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wimpy diode

Started by John Larkin December 7, 2015
On Tue, 8 Dec 2015 04:50:40 -0800 (PST), tabbypurr@gmail.com wrote:

>On Tuesday, 8 December 2015 02:05:47 UTC, John Larkin wrote: >> On Mon, 7 Dec 2015 16:52:20 -0800 (PST), nt wrote: >> >On Monday, 7 December 2015 21:26:45 UTC, whit3rd wrote: >> >> On Monday, December 7, 2015 at 12:49:23 PM UTC-8, nt wrote: >> >> > On Monday, 7 December 2015 20:33:47 UTC, John Larkin wrote: >> >> > > On Mon, 7 Dec 2015 12:04:08 -0800 (PST), George Herold >> >> > > <gherold@teachspin.com> wrote: >> > >> >> > > >An LED? or is that too much voltage? >> >> >> >> > > I want about 0.8 volts >> >> >> >> > Aren't 2 diodes cheaper? >> > >> >that's misleading editing. I was talking about use in current sources, not the former application that wanted 0.8v >> >> OK, but that will have a pretty bad tempco, roughly 0.3% per degree C. >> The LED thing is way better. 2 diodes is OK if accuracy doesn't matter >> too much. > >If base diodes temp rise is half as much as the tr, you're good :) Easily arranged, but of course short term power dissipation variations will not go so well, and T_amb is uncompensated. > > >NT
Add a TEC to compensate! It's true that power dissipation in the transistor will warm it up, and change its Vbe. So a trick like this will never be really accurate. But it is cute.
Den tirsdag den 8. december 2015 kl. 13.53.09 UTC+1 skrev tabb...@gmail.com:
> On Tuesday, 8 December 2015 02:34:16 UTC, Lasse Langwadt Christensen wrote: > > Den tirsdag den 8. december 2015 kl. 01.52.25 UTC+1 skrev nt: > > > On Monday, 7 December 2015 21:26:45 UTC, whit3rd wrote: > > > > On Monday, December 7, 2015 at 12:49:23 PM UTC-8, nt wrote: > > > > > On Monday, 7 December 2015 20:33:47 UTC, John Larkin wrote: > > > > > > On Mon, 7 Dec 2015 12:04:08 -0800 (PST), George Herold > > > > > > <gherold@teachspin.com> wrote: > > > > > > > > > >An LED? or is that too much voltage? > > > > > > > > > > I want about 0.8 volts > > > > > > > > > Aren't 2 diodes cheaper? > > > > > > that's misleading editing. I was talking about use in current sources, not the former application that wanted 0.8v > > > > without thinking too much about I think 2 diodes will have ~2x2mV/K > > with the right color LED I think you can find one that is closer to ~2mV/K > > One of those 2mVs nicely compensates the tr. So halve the diode temp rise... >
or make the temp rise equal with 2mV LED and tamb is also compensated -Lasse
On Mon, 07 Dec 2015 21:31:47 -0800, Robert Baer
<robertbaer@localnet.com> wrote:

>John Larkin wrote: >> >> I need a wimpy diode, with lots of forward drop at a few mA, to bias >> up an e-phemt. Thought I'd try a small-geometry NPN transistor. >> >> https://dl.dropboxusercontent.com/u/53724080/Parts/Transistors/BCX70_as_Diode.JPG >> >> Looks good. The b-e and b-c junctions look symmetric. >> >> The middle config looks like the classic 60 mV/decade-current, and the >> others are maybe a little ohmic. We pay 2.4 cents each for these, so I >> don't mind wasting the transistor action. >> >> >> >> >> > How come you seem to be the only one that shows READABLE (_not_ grey >on grey) drawings?
Sorry, I've never figured out how to do that. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Tue, 8 Dec 2015 09:45:03 +0000, piglet <erichpwagner@hotmail.com>
wrote:

>On 07/12/2015 19:47, John Larkin wrote: >> >> I need a wimpy diode, with lots of forward drop at a few mA, to bias >> up an e-phemt. Thought I'd try a small-geometry NPN transistor. >> > >How is that e-phemt Vgs bias point to change with temperature? Would the >B-E junction TC even be in the right direction? > >piglet >
I suspect that it's about the right amount and direction. I suppose I should measure it. Snarl. Snap. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On 08 Dec 2015 11:22:12 GMT, Allan Herriman
<allanherriman@hotmail.com> wrote:

>On Mon, 07 Dec 2015 11:47:48 -0800, John Larkin wrote: > >> I need a wimpy diode, with lots of forward drop at a few mA, to bias up >> an e-phemt. Thought I'd try a small-geometry NPN transistor. >> >> https://dl.dropboxusercontent.com/u/53724080/Parts/Transistors/ >BCX70_as_Diode.JPG >> >> Looks good. The b-e and b-c junctions look symmetric. >> >> The middle config looks like the classic 60 mV/decade-current, and the >> others are maybe a little ohmic. We pay 2.4 cents each for these, so I >> don't mind wasting the transistor action. > > >I use a BAV199W if I want a high forward drop or vanishingly small >reverse current. Vf is not very well constrained by worst case limits in >the datasheet though. > >Allan
If you want crazy low reverse current, consider the c-b junction of a BFT25. https://dl.dropboxusercontent.com/u/53724080/Parts/Transistors/BFT25.JPG That reverse leakage was about at my measurement resolution limit. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Tue, 8 Dec 2015 04:56:30 -0800 (PST), tabbypurr@gmail.com wrote:

>On Tuesday, 8 December 2015 11:22:17 UTC, Allan Herriman wrote: >> On Mon, 07 Dec 2015 11:47:48 -0800, John Larkin wrote: >> >> > I need a wimpy diode, with lots of forward drop at a few mA, to bias up >> > an e-phemt. Thought I'd try a small-geometry NPN transistor. >> > >> > https://dl.dropboxusercontent.com/u/53724080/Parts/Transistors/ >> BCX70_as_Diode.JPG >> > >> > Looks good. The b-e and b-c junctions look symmetric. >> > >> > The middle config looks like the classic 60 mV/decade-current, and the >> > others are maybe a little ohmic. We pay 2.4 cents each for these, so I >> > don't mind wasting the transistor action. >> >> >> I use a BAV199W if I want a high forward drop or vanishingly small >> reverse current. Vf is not very well constrained by worst case limits in >> the datasheet though. >> >> Allan > >I've got a graph from who knows where that shows 1N458 0.8v @ 10mA. FWIW. > > >NT
I think for exactly 800mV I'd be tempted to just slap an AP2127 down there- SOT-23-5 and you get 800mV +/-2% with no additional parts (just an output cap). Less than 10 cents in quantity, and it can be used in other applications. Even has a shutdown input. --sp -- Best regards, Spehro Pefhany Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8 Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48
On Tue, 8 Dec 2015 22:52:08 +1100, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

>On 08/12/2015 08:08, John Larkin wrote: >> On Mon, 7 Dec 2015 12:49:17 -0800 (PST), tabbypurr@gmail.com wrote: >> >>> On Monday, 7 December 2015 20:33:47 UTC, John Larkin wrote: >>>> On Mon, 7 Dec 2015 12:04:08 -0800 (PST), George Herold >>>> <gherold@teachspin.com> wrote: >>>> >>>>> On Monday, December 7, 2015 at 2:47:59 PM UTC-5, John Larkin wrote: >>>>>> I need a wimpy diode, with lots of forward drop at a few mA, to bias >>>>>> up an e-phemt. Thought I'd try a small-geometry NPN transistor. >>>>>> >>>>>> https://dl.dropboxusercontent.com/u/53724080/Parts/Transistors/BCX70_as_Diode.JPG >>>>>> >>>>>> Looks good. The b-e and b-c junctions look symmetric. >>>>>> >>>>>> The middle config looks like the classic 60 mV/decade-current, and the >>>>>> others are maybe a little ohmic. We pay 2.4 cents each for these, so I >>>>>> don't mind wasting the transistor action. >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> -- >>>>>> >>>>>> John Larkin Highland Technology, Inc >>>>>> picosecond timing precision measurement >>>>>> >>>>>> jlarkin att highlandtechnology dott com >>>>>> http://www.highlandtechnology.com >>>>> >>>>> An LED? or is that too much voltage? >>>>> >>>>> George H. >>>> >>>> I want about 0.8 volts, to be a little above the gate turn-on voltage >>>> of the enhancement phemt. It would be cool to see it glow, but a >>>> visible LED would be 1.8 volts or so. >>>> >>>> I have used LEDs as the voltage reference for simple current sources, >>>> where they are just right. >>> >>> Aren't 2 diodes cheaper? >>> >>> >>> NT >> >> The right LED at the right current will have a temperature coefficient >> that almost cancels the Vbe of a transistor. >> >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Current_Sources/LED_Isrc_bb.JPG >> >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Current_Sources/LED_Isrc_data.JPG >> >> Plus, it glows in the dark! >> >> > >In some companies there could be a risk that someone in Marketing will >change the LED colour to blue without asking you, because the blue LEDs >fitted under his car look so good. >
I find that Blue LED's create a blue fog of dispersed light vs. Green, amber or Red LED's. It's particularly annoying. Cheers
On Tue, 08 Dec 2015 14:31:08 -0500, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

>On Tue, 8 Dec 2015 04:56:30 -0800 (PST), tabbypurr@gmail.com wrote: > >>On Tuesday, 8 December 2015 11:22:17 UTC, Allan Herriman wrote: >>> On Mon, 07 Dec 2015 11:47:48 -0800, John Larkin wrote: >>> >>> > I need a wimpy diode, with lots of forward drop at a few mA, to bias up >>> > an e-phemt. Thought I'd try a small-geometry NPN transistor. >>> > >>> > https://dl.dropboxusercontent.com/u/53724080/Parts/Transistors/ >>> BCX70_as_Diode.JPG >>> > >>> > Looks good. The b-e and b-c junctions look symmetric. >>> > >>> > The middle config looks like the classic 60 mV/decade-current, and the >>> > others are maybe a little ohmic. We pay 2.4 cents each for these, so I >>> > don't mind wasting the transistor action. >>> >>> >>> I use a BAV199W if I want a high forward drop or vanishingly small >>> reverse current. Vf is not very well constrained by worst case limits in >>> the datasheet though. >>> >>> Allan >> >>I've got a graph from who knows where that shows 1N458 0.8v @ 10mA. FWIW. >> >> >>NT > > >I think for exactly 800mV I'd be tempted to just slap an AP2127 down >there- SOT-23-5 and you get 800mV +/-2% with no additional parts (just >an output cap). Less than 10 cents in quantity, and it can be used in >other applications. Even has a shutdown input. > >
Thanks, but I don't need that precision, and we have the BCX70 in stock. Putting more parts into stock is generally avoided. We have 6469 parts already, 3.03 million pieces.
On Wednesday, 9 December 2015 04:10:16 UTC, John Larkin  wrote:

> Thanks, but I don't need that precision, and we have the BCX70 in > stock. > > Putting more parts into stock is generally avoided. We have 6469 parts > already, 3.03 million pieces.
Ouch. JIT. NT
On Wed, 9 Dec 2015 01:51:53 -0800 (PST), the renowned
tabbypurr@gmail.com wrote:

>On Wednesday, 9 December 2015 04:10:16 UTC, John Larkin wrote: > >> Thanks, but I don't need that precision, and we have the BCX70 in >> stock. >> >> Putting more parts into stock is generally avoided. We have 6469 parts >> already, 3.03 million pieces. > >Ouch. JIT. > > >NT
I think at one time JL said they had more than 100 types of LDO alone in stock. --sp -- Best regards, Spehro Pefhany Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8 Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48