Photodiode Question
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From: Phil Hobbs on 22 Mar 2010 08:01 On 3/21/2010 11:10 PM, Jim Thompson wrote: > What current would one expect to see from a photodiode exposed to > typical bright room light (used to control a backlight proportional to > room light)? > > As usual, my customer has no clue. My only concern, design wise, is > to build a micropower transconductance amplifier with sufficient > output drive to handle the maximum available photo current. > > ...Jim Thompson Those guys are all nuts. For backlight control, you can use one of the ambient light sensors such as the Intersil ISL29000 or TAOS TSL2560, for instance. They look after all of that nonsense, and produce an analogue or digital output proportional to just the perceived brightness (i.e. luminous intensity), without being fooled by all the IR from incandescents, for instance. Full zenith sunlight is about 800W/m**2, so figuring 0.3 A/W average responsivity, a typical 2.3 mm square photodiode such as a BPW34 will never produce more than ~1.3 mA without optical concentration of some sort. Indoors it's a factor of 10**3 to 10**4 dimmer than that, so you're looking at a few microamps in bright room lights. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: MooseFET on 22 Mar 2010 09:28 On Mar 21, 8:10 pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- Web-Site.com> wrote: > What current would one expect to see from a photodiode exposed to > typical bright room light (used to control a backlight proportional to > room light)? I would expect a little under 0.5A per watt of light power hitting the diode. How big is the diode? > As usual, my customer has no clue. My only concern, design wise, is > to build a micropower transconductance amplifier with sufficient > output drive to handle the maximum available photo current. Since you are working at low frequencies, you can run the diode at zero bias or some other value the circuit likes. This makes it a lot easier to do the amplifier design.
From: GregS on 22 Mar 2010 10:44 In article <a8e2d54d-cd89-4910-a6e5-2091bab0ce85(a)f13g2000pra.googlegroups.com>, MooseFET <kensmith(a)rahul.net> wrote: >On Mar 21, 8:10=A0pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- >Web-Site.com> wrote: >> What current would one expect to see from a photodiode exposed to >> typical bright room light (used to control a backlight proportional to >> room light)? > >I would expect a little under 0.5A per watt of light power hitting >the diode. > >How big is the diode? > >> As usual, my customer has no clue. =A0My only concern, design wise, is >> to build a micropower transconductance amplifier with sufficient >> output drive to handle the maximum available photo current. > >Since you are working at low frequencies, you can run the diode at >zero bias or some other value the circuit likes. This makes it a >lot easier to do the amplifier design. I would just MEASURE it with some diode. I just measured 2 mv p-p into a scope 1M. Directly pointing at the light is 14 mv p-p. Thats 2 micro volta to 14 micro volts. I don't know what the diode is, but I have been using it to measure light noise. greg
From: John Larkin on 22 Mar 2010 09:52 On Mon, 22 Mar 2010 08:01:35 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >On 3/21/2010 11:10 PM, Jim Thompson wrote: >> What current would one expect to see from a photodiode exposed to >> typical bright room light (used to control a backlight proportional to >> room light)? >> >> As usual, my customer has no clue. My only concern, design wise, is >> to build a micropower transconductance amplifier with sufficient >> output drive to handle the maximum available photo current. >> >> ...Jim Thompson > >Those guys are all nuts. Then I'm sure glad I didn't make any suggestions. John
From: Phil Hobbs on 22 Mar 2010 09:58
On 3/22/2010 9:52 AM, John Larkin wrote: > On Mon, 22 Mar 2010 08:01:35 -0400, Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> wrote: > >> On 3/21/2010 11:10 PM, Jim Thompson wrote: >>> What current would one expect to see from a photodiode exposed to >>> typical bright room light (used to control a backlight proportional to >>> room light)? >>> >>> As usual, my customer has no clue. My only concern, design wise, is >>> to build a micropower transconductance amplifier with sufficient >>> output drive to handle the maximum available photo current. >>> >>> ...Jim Thompson >> >> Those guys are all nuts. > > Then I'm sure glad I didn't make any suggestions. > > John > Nah, it's just my rhetorical way of trying to get people to do a bit of calculation (or even experiment) before giving advice. I know, I know, I'm on Usenet, but we must live in hope. ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net |