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Old 08-14-2010, 02:57 AM
edvard_m edvard_m is offline
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Question Diode as voltage regulator problem

Hi everyone. I'm trying to build a cheap voltage regulator to have a regulated voltage of 1.4V from a 3V source. I was trying to do something similar to what was presented here: http://www.reuk.co.uk/Zener-Diode-Voltage-Regulator.htm but I couldn't find a zener diode on Digikey with a Zener voltage of 1.4V (or .7V to put in series).

Now I'm just looking for any through hole axial diode to get a 1.4 drop across. The problem is on Digikey, the parts say Forward Voltage .7V @ 1A or .7V @ 5A etc. I know I want .7V Forward Voltage, but how do I know what current will be going through the diodes?

Thanks.
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Old 08-14-2010, 03:00 AM
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marshallf3 marshallf3 is offline
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Just ignore the current rating, virtually all regular silicon (but not schottky) rectifiers will drop between 0.6 - 0.7V across the junction if the current is kept within it's its operating range.
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Old 08-14-2010, 03:03 AM
Ghar Ghar is offline
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Check out the diode datasheet to see a plot of voltage vs current.
You'll see there's quite a bit of variation even in the same part number (with temperature, current, whatever)

You set the current using a resistor, usually called the 'current limiting resistor'. There are a lot of calculators and explanations online on how to do this.

Last edited by Ghar; 08-14-2010 at 03:11 AM.
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Old 08-14-2010, 03:12 AM
edvard_m edvard_m is offline
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Ok. I know my load draws about 1mA. How do i know if the current will be within its operating range if I don't know what the current running through the diodes is? If it is too low, then won't i get a smaller voltage drop across each diode based on the i-v characteristics of a diode?
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Old 08-14-2010, 03:21 AM
Ghar Ghar is offline
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The current in the diode will be:

(Vsource - Vdiode)/R - ILoad

Vdiode won't be changing that much for small changes in current but you can follow the graph to see where you are.
The change is something like 0.1 V for every decade of current (factor of 10), it depends on the exact diode.
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Old 08-14-2010, 04:11 AM
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Unless you're shooting for absolute precision I consider it to be a non-issue in most simple circuitry.
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Old 08-14-2010, 07:07 AM
edvard_m edvard_m is offline
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Ok. If I chose a 0.7V Forward Voltage (FV) @ 1A, if I ran it at .1A, I'd already be down to about 0.4V FV.

So I found another diode that operated at 1.7V FV @ 1A. I looked at the i-v curve and it looks like if the diode current is 10mA, I can get the FV to be 0.7V.

http://www.diodes.com/datasheets/ds25002.pdf The UF1005

I1 = Idiode + Iload
(3V-1.4V)/R1 = 10mA + 1mA = 11mA

This means I need R1 to be 14.5Ω. Is this calculation correct? I want to run as little current as possible through the diodes to save the 2AA batteries, but it looks like this is the least current I can supply from what I have to choose from on Digikey.
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Old 08-14-2010, 04:05 PM
Ghar Ghar is offline
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You're off by a factor of 10, it's 145 ohms.
You should plug the value back in to check especially when it's such a short equation.

How close do you need to be though? You could cut power consumption in half and lose less than 0.1V.
The diodes themselves will be off from the datasheet number as well, so you'll probably need to adjust it if you want exactly 1.4V
It'll also change with your load unless it's a constant 1 mA.
Marshallf does have a good point.

You could also look into putting 3 Schottkys in series, you might be able to find ones that work with 1.4V at a lower current.

Last edited by Ghar; 08-14-2010 at 04:14 PM.
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Old 08-15-2010, 01:00 AM
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A diode is not a voltage regulator.
The voltage across a diode changes when its current and temperature changes.
You should use a low current voltage reference IC.
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Old 08-16-2010, 12:38 PM
sage.radachowsky sage.radachowsky is offline
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Yes, here is a 1.5V reference.
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