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From: Bernhard Kuemel on 29 Jan 2010 06:37
Jon Kirwan wrote:
> On Thu, 28 Jan 2010 21:04:57 +0100, Bernhard Kuemel
> <bernhard(a)bksys.at> wrote:
>
>> Hi seb!
>>
>> I used LEDs experimentally as photo diodes, yielding up to 1,4V with
>> green LEDs when near a 60W incandescent light bulb.
>
> Not disagreeing, but how did you measure the 1,4V?

I made some new measurements. DMM is Metex M-3800, DC 2V range, negative
probe connected to LED cathode via alligator clip test leads, positive
probe to anode. I held the LED at the insulation of the alligator clips
to avoid creepage current through my fingers. LEDs were held
horizontally at the 60 W incandescent lightbulb (brand Lightway),
touching it just opposite to the socket. The leds top is a flat, frosted
arrow shape. I tried to find the position of maximum voltage and
sometimes it appeared the voltage rised with time (within 20s). Maybe
there was an input capacitance being charged up.


green LED (V):
1.288

yellow LEDs (some have an orange tint; mV). The first 2 or 3 are not
part of the 10 series LEDs.
270
530
550
680
590
690
560
590
580

>> Right now I put 10
>> LEDs in series to get higher voltages, but the voltages don't add?! All
>> I get is a voltage similar to the individual voltages - up to about 400
>> mV with yellow LEDs.
>>
>> How can that be?
>
> Probably would help to know more about what you are doing to
> measure things as well as the general setup.

The 10 yellow LEDs are put in a breadboard. Same DMM, alligator test
leads to the anode of the first LED and cathode of the last LED. The 60
W light bulb may not have been as close as with the measurements above.

To exclude any effects of the bread board I soldered 2 yellow LEDs in
series. Just soldered a cathode and an anode terminal together, no PCB
etc. The individual LEDs produced 490 mV and 650 mV respectively. Both
in series measured 500 mV.

Anyone care to reproduce this experiment. Connect 2 LEDs in series and
measure the individual and series photovoltages close to (touching) an
incandescent light bulb?

>> No current is measurable when I close the circuit with the multimeter
>> even at the 200 uA range.
>
> The LED die is often tiny and the currents are likely fairly
> small.

Thanks for your effort of explaining the unmeasurably small short
curcuit current. This is no big problem for me, since I have no reason
to believe it should be otherwise. But what puzzles me is that the
photovoltages don't add up.

Bernhard
From: Bernhard Kuemel on 29 Jan 2010 07:17
Jasen Betts wrote:
> On 2010-01-28, Bernhard Kuemel <bernhard(a)bksys.at> wrote:
>> Hi seb!
>>
>> I used LEDs experimentally as photo diodes, yielding up to 1,4V with
>> green LEDs when near a 60W incandescent light bulb. Right now I put 10
>> LEDs in series to get higher voltages, but the voltages don't add?! All
>> I get is a voltage similar to the individual voltages - up to about 400
>> mV with yellow LEDs.
>>
>> How can that be?
>
> not enough current to produce a higher voltage into the load your
> meter presents
>
> try a series-parallel arrangement.

5 pairs of parallel LEDs connected in series give 1065 mV. Clearly above
individual photovoltage, although not 5 times as much. A 3x3
configuration gave me 1700 mV, pretty much 3 times the individual voltage.

About the 2 diodes that individually give 490 mV and 650 mV individually
and 500 mV in series ... could it be that the diodes only allow their
individual photocurrent so the total current would be limited by the
weaker diode and my DMM would measure the voltage across a sense
resistor and so rather measures current than voltag? I guess a FET might
measure voltage more correctly.

Bernhard
From: Bernhard Kuemel on 29 Jan 2010 07:40
Bernhard Kuemel wrote:
> Jasen Betts wrote:
>> On 2010-01-28, Bernhard Kuemel <bernhard(a)bksys.at> wrote:
>>> Hi seb!
>>>
>>> I used LEDs experimentally as photo diodes, yielding up to 1,4V with
>>> green LEDs when near a 60W incandescent light bulb. Right now I put 10
>>> LEDs in series to get higher voltages, but the voltages don't add?! All
>>> I get is a voltage similar to the individual voltages - up to about 400
>>> mV with yellow LEDs.
>>>
>>> How can that be?
>> not enough current to produce a higher voltage into the load your
>> meter presents
>>
>> try a series-parallel arrangement.
>
> 5 pairs of parallel LEDs connected in series give 1065 mV. Clearly above
> individual photovoltage, although not 5 times as much. A 3x3
> configuration gave me 1700 mV, pretty much 3 times the individual voltage.

The 3x3 config measures 0.1 uA (the minimum on my DMM). 10 LEDs parallel
measure 0.5 uA. Photovoltage of 10 paralell yellow LEDs is 1300 mV.

Bernhard
From: George Herold on 29 Jan 2010 11:57
On Jan 29, 5:20 am, Jasen Betts <ja...(a)xnet.co.nz> wrote:
> On 2010-01-28, Bernhard Kuemel <bernh...(a)bksys.at> wrote:
>
> > Hi seb!
>
> > I used LEDs experimentally as photo diodes, yielding up to 1,4V with
> > green LEDs when near a 60W incandescent light bulb. Right now I put 10
> > LEDs in series to get higher voltages, but the voltages don't add?! All
> > I get is a voltage similar to the individual voltages - up to about 400
> > mV with yellow LEDs.
>
> > How can that be?
>
> not enough current to produce a higher voltage into the load your
> meter presents
>
> try a series-parallel arrangement.
>
> FWIW, I've seen weak photo-diode behaviour from glass-cased 1N914
> diodes too.
>
> --- news://freenews.netfront.net/ - complaints: n...(a)netfront.net ---

This sounds like the answer. The 10Meg (or lower) input impedance of
your meter is shorting out all the current. If you've got any large
value resistors (100 Meg or 1 G ohm) you could try putting one in
series with the meter. The meter is then the bottom of a voltage
divider and you can do the math to figure out the voltage from the
LEDS... course you're still drawing some current from them.....
otherwise perhaps a FET opamp as buffer.

George H.

From: Michael Black on 29 Jan 2010 13:53
On Fri, 29 Jan 2010, George Herold wrote:

> On Jan 29, 5:20 am, Jasen Betts <ja...(a)xnet.co.nz> wrote:
>> On 2010-01-28, Bernhard Kuemel <bernh...(a)bksys.at> wrote:
>>
>>> Hi seb!
>>
>>> I used LEDs experimentally as photo diodes, yielding up to 1,4V with
>>> green LEDs when near a 60W incandescent light bulb. Right now I put 10
>>> LEDs in series to get higher voltages, but the voltages don't add?! All
>>> I get is a voltage similar to the individual voltages - up to about 400
>>> mV with yellow LEDs.
>>
>>> How can that be?
>>
>> not enough current to produce a higher voltage into the load your
>> meter presents
>>
>> try a series-parallel arrangement.
>>
>> FWIW, I've seen weak photo-diode behaviour from glass-cased 1N914
>> diodes too.
>>
>> --- news://freenews.netfront.net/ - complaints: n...(a)netfront.net ---
>
> This sounds like the answer. The 10Meg (or lower) input impedance of
> your meter is shorting out all the current. If you've got any large
> value resistors (100 Meg or 1 G ohm) you could try putting one in
> series with the meter. The meter is then the bottom of a voltage
> divider and you can do the math to figure out the voltage from the
> LEDS... course you're still drawing some current from them.....
> otherwise perhaps a FET opamp as buffer.
>
That's a point. The fact that he put them in series for higher voltage
suggests he's not interested in them as "photo-diodes" but is thinking
in terms of solar cells, ie a generator of power. And that's obviously
not going to happen, way too little current at any point.

I was certainly wondering if putting them in series was forward biasing
them to some extent which then causes less output as a photo diode.

Michael
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