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From: George Herold on 9 Jul 2010 14:40
On Jul 9, 2:10 pm, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net>
wrote:
> George Herold wrote:
> > Right, who would do such a stupid thing?
>
> > This is for a piece of equipment that will be used by students.  They
> > get to hook up the photodiode and it's guaranteed that 1/2 of them
> > will do it backwards the first time.  (And the other half will do it
> > backwards the second time.)  The power supply that biases the PD is
> > current limited at about 250mA.  With this much current the forward
> > drop across the PD (PIN-3CD from OSI) is 1.35 Volts at room temp.
> > (0.34 Watts.)
> > I’ve been cooking the PD on my bench for several hours.  It’s a bit
> > warm, forward voltage drop is now 1.29 V.  (It’s nice that it goes
> > down.)  From which I guesstimate the temperature rise to be about 30
> > C.  (60 mV)
>
> > Is this going to damage the photodiode in any way?   Seems like it
> > should be fine.
>
> > Thanks
>
> > George H.
>
> Depends on the PD.  Some have really thin epi for good blue response,
> which leads to a lot of current crowding near the contact, so that they
> die with an applied forward bias but easily survive the same size
> photocurrent.  If it doesn't die right away, and the leakage doesn't get
> degraded by the experience, you should be fine.
>
> 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 nethttp://electrooptical.net- Hide quoted text -
>
> - Show quoted text -

Thanks, I'll measure the leakage current after the abuse.
Perhaps I should 'test' a few more and measure leakage before and
after.

George H.
From: John Larkin on 9 Jul 2010 14:50
On Fri, 9 Jul 2010 10:26:57 -0700 (PDT), George Herold
<gherold(a)teachspin.com> wrote:

>Right, who would do such a stupid thing?
>
>This is for a piece of equipment that will be used by students. They
>get to hook up the photodiode and it's guaranteed that 1/2 of them
>will do it backwards the first time. (And the other half will do it
>backwards the second time.) The power supply that biases the PD is
>current limited at about 250mA. With this much current the forward
>drop across the PD (PIN-3CD from OSI) is 1.35 Volts at room temp.
>(0.34 Watts.)
>I�ve been cooking the PD on my bench for several hours. It�s a bit
>warm, forward voltage drop is now 1.29 V. (It�s nice that it goes
>down.) From which I guesstimate the temperature rise to be about 30
>C. (60 mV)
>
>Is this going to damage the photodiode in any way? Seems like it
>should be fine.
>
>Thanks
>
>George H.

You can't add a current limiter to the PD? Like a polyfuse or a
depletion fet? Or even a resistor?

It is a big chip, lots of wire bonds, so maybe it's OK.

John



From: Spehro Pefhany on 9 Jul 2010 15:10
On Fri, 9 Jul 2010 10:26:57 -0700 (PDT), George Herold
<gherold(a)teachspin.com> wrote:

>Right, who would do such a stupid thing?
>
>This is for a piece of equipment that will be used by students. They
>get to hook up the photodiode and it's guaranteed that 1/2 of them
>will do it backwards the first time. (And the other half will do it
>backwards the second time.) The power supply that biases the PD is
>current limited at about 250mA. With this much current the forward
>drop across the PD (PIN-3CD from OSI) is 1.35 Volts at room temp.
>(0.34 Watts.)
>I�ve been cooking the PD on my bench for several hours. It�s a bit
>warm, forward voltage drop is now 1.29 V. (It�s nice that it goes
>down.) From which I guesstimate the temperature rise to be about 30
>C. (60 mV)
>
>Is this going to damage the photodiode in any way? Seems like it
>should be fine.
>
>Thanks
>
>George H.

Could they connect the PD backwards with the power supply already
turned on? There may be enough capacitance on the output side of the
current limiting to saut� la jonction toute de suite.

From: Joerg on 9 Jul 2010 15:52
George Herold wrote:
> On Jul 9, 1:40 pm, Joerg <inva...(a)invalid.invalid> wrote:
>> George Herold wrote:
>>> Right, who would do such a stupid thing?
>>> This is for a piece of equipment that will be used by students. They
>>> get to hook up the photodiode and it's guaranteed that 1/2 of them
>>> will do it backwards the first time. (And the other half will do it
>>> backwards the second time.) The power supply that biases the PD is
>>> current limited at about 250mA. ...
>> 250mA? Yikes! Why?
>
> Why 250 mA? Well the power supply can used for other things also.
> The biggest power hog is running a light bulb.


I'd add an angst resistor towards the PD, especially when dealing with
students. Or at least a fast 50mA fuse.


>>> ... With this much current the forward
>>> drop across the PD (PIN-3CD from OSI) is 1.35 Volts at room temp.
>>> (0.34 Watts.)
>>> I�ve been cooking the PD on my bench for several hours. It�s a bit
>>> warm, forward voltage drop is now 1.29 V. (It�s nice that it goes
>>> down.) From which I guesstimate the temperature rise to be about 30
>>> C. (60 mV)
>>> Is this going to damage the photodiode in any way? Seems like it
>>> should be fine.
>> It's certainly not a healthy modus operandi. What does the datasheet say
>> under abs max?
>
> Data sheet says nothing... Some list a maxiumum short circuit photo
> current but that's sure to be much less than 250 mA. At full light I
> can only get about 10mA from the diode.
>

10mA? That is one fat diode.


> George H.
>
> (Hmm I guess I could ask the manufacturer)


Good idea :-)

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: George Herold on 9 Jul 2010 15:56
On Jul 9, 2:50 pm, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Fri, 9 Jul 2010 10:26:57 -0700 (PDT), George Herold
>
>
>
>
>
> <gher...(a)teachspin.com> wrote:
> >Right, who would do such a stupid thing?
>
> >This is for a piece of equipment that will be used by students.  They
> >get to hook up the photodiode and it's guaranteed that 1/2 of them
> >will do it backwards the first time.  (And the other half will do it
> >backwards the second time.)  The power supply that biases the PD is
> >current limited at about 250mA.  With this much current the forward
> >drop across the PD (PIN-3CD from OSI) is 1.35 Volts at room temp.
> >(0.34 Watts.)
> >I’ve been cooking the PD on my bench for several hours.  It’s a bit
> >warm, forward voltage drop is now 1.29 V.  (It’s nice that it goes
> >down.)  From which I guesstimate the temperature rise to be about 30
> >C.  (60 mV)
>
> >Is this going to damage the photodiode in any way?   Seems like it
> >should be fine.
>
> >Thanks
>
> >George H.
>
> You can't add a current limiter to the PD? Like a polyfuse or a
> depletion fet? Or even a resistor?
>
> It is a big chip, lots of wire bonds, so maybe it's OK.
>
> John- Hide quoted text -
>
> - Show quoted text -

I was thinking I might have to add some sort of current limit. It
would just be much easier if I didn't have to. OK I just tried adding
a bit of R in series. 50 ohms limited the current to 150 mA with a
1.23 voltage drop across the diode. A bit of an improvement. With
100 ohms it's down to 90mA. The resistor is getting hot now. This is
not enough current to activate the current limit on the power
supply. Does putting a bit of R (100 ohms) in the bias arm of a PD
circit effect the response time?

I'm going to try Phil's idea of measuring the leakage currents on
Monday.

George H.
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