diodes for breakfast Jan 1, 2006

(D) There is an additional resistive drop in series with the diode junction that has a significant but different tempco than the junction, but its drop becomes significant only at higher current.

I would try several lower currents to see if the tempco is stable below some limiting current. Do you insulate the diodes or are they in contact with the water?

I just dunked the diodes right in the water. They were soldered together in a string with a current limiting resistor, which I kept out of the water.

application (I'm building a voltage regulator), the diode string will have very low current, probably no more than a mA. Thanks for the heads up about series resistance, I didn't think about that.

What about the conductance of the water? Won't that lead to unreliable results?

Regards

Klaus

Better to place them i your kitchen oven :-)

A nice simple experiment. It should be noted that the LEDs are effectively better than the diodes as a reference, because they have a much higher voltage drop, and hence a much lower % change with temperature. Also, some people use an LED as the reference for a current source, using a transistor and resistor, wherein the transistor's tempco nearly cancels the LED's tempco. Cool.

--
 Thanks,
    - Win

It should be further noted that the LED zero-tempco current-source trick can't be done with a stack of diodes to bias the transistor, which is quite popular and commonly seen, because each diode in the stack adds another -2mV/C to the tempco. Furthermore, the Vbe reference trick, wherein two resistors are used with a transistor to make an arbitrary reference voltage to bias a current-source transistor, also suffers from a multiplied tempco. The only low- cost part (or set of parts) that works is one LED. Some, like me, find it slightly bizzarre to use an LED as a voltage reference, and shy away from using this trick. Others consider it elegant.

--
 Thanks,
    - Win

I think you can see the effect of non ideal diode resistance in the data sheet curve of voltage drop versus current at the current where the voltage no longer follows a logarithmic shape, but tends toward a linear one.

For instance, lay a straight edge along th line in figure 1 of:

It looks to me that the curve deviates from a log starting at about 10 mA. The ideal diode formula, including the tempco is more likely to be accurate along the straight (logarithmic) part of that curve.

If the experiment is worth doing, I think you should try to keep the diodes dry during the test. Either place them in the oven with a thermometer beside in a glass baking dish, or put them in a test tube or something similar for the dunk test. This will let you take data points at much lower current (around the range your application will use), without having to worry if leakage current through the water is involved.

In article , Winfield Hill wrote: [...]

They also make a cool over current clamp:

! !/ c ---+----! ! !\ e V ! --- / ! \ ! / ! ! ------

The LED lights to tell you about the over current condition and does the clamping too.

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kensmith@rahul.net   forging knowledge

The nicest thing about forward-biased diodes as voltage references is that you can get pretty low noise--subtracting a 0.7V Vbe from 1.7V LED makes a

1-V reference that's a good 15 or 20 dB quieter than a bandgap, at the price of lower accuracy and higher drift, which is sometimes a big win.

The other thing it would be nice to know is whether the logarithmic slope of V vs I in the LEDs is closer to kT/e than in the 1N4148s. It's one of the niggling annoyances of electronics that diodes don't obey the diode equation, but diode-connected transistors do!

Cheers,

Phil Hobbs

It is! Besides, it's cool to have SOT-23 LEDs all over your boards, looking like the LA skyline. Mandatory around here is an led per Xilinx chip, on the DONE line, showing that it configured peoperly.

That's nice. What happens with different colored LEDs?

+--------------+------------------------+ | | | | | | r | | | c | +------------b | | e | | | | a +----------- + | led k | | | r opamp out----+------ | | | | | +---- - r | | | | | | | | | | +-----------------+ | | | | | r | | | +--------------+------+---gnd

And some way to get it started, of course.

John

Well, I of course rush off to simulate this, and run into a problem: The LED models I have all exhibit a positive Vf tempco. I tried the standard ltspice LED models, plus some other models I picked up from the ltspice group on yahoo.

Here is one of the models (although they all exhibit the same symptom.)

*Typ RED GaAs LED: Vf=1.7V Vr=4V If=40mA trr=3uS .MODEL RED_LED D (IS=93.2P RS=42M N=3.73 BV=4 IBV=10U

• CJO=2.97P VJ=.75 M=.333 TT=4.32U

• Vpk=1.7V Iave=40mA type=LED)

I wonder if there is some simple correction I can make to the model?

--
Regards,
  Bob Monsen

Zero is the number of objects that satisfy a condition that is never 
satisfied.  But as never means "in no case", I do not see that any progress 
has been made.
- Poincare

["Followup-To:" header set to sci.electronics.design.] On 1 Jan 2006 09:06:10 -0800, Winfield Hill wrote in Msg.

I've done it many times and I really like it. Also, for red LEDs, the voltage drop across the current-setting resistor is approx. 1V, making the math really easy ;-)

robert

Another nice thing about LEDs instead of Zeners is that the knee is *much* sharper at low currents.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Home of DaqGen, the FREEWARE signal generator

:-)

Yes, here again (it's been a few years since I posted this link) are some old diode, transistor and LED forward-voltage plots I made over a 1pA to 10mA range. Sorry they cut off above 1.25 V, but you can see the red LED is much sharper (110mV/decade) than Ebers-Moll predicts (60mV/decade) over the 0.1nA to 0.2uA range.

I'll have to install Corel's Quattro-Pro spreadsheet on my new "updated" WinXp computer, so I can look at the orginal file to see what the LED did at higher currents.

--
 Thanks,
    - Win

Well, (a), I wonder about how photosensitivity might affect these readings and (b), I wonder about nonlinearity - i.e., did kell also dip them into a container of ice water and check their cold Vf? It might not be a straight line, you know. :-)

Of course, a real thermometer for ambient would help a lot. :-)

Cheers! Rich

Get a black magic marker. ;-)

Cheers! Rich

I tried that experiment again and couldn't get the led's to settle down to a steady voltage when I had them in the water. I decided to try a different way of testing it. I put a series of led's in parallel with a series of an equal number of

1N400X diodes, tied to a common ground at the bottom of the strings with a small current fed to the top of each string. I attached one lead from a voltmeter to the top of each string of diodes. Now when I heated the strings of diodes with a hair drier, IF the led's had the same tempco as the diodes, the voltage on the meter would stay unchanged, because the two strings, having an equal number of elements, would have equal temperature-related drops. As it turned out, the voltage did change. Now to find out the temperature coefficient of the led's, I need to find a ratio between the number of led's and diodes where the tempcos cancel out. For example, if 7 led's balance out with 6 1N400X diodes and show no voltage change when heated with the hair drier, I could figure the led tempco would be six sevenths the diode tempco, which is known. This is about the most reliable way I could think of to really pin down the led tempco; if I put them in the oven it would just be a mess.

The red LED shows a steady +120mV/decade I-V slope over four decades from 0.1nA to 1uA, where Vf is about 1.34V, then the I-V slope *decreases* to +60mV/decade for four more decades to 10mA, the highest value I measured.

--
 Thanks,
    - Win