USB 5V to 1.5V

[mvs sarma]

A 1k resistor in series with the LED gave 200mA? I don't believe you!

What's the LED rated to?

Did you use a series resistor when it drew 1300mA?

Hi, As i understand from the post , it is 200mA with !K series resistor or 1300mA dirctly-- and he comments- either way more than the specified 100mA.

 

[Hero999]

Unless I'm misunderstanding something, ohm's law dictates that only 1.5mA should flow assuming the output from the USB is 5V and the forward LED voltage drop is 3.5V.

 

[mvs sarma]

I agree with you HERO999

 

[iONic]

Digital multimeters are darn accurate. The 1300mA was see when the Luxeon LED was connected without a resistor. The LED got warm within 5 seconds.

vF = 3.9v, Max I = 1000mA

Specs for the LED are here: https://www.luxeonstar.com/item.php?id=386&link_str=196::198&partno=LXHL-LW3C

 

[Nigel Goodwin]

Digital multimeters are darn accurate.

You also said "with a 1K resistor perhaps 200mA", which was complete drivel!!.

 

[iONic]

My apologies, I just remeasured this current and I do believe it's time to put FOOT-IN-MOUTH(yes I fell down and bumped my head), it is 2.25mA.

 

[Hero999]

May this bee a lesson to you in estimating figures.

2.25mA sounds about right when dealing with values of 5V and 1k, 200mA sounds far to large when dealing with those sorts of values..

 

[Nigel Goodwin]

May this bee a lesson to you in estimating figures.

2.25mA sounds about right when dealing with values of 5V and 1k, 200mA sounds far to large when dealing with those sorts of values..

1K is a nice easy value, because it's 1mA/V - so with a 5V supply (and no LED) the absolute maximum possible current is only 5mA.

 

[iONic]

Just be careful, they are teeny!

Indeed they are! does anyone have any suggestion on how to solder these things? I have some conductive adhesive that supposed to be good or would you have another way. All I have is a 35W soldering iron wit tips that are too big to be accurate.

 

[Speakerguy]

You need a narrow tip and preferably .015" solder to solder it right. It's the usual put some solder on a pad, tack down one pin to that pad, then solder the other pins. Conductive epoxy can work but getting a correct tip would be better. We've used conductive epoxy to fix some specialized FET's at work where the wire inside the package came off the silicon, but that's about the only place I've used it.

Alternatively, you can use the "horrible but will probably work" method of just flooding all the pins on one side with solder and letting them bridge, then wicking off the excess. Just try not to get the component too hot.

 

[iONic]

My friend has just finished building his CNC machine and I think I wil wait for him to scribe me a tiny board for the chip and SMD caps, then I will see what I can burn up!!!