LED curiosities of a non-expert

On a sunny day (Wed, 14 Nov 2007 12:19:25 -0500) it happened "Peter S. May" wrote in :

The internal resistance of the LED, or, if you want, the diode chracteristic of its junction (I versus U).

Uc outputs do not have zero ohm when on (in pull up, or pull down). Those MOSFETS will limit the current, But that limit depends on temperature, supply voltage, etc...

Better have a resistor or driver.

Its I-V characteristic curve. For an ideal diode, the I/V relationship is roughly exponential; see:

At higher currents, the cell's internal resistance will also be a factor.

The simplified model of a diode as a switch, with zero conductance below the turn-on voltage and infinite conductance above it falls down in the case where you're connecting a low impedance voltage source directly.

Such situations are generally avoided, as the current varies significantly with small changes in both the applied voltage and the diode's turn-on voltage, resulting in circuits which tend to work significantly less well in practice than in theory.

In the case of R=0, the LED current is due to semiconductor physics. See the I(V) graph on the LED datasheet. It follows a diodelike transfer function. Temperature dependant too.

Sure LED direct drive by 5V logic with a continous max output current spec that doesn't blow the LED can limit the current but at a price....higher IC temp. If the IC can handle the heat and works ok, it's ok.

D from BC

The relevant part of the output structure on the micro in this case is a p-channel MOSFET which is likely operating as a more-or-less constant-current source without the resistor (because Vds >> Vth). The problem is that the current is not well defined and could be too large for reliable operation of the micro and/or the LED. Chances are very good it will work most or all of the time with some chips, in fact the early Sinclair calculator omitted the drivers and series resistors in just this way to simplify the circuit and meet a GBP 99 (IIRC) price point. Something like a 5V PIC might not have as happy a result since their short-circuit current will likely be excess of LED and output rating. It may or may not work for a fair length of time at room temperature with a given chip. Microchip do not provide even typical graphs for their current chips, however the information was included on early datasheets. See, for example: (page 135)

For real designs that have to work reliably, it's best avoided, however that doesn't mean it won't work sometimes.

Best regards, Spehro Pefhany

Not just that but I recall there is a max current you can pull through the PIC's power pins. Even though a single PIC output may be able to source 25mA, the PIC can't source 25mA on all pins simultaneously. IIRC, YMMV, etc

Quite true, and with some micros there is also a per-port maximum, IIRC.

Best regards, Spehro Pefhany

"Peter S. May" wrote in news:3- SdnRcm0tuCs6banZ2dnUVZ snipped-for-privacy@comcast.com:

internal impedance of the cell. That's how those keychain LED lights limit LED current.