Minimags - do they boost current?

[Black Rose]

I received some 5mm red LEDs yesterday. They power up nicely with a 3V CR2016 cell, so I thought I'd plug one into a 2xAAA minimag to see if it would be usable for a star chart light, since it's also 3V.

Well...the LED flickered, faded, and gave off a nasty smell. The minimag still works fine with the regular bulb, but the LED no longer works.

From what I have read about 5mm LEDs, reversing polarity is not supposed to damage them, so it would appear that I've killed the LED by applying too much current.

Do the minimags boost the current in some way in order to power up the stock bulbs?

 

[chimo]

I received some 5mm red LEDs yesterday. They power up nicely with a 3V CR2016 cell, so I thought I'd plug one into a 2xAAA minimag to see if it would be usable for a star chart light, since it's also 3V.

Well...the LED flickered, faded, and gave off a nasty smell. The minimag still works fine with the regular bulb, but the LED no longer works.

From what I have read about 5mm LEDs, reversing polarity is not supposed to damage them, so it would appear that I've killed the LED by applying too much current.

Do the minimags boost the current in some way in order to power up the stock bulbs?

Red LEDs have a lower operating voltage than white/blue LEDs. The reason it works with the coin cell batteries is that they cannot provide very much current without their voltage sagging. The minimag can and will provide enough current to burn out a red LED - an incan minimag does not boost voltage.

 

[shadowjk]

They do not boost, but 2xAA will be able to provide much more current than a tiny 2016 cell. If you've got some of those LEDs left, it'd be interesting to measure the voltage when you run them on the coin cell, as well as measure the current.

 

[Black Rose]

They do not boost, but 2xAA will be able to provide much more current than a tiny 2016 cell. If you've got some of those LEDs left, it'd be interesting to measure the voltage when you run them on the coin cell, as well as measure the current.

The light that I originally intended to use these red LEDs in is powered by 2 CR2016 cells connected in parallel.

When testing the existing white 5mm LED in the light, the voltage is 3.03 V and a current of 30 mA at the LED.

When testing the red LED powered by a single CR2016 cells, the voltage is 1.91 V and a current of 30 mA.

I connected a red LED to the intended light, the voltage is 1.96 V and a current of 26 mA at the LED.

 

[Marduke]

One of my most used lights is a 2xAAA MiniMag running a single 5mm deep red LED direct drive off two NiMH cells. It's Vf is 1.6v IIRC. Yes, it does get warm, and no, there isn't much heatsinking. But at the same time, I rarely use it more than a few minutes, and in my experience (and understanding) 5mm red LED's are often able to tolerate much more abuse than the relatively much more delicate 5mm white LED's. Plus, even if/when it burns out, it's less than $2 at Radio Shack (less than 13 cents on DX) to put a new one in there. That's cheaper, and much longer lasting, than the stock 2-cell incan bulbs.

I also have several fauxtons running a red off 1xCR2032

 

[Black Rose]

These LEDs are DX sku 9075.

They have no specs at all listed with them, just saying they are 1000-1500 MCD.

 

[Gunner12]

The Minimaglites have no circuitry in them, just the bulb connected to a pressure switch.

Since the AAA batteries have much more capacity and can take much more draw then CR2016 batteries before sagging in voltage. This difference killed the LED(as the other members posted).

Most Red LEDs run at around 1.6v. Higher Vf ones might get up to 2v.

You can probably try almost dead (1v should be good) AAA batteries. Those might work with the LED.

 

[Darkpower]

I think you need a current limiting resistor in series with the LED. I don't know why it didn't burn out on you when you did it with the CR2016 other then the button cell just can't source enough current to burn it out.

 

[Black Rose]

Most Red LEDs run at around 1.6v. Higher Vf ones might get up to 2v.

You can probably try almost dead (1v should be good) AAA batteries. Those might work with the LED.

I tried a 1.5V button cell....the LED wouldn't even light up.

 

[Black Rose]

I think you need a current limiting resistor in series with the LED. I don't know why it didn't burn out on you when you did it with the CR2016 other then the button cell just can't source enough current to burn it out.

There is no room inside the body for a resistor or anything else...it's extremely tight and flat.

I modified two of these light tonight and both are "working".
The second light has an even stronger pair of CR2016 batteries in it.

 

[Mr Happy]

I tried a 1.5V button cell....the LED wouldn't even light up.

Yes, that's normal. 1.5 V would not exceed the Vf of the LED. You would need the 2 V or so that two dead AAA batteries provide.

The thing is that LEDs don't have a linear relationship between voltage and current. As you increase the voltage on an LED the current goes like this:

Zero - zero - zero - zero - lots - too much -

To stabilize the current somewhere between zero and lots you need some form of external current restriction like a resistor, cells with high internal resistance, or a regulator circuit.

Button cells work with direct drive because they just don't have the power output to smoke an LED like big cells do.

 

[TigerhawkT3]

There is no room inside the body for a resistor or anything else...it's extremely tight and flat.

I modified two of these light tonight and both are "working".
The second light has an even stronger pair of CR2016 batteries in it.

I think he meant that you could put a resistor into the Minimag, not into the keychain light.

 

[Black Rose]

Any recommendations on the right resistor to use?

I was interested in electronics in my early teens and then discovered other things to do. It wasn't until I joined here that the bug was resurrected....now I need to recall what I learned oh so many years ago.

 

[Mr Happy]

You can work with your measured values of 1.91 V and 30 mA from the CR2016 cell that seemed to run the LED nicely.

Suppose a pair of fresh alkaline batteries provide 3 V. To limit the current to 30 mA you want to drop about 1 V through the resistor, which would make the resistor value R = V/I = 1 / 0.03 = 33 ohms.

This resistor value isn't critical, so you could experiment with values in the 20 - 40 ohm range and see what gives the best brightness. You will most likely find that when you reduce the resistance and increase the current beyond a certain point the LED stops getting any brighter and may even get dimmer. The best choice of resistor is the one that gives maximum brightness on fresh batteries.

 

[Black Rose]

Thanks for that information Mr. Happy.

I'll drop by the electronics store tomorrow and pick some up to experiment with along with some other stuff.

 

[Marduke]

I recently got a Proton Pro, so if you're interested, you can PM me about obtaining my 2xAAA MiniMag with a deep red LED. Works great on NiMH cells.

 

[Ron Schroeder]

If you are going to use this for star charts with dark adapted eyes, you may want to run it at a much lower current. I would start with a 100 ohm resistor and see how bright it is in the actual aplication.