bcarso
Well-known member
To All: If anyone has some of these parts could you find out where the Vbr of the LEDs are? The experiment would place a 100k or so R in series with a reasonably high voltage d.c. supply and observe what the voltage across the LED is with a decent high-Z (say a typical 10M input R) voltmeter. If you read about 1.5V then reverse the connections to the LED ;-).
For loose LEDs of various types I've been measuring, the Vbr is anywhere from 25 to 40V. I sent Perkin Elmer an email but not surprisingly I've gotten no response. I may try to get them on the phone, but they may not know the answer themselves and might well be unwilling to divulge who their LED supplier(s) is. All any vendor will say is don't hit the part with more than 3V reverse, or in some cases as high as 5V. Of course they are assuming a voltage source, with no intrinsic current limiting to protect the part from excessive power dissipation if the Vbr is exceeded.
I've also been unable to find any data on potential degradation of performance due to reverse current---the only parameter cited for loss of light output over time is die temperature.
In any event, this is a crucial parameter when using a circuit such as presented recently, in which the LED is driven solely by a series capacitor from a current-limited but large voltage swing source (such as the 12AU7 cathode follower with 22k cathode load R). As outlined by PRR such a circuit functions by routinely reverse-biasing the LED into breakdown to discharge the capacitor. It will affect threshold but even more will tend to make the LED drive circuit more of a peak detector when the breakdown voltage is large relative to the nominal forward voltage. This will have a very significant effect on the audio performance.
For loose LEDs of various types I've been measuring, the Vbr is anywhere from 25 to 40V. I sent Perkin Elmer an email but not surprisingly I've gotten no response. I may try to get them on the phone, but they may not know the answer themselves and might well be unwilling to divulge who their LED supplier(s) is. All any vendor will say is don't hit the part with more than 3V reverse, or in some cases as high as 5V. Of course they are assuming a voltage source, with no intrinsic current limiting to protect the part from excessive power dissipation if the Vbr is exceeded.
I've also been unable to find any data on potential degradation of performance due to reverse current---the only parameter cited for loss of light output over time is die temperature.
In any event, this is a crucial parameter when using a circuit such as presented recently, in which the LED is driven solely by a series capacitor from a current-limited but large voltage swing source (such as the 12AU7 cathode follower with 22k cathode load R). As outlined by PRR such a circuit functions by routinely reverse-biasing the LED into breakdown to discharge the capacitor. It will affect threshold but even more will tend to make the LED drive circuit more of a peak detector when the breakdown voltage is large relative to the nominal forward voltage. This will have a very significant effect on the audio performance.