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zener diode voltage divider

zener diode voltage divider

zener diode voltage divider

(OP)
hello all again,
quick qs

I need to get a 1.5v supply from a 9v battery.
I want to use a zener diode to do this.
attached are two drawings 1st with a 7.5V zener in series, the second with a 1.5v zener in parallel. Am I right in thinking that you cannot get a 1.5V zener diode. Therefore does the 7.5V zener in series do the same job of creating a 1.5V supply?

RE: zener diode voltage divider

The problem with the first option, dropping 7.5 across the zener, is that it will be fairly sensitive to variations in the "9 volts". If the 9 is actually 9.2 to start, then your output will be 1.7. As soon as the battery drops to 8.8, then your output drops to 1.3. It is very sensitive because of the 'leverage' of the approach.

The second approach needs a series resistor. It will provide a very stable voltage until the 9-v battery has rotted away. But it draws a continuous current so the battery wouldn't last very long.

Any chance of using a 1.5 volt battery?

RE: zener diode voltage divider

(OP)
Thanks for the reply VE1BLL,
The problem this is just an off-shoot of a larger ciruit. The 9V is needed to run an amplifier so I cannot use a 1.5V battery unless I add this in a almost a seperate circuit but this will take up too much space within the device. I would prefer to use the second option for the reasons that you describe but can't find a 1.5V zener diode. I need a stable 1.5v hence the use of a zener diode.

RE: zener diode voltage divider

Some of the standard 3-terminal regulators are actually 1.4V regulators, which drop the remaining voltage across its resistor to ground.

TTFN

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RE: zener diode voltage divider

How much current do you require?

RE: zener diode voltage divider

(OP)
I don't need a lot of current approx 300uA.

RE: zener diode voltage divider

For the space, I would think a 3-terminal regulator would be perfect here... a few pennies more, but rock-solid voltage.  You may even get away with using its very own leakage current to power your circuit... that would be an interesting circuit to try and make work!

Dan - Owner
http://www.Hi-TecDesigns.com

RE: zener diode voltage divider

Or two resistors in a voltage divider. Depending on the variation of that 300uA, you can set the current through the resistors to swamp out the voltage variations within your limits. If the 300uA doesn't vary much, then you won't waste very much current. Might even be less than some 3-terminal regulators would use.

RE: zener diode voltage divider

1.5V +/- how much?

RE: zener diode voltage divider

Two silicon diodes in series with the band pointing to the negative and a 120K dropping resistor should be sufficient to run the microphone.  They aren't very critical.  Didn't see the resistor in your drawing.

RE: zener diode voltage divider

(OP)
thanks all for you replies. I did not want to use a set of resistors for the voltage divider as I wantd a very stable voltage. Also I was trying to avoid using a voltage regulator for something cheaper. What sort of silicon diodes would I need to use for this application?
Many thanks Andrew

RE: zener diode voltage divider

Perhaps you should quantify what the specific performance requirements are.  As usual, doing this will avoid further attempts at solutions that don't meet your unstated requirements.

For example, what is your cost bogey?  The example here: http://www.national.com/pf//LM/LM2736.html probably runs less than $2.  You've already spent that much on this thread.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: zener diode voltage divider

Any two small silicon glass signal diodes would do up to a 1N4001, 02,...07 type.  It has to be cheap to do because I have a clock radio that they mounted a standard quartz clock unit in.  Instead of an AA battery, that is what they did to supply power.

RE: zener diode voltage divider

Given your low current requirement, the LMV431 from National seems like a good candidate.  It integrates a 1.2V reference zener and a linear comparator to vary the ouput to anything you like, using two resistors.  It can be purchased in qty one if wished at places like Digikey.  Available in both SMT and lead-thru packages.

RE: zener diode voltage divider

I love the LM31, got a couple hundred of them.   There are a lot of neat things you can do with them, but one of the things they can't do is go below 2.5 volts.

RE: zener diode voltage divider

We all know that zener diodes are only available down to 2.4V. Below that you either use one or two FORWARD biased silicon diodes (zeners are of course reverse biassed for zener operation).

If the load is 300µA then you can't use a shunt regulator diode and a 120K resistor (as suggested by operahouse) since the maximum current from that would be 62µA. clearly you need a resistor lower than (9 – 1.5)/300µ = 25K. Typically you would allow at least 200µA for the shunt regulator, but this all depends on how much variation there is in this "300µA" load and how much voltage ripple on the 1.5V rail is acceptable.

Whilst ordinary silicon diodes are not really specified for forward operation as shunt regulators, there is a diode specially designed for this purpose. Example NXP BAS17 is a single diode "stabistor". All it is, is a single silicon diode for operation forward biassed, BUT it gives data for using it in this way which you will not find for an ordinary 1N4148, BAV99 or whatever.

RE: zener diode voltage divider

9V batteries usually have the worst energy density of any common battery types and highest energy cost. Any reason why one 1.5V cell can't be used, or two in series, or a 3V lithium cell? Regulating 9V down to 1.5V using a linear regulator seems kinda pointless when you can easily start with a supply voltage so much closer to what you need which will give you better efficiency and lower running costs.
 

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If we learn from our mistakes I'm getting a great education!

RE: zener diode voltage divider

OOPS...off a decimal point.  Thought I read 250K on the calculator.  It is probably just a little goofy electret microphone.  If it is a two wire type, they can operate on higher voltage.  If a 3 wire type, then I would consider changing it for a two wire unit.

RE: zener diode voltage divider

Scotty, I believe the OP mentioned there was a 9V supply already on the board, hence the desire to use it rather than adding another battery.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: zener diode voltage divider

macgyver,

I believe you are right. Where did that come from? It wasn't there when I looked last night! tongue

Sorry folks, I'll get my coat...
 

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If we learn from our mistakes I'm getting a great education!

RE: zener diode voltage divider

Hiya-

This is "out of the box" and might not field your specific requirements for your design.  Small LEDs usually have a forward voltage drop around 1.7V.

If the OP has an LED somewhere in the device and if the voltage "regulation" is rough, then this just might be something to check into.

Of course, there might be significant variances on the forward voltage drop due to process changes.

The series resistance of the LEDs (the internal resistance) might be low enough to provide enough regulation for the desired purpose.

Hope this helps!

  Cheers,

   Rich S.

RE: zener diode voltage divider

I have used a pair of 1N4002 before now , LEDs can be noisy .
Using a Zener 7v5 drop is risky (even if the 9v was stabilized ) as Zeners fail S/C .

 Vary the series resistor for the best stability vs Battery life ( as the 1N4002s can cope with 200mA easily )

 A cheap DMM (Not an AVO8 ) is essential to monitor 1.4 - 1.5V

RE: zener diode voltage divider

Why does people keep saying that Zener diode are not available below 2.4V?

If you go at Digikey, you can find Zener diode as low as 1.8V.

RE: zener diode voltage divider

Yes, but they are not true 'zeners' and they have a horrible soft kneepoint which makes them a poor choice for regulation.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: zener diode voltage divider

Right. They are series connected Si diodes. Terrible.

A forward LED has a much better characteristic. Select IR for around 1.3 V a red for 1.6 a green for around 1.8 V.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

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