Another Scout Project

[ban-all-sheds]

I thought their motto was "be prepared"?

What is it best to be equipped with in order to be prepared - a PP3 battery and some wire wool, or one of these:

 

[securespark]

OK. So I've ordered some tabbed 2032 cells and various 3.4V LED's.

But, if I want to run 2 LED's off a 3V cell without a resistor (without doubling up the cells) could I use two 1.7V LED's?

That'd work, wouldn't it?

 

[JohnW2]

OK. So I've ordered some tabbed 2032 cells and various 3.4V LED's. But, if I want to run 2 LED's off a 3V cell without a resistor (without doubling up the cells) could I use two 1.7V LED's? That'd work, wouldn't it?

Are you talking about just 'bare' LEDs? If so, as I'm sure others will have said, to run them without any resistor is very iffy - since the current through the LEDs (assuming that the voltage across each is at least high enough for it to work at all) will then only be controlled/limited by the internal resistance of the battery.

Kind Regards, John

 

[securespark]

It's OK, as long as you ensure the voltage of the LED slightly exceeds the cell voltage.

 

[Monkeh]

It's OK, as long as you ensure the voltage of the LED slightly exceeds the cell voltage.

If the LED Vf exceeds the supply voltage it won't light at all.

 

[JohnW2]

It's OK, as long as you ensure the voltage of the LED slightly exceeds the cell voltage.

Again assuming you're talking of 'bare LEDs' it's the opposite way around from that - the cell voltage has to exceed the Vf of the LED (otherwise,a s Monkeh has said, the LED will not light at all). However, as I said, the current will then be approximately equal to the difference between cell voltage and LED Vf, divided by the internal resistance of battery, whatever that might be.

Kind Regards, John

 

[securespark]

http://www.instructables.com/id/LEDs-for-Beginners/step5/

 

[Monkeh]

And thus, why datasheets exist.

http://www.farnell.com/datasheets/461255.pdf

Page 5.

 

[JohnW2]

http://www.instructables.com/id/LEDs-for-Beginners/step5/

Well, just like your post (I guess that's where you got it from) they have a "1.7V LED" lighting with a a 1.5V battery - so goodness knows what they mean by "1.7V LED". It can't be an LED with a minimum Vf of 1.7V since, as Monkeh and I have said, that would not light at all with a 1.5V battery.

However, even with a Vf <1.5V, I can but repeat that, without a resistor, you'll be gambling on what the current is going to be.

When you get the "1.7V LEDs", I suggest you put one in series with a resistor of 47&#937; - 100&#937; or thereabouts across a 3V source and measure the voltage across the LED - that will give you an idea of it's actual Vf

Kind Regards, John

 

[Monkeh]

Depending on the chemistry of the lithium cells, you might light up two ordinary red LEDs in series. They won't be very bright if they do. Same goes for '3.4V' LEDs. The datasheet and a measurement of the voltage of the cell under a small load will tell you, as will simply trying it (good luck measuring the current, though, you won't have a low enough burden voltage to do it).

When you get the "1.7V LEDs", I suggest you put one in series with a resistor of 47&#937; - 100&#937; or thereabouts across a 3V source and measure the voltage across the LED - that will give you an idea of it's actual Vf

Kind Regards, John

That'll tell you the Vf you get with whatever the current ends up being. It is not a fixed value. It will also be out of spec for most small LEDs.

 

[JohnW2]

When you get the "1.7V LEDs", I suggest you put one in series with a resistor of 47&#937; - 100&#937; or thereabouts across a 3V source and measure the voltage across the LED - that will give you an idea of it's actual Vf

That'll tell you the Vf you get with whatever the current ends up being. It is not a fixed value.

Indeed, but (if the Vf at such currents is, as is being suggested, a bit below 1.7V) the resistor values I suggested ought to give 15-30mA, which is not an unreasonable sort of current at which to operate, and hence for which to determine the Vf, wouldn't you agree?

Kind Regards, John

 

[Monkeh]

When you get the "1.7V LEDs", I suggest you put one in series with a resistor of 47&#937; - 100&#937; or thereabouts across a 3V source and measure the voltage across the LED - that will give you an idea of it's actual Vf

That'll tell you the Vf you get with whatever the current ends up being. It is not a fixed value.

Indeed, but (if the Vf at such currents is, as is being suggested, a bit below 1.7V) the resistor values I suggested ought to give 15-30mA, which is not an unreasonable sort of current at which to operate, and hence for which to determine the Vf, wouldn't you agree?

Kind Regards, John

15-30mA is quite a lot. 2mA is pretty obvious when lit, 7-10mA is the max for most small LEDs, 20mA from a 5mm blue LED is an effective nightlight.

 

[JohnW2]

15-30mA is quite a lot. 2mA is pretty obvious when lit, 7-10mA is the max for most small LEDs, 20mA from a 5mm blue LED is an effective nightlight.

OK - put the resistor up to a couple of hundred ohms or so, then.

Kind Regards, John

 

[Monkeh]

I'd employ a constant current supply.

Saying that, I happen to have adjustable constant current supplies. Not a normal household item.

 

[JohnW2]

I'd employ a constant current supply. Saying that, I happen to have adjustable constant current supplies. Not a normal household item.

So would I - and, like you, I 'happen' to have one. However, most people in this forum probably don't, which is why I suggested just a resistor!

I'f I'd wanted to be more clever, I might have suggested two resistors, a zener and a transistor (plus a suitable voltage source) (i.e. a simple constant-current source), but a resistor seemed quite adequate for what I was proposing!

Kind Regards, John