Getting the correct current through an LED

[Froskoy]

Hi,

I've got a 5V supply and an LED. The on current through the LED should not exceed 20mA at which point the voltage drop across the LED is 1.5V. Given an LED and 5 silicon diodes, connect the circuit such that the LED lights correctly.

So if the LEDs and all 5 diodes are connected in series, will this ensure that the current through the LED is limited to 20mA.

Many thanks,

Froskoy.

 

[Nigel Goodwin]

Sorry, but it's a REALLY, REALLY badly written question, and obviously written by someone with no understanding of electrionics.

There's no way to guarantee the current using just diodes, and it would be an absolutely stupid way to try and do it.

 

[Reloadron]

I strongly agree with Nigel. What the question is likely getting at is an assumption (a bad assumption) the forward voltage drop on each silicon diode is .7 volts so 5 * 0.7 = 3.5 volts. That would leave 1.5 of the 5 total for the LED. This is really a poor example and not very well thought out. Actually it is a crap shoot and a very good example of what not to do.

Ron

 

[DerStrom8]

Yeah, diodes are a terrible way to do this, and are absolutely never used in the real world for this application. Maybe you could bring this question to your teacher/professor and point out the errors. You may be able to get some extra credit Resistors really should be used to limit current through an LED, not diodes.

Der Strom

 

[Nigel Goodwin]

Any bother with him, ask him to post on here - or see if he will allow you to post his email address and we'll discuss it with him.

To be fair, in the UK it's dead common for electronics to be taught in schools by someone who knows absolutely nothing at all about it. You send a geography teacher on a one or two day course, and that's it - a qualified electronics teacher!.

 

[Diver300]

LED current seems to be one of those things that there is loads of poor information about. Often circuits take no account of component or power supply variation.

For instance searching for "led calculator" comes up with this page **broken link removed**

The calculations take no account of variations and are often wrong.

If you ask for an array of 8 LEDs at 2 V each, running from 12 V at 10 mA, the first suggestion is to put 6 in series with a 1 ohm resistor, then 2 in series with an 820 ohm resistor.

Change the current to 1 mA, and the 6 LEDs still need a 1 ohm resistor, while the 2 now need 8200 ohms.

The configuration is really bad for real-world conditions, where 6 LEDs in series on a 12 V supply, with effectively no series resistor could have just about any current. All LED arrays where the current is controlled by the resistor should have at least 10% of the supply voltage dropped across the resistor, and more if the supply isn't well regulated, but that rule isn't instituted in the calculator.