From: Phil Hobbs on
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
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
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
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
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