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From: George Herold on 9 Jul 2010 16:10
On Jul 9, 3:10 pm, Spehro Pefhany <speffS...(a)interlogDOTyou.knowwhat>
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.
>
> 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 sauté la jonction toute de suite.- Hide quoted text -
>
> - Show quoted text -

Yeah there is a switch and indicator LED, so hoepfully they turn
things off before screwing around... But of course some will forget to
turn it off.

But I'll do a bit of live swtiching and see.... Well that might have
cooked it. After several rapid forward biases something has changed.
The forward voltage drop is now less than it was before...Hmm

George H.
From: whit3rd on 9 Jul 2010 16:13
On Jul 9, 10:26 am, George Herold <gher...(a)teachspin.com> wrote:

> 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.

> Is this going to damage the photodiode in any way?   Seems like it
> should be fine.

It depends on the photodiode, but some of the precautions against
aging and surface dirt (photodiodes have a LOT of surface) are
field-dependent. So forward bias can turn photodiodes leaky and
noisy. Maybe reverse bias/heat/time will fix, maybe not.
From: George Herold on 9 Jul 2010 16:13
On Jul 9, 3:52 pm, Joerg <inva...(a)invalid.invalid> wrote:
> 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.

Yeah I think a bit of R will be a good idea.
>
>
>
>
>
> >>>                        ... 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.

Opps sorry, that's only 1 mA and in fact a bit less than that.
(700uA??)

George H.
>
> > 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.- Hide quoted text -
>
> - Show quoted text -

From: George Herold on 9 Jul 2010 16:38
On Jul 9, 4:13 pm, whit3rd <whit...(a)gmail.com> wrote:
> On Jul 9, 10:26 am, George Herold <gher...(a)teachspin.com> wrote:
>
> > 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.
> > Is this going to damage the photodiode in any way?   Seems like it
> > should be fine.
>
> It depends on the photodiode, but some of the precautions against
> aging and surface dirt (photodiodes have a LOT of surface) are
> field-dependent.  So forward bias can turn photodiodes  leaky and
> noisy.   Maybe reverse bias/heat/time will fix, maybe not.

Thanks Whit3rd, I'll check it out. To be honest I made this mistake
myself a month or so ago. (I hooked up the PD backwards.) I took me a
few minutes to figure out what was wrong and afterwards the PD worked
just fine. (But I didn't look at the leakage current!)

George H.
From: Spehro Pefhany on 9 Jul 2010 17:06
On Fri, 9 Jul 2010 13:10:21 -0700 (PDT), George Herold
<gherold(a)teachspin.com> wrote:

>On Jul 9, 3:10�pm, Spehro Pefhany <speffS...(a)interlogDOTyou.knowwhat>
>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.
>>
>> 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.- Hide quoted text -
>>
>> - Show quoted text -
>
>Yeah there is a switch and indicator LED, so hoepfully they turn
>things off before screwing around... But of course some will forget to
>turn it off.
>
>But I'll do a bit of live swtiching and see.... Well that might have
>cooked it. After several rapid forward biases something has changed.
>The forward voltage drop is now less than it was before...Hmm

Glad to be of help. ;-)


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