 'ballpark' figures.

Very laudable, I have to admit, but for the exercise in question 'ballparks' were really all we needed. Having said that, how 'usefully accurate' do you want? - a 2mV insertion loss when measuring 200μA is surely not going to impact significantly on 'useful accuracy', even with the small voltages we're talking about, is it?

Kind Regards, John

Down at the lower end of the range, less of a problem. Once you get up to the higher end of the range, it can be a major issue.

Try to remotely accurately calculate battery life on a 1.8V microcontroller with your DMM some time, then test the numbers. Assuming it doesn't trigger a brown-out reset when active.

 

[securespark]

I say semi posh because it cost a fair bit more than a cheap DMM.

It's an Alphatek TEK 265. I know you can get them cheaper now but when I got mine it was a good piece of kit and cost in excess of £50 including the dreaded.

It reads zero.

 

[Monkeh]

I say semi posh because it cost a fair bit more than a cheap DMM.

It's an Alphatek TEK 265. I know you can get them cheaper now but when I got mine it was a good piece of kit and cost in excess of £50 including the dreaded.

It reads zero.

Aww, how cute. Sorry to say internally it's as cheap and crap as the £5 ones. Does at least have a fuse, though..

 

[JohnW2]

It reads zero.

On what range (or is it auto-ranging?) - and do the LEDs go out (or reduce in brightness) when you insert the meter into the circuit?

Kind Regards, John

 

[Monkeh]

It's not autoranging..

It's likely got at least 1k impedance in the mV range.

 

[JohnW2]

It's not autoranging.. It's likely got at least 1k impedance in the mV range.

I presume you mean mA. My cheapo one seems to have very neat numbers - almost exactly 1kΩ on 200μA range, 100Ω on 2mA range, 10Ω on 20mA and 200mA ranges.

However, I still want to know what happens to his LEDs when he puts his 'posh' meter into the circuit!

Kind Regards, John

 

[Monkeh]

It's not autoranging.. It's likely got at least 1k impedance in the mV range.

I presume you mean mA. My cheapo one seems to have very neat numbers - almost exactly 1kΩ on 200μA range, 100Ω on 2mA range, 10Ω on 20mA and 200mA ranges.

Yes, sorry.

 

[securespark]

Sorry....I've been AFK trying to stem the flood of tears after that cruel, hard insult from Monkeh....

My eyes are not so good. Even when they're not full of tears....

The LED's certainly don't go out, but they may well dim a bit!