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9vdc voltage limiter

Tony Knerr

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I've asked about this several other places, but no one seems to know the answer.

I would like to limit a speed controlled 12vdc fan to 9v max. The caveat being that anything below 9v be passed at the same voltage in-and-out, so a simple resistor wont do. Will a dropout voltage regulator correctly perform this function? Something like this? http://www.jameco.com/1/1/34873-ld1...drop-positive-fixed-voltage-regulator-9v.html

Or is a more complex circuit required?

Thanks,

Tony
 

modem7

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You could try the 'Zener Limiter' at [here]. 9V zener. It is a common circuit, often introduced to people during electronics training. There will be information on the Internet about how to calculate the component values (including power rating).
 

Chuck(G)

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Since this is a fan, an LM7809 should do just fine--cheaper than the LDO variety, even if the regulation at a line supply of +12V isn't as good. Its's a fan, after all.

Alternatively, you could use a variable regulator such as an LM317 to provide fine control.

Or you could stick a resistor in series with it (the fan nameplate should show power consumption).

Or you could go exotic and use a PWM regulator....
 

Tony Knerr

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The 'Zener Limiter' circuit looks like the only thing that fits the requirements so far. Thank you.

I had hoped to use a dropout regulator for this though. I guess my question regarding the dropout regulator will go unanswered. :( Probably the only way I'll find out the true nature of this device is to try it for myself.

For those that didn't understand the inquiry, here's a further explanation. If the device has 1v on it's input, the output should be 1v. If the device has 2v on it's input, the output should be 2v. If the device has 3v on it's input, the output should be 3v. ...and so on up to where If the device has 9v on it's input, the output should be 9v. But if the device has 10v or above on it's input, the output is limited to 9v. I hope this clarifies my inquiry.

Tony
 

luckybob

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thats what I linked to you...

linear voltage regulators will lose some voltage, no matter what. The sheet you linked tells you it happens to be 1.3v. That means, NO MATTER WHAT you lose 1.3v So you need a minimum of 10.3v to get 9v out.

just buy that little module i linked.
 

Chuck(G)

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Every active bipolar device exhibits a certain amount of loss due to the presence of diode junctions, which runs about 0.6-0.7V per junction. Look at the schematic of the LM7800 series of regulators and you'll see that the drop from line to load is about 2V, or about 3 junctions. If you look at the LM7809 quick spec sheet here, you'll see that the minimum supply voltage to get 9V out is 11.2V, which, from a 12V regulated SMPSU, is more than adequate. At 71 cents, it's hard to see how you could be more cost-effective.

An LDO uses a somewhat different implementation that cuts the drop to about half that or less of the tranditional bipolar device, so it's handy if you're trying to get a 3.3V supply from a 5V rail. They still come in 3-terminal configurations.
 

xprt

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The 'Zener Limiter' circuit looks like the only thing that fits the requirements so far. Thank you.

I had hoped to use a dropout regulator for this though. I guess my question regarding the dropout regulator will go unanswered. :( Probably the only way I'll find out the true nature of this device is to try it for myself.

For those that didn't understand the inquiry, here's a further explanation. If the device has 1v on it's input, the output should be 1v. If the device has 2v on it's input, the output should be 2v. If the device has 3v on it's input, the output should be 3v. ...and so on up to where If the device has 9v on it's input, the output should be 9v. But if the device has 10v or above on it's input, the output is limited to 9v. I hope this clarifies my inquiry.

Tony

How about an LT1963A? Lower dropout than the LD1086. If your fan draws about 200mA, the dropout is only about 0.15V. A problem with the low dropout regulator for your application is the behavior when it drops out is not well specified. The output voltage is not likely to drop nice and linearly as the input voltage drops. At some point as you lower the input voltage the output voltage might just collapse.
 

xprt

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Last edited:

Chuck(G)

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Well, I have to admit that I'm puzzled by all of this.

If you want to make the output truly independent of the input voltage, then use a buck-boost converter--there are plenty. But I still can't understand why the input voltage to the regulator is expected to drop much below 12V. Are you planning to arc-weld with it in your off-time?

If this isn't a computer application, then it's miscategorized.
 

Tony Knerr

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The end result was that I tried the 9v dropout regulator and it worked as expected. When the computer supplies a voltage less than 9v to the fan, the regulator passes the same voltage to the fan as on the regulator's input. But when the computer supplies more than 9v, the regulator limits the output to 9v. No more B52 bomber taking off on my desk. :)

To Chuck - The input voltage to the fan can be less than 12v because the voltage is controlled by the computer. There is a temperature sensor in the computer that runs the fan faster when the computer gets warmer, and runs the fan slower when the computer cools down. I didn't want to make the voltages independant of each other, I just wanted to limit the maximum voltage to the fan to 9v instead of 12v. A buck-boost converter would have had the exact opposite effect of what I was looking for. I sincerely hope you understand the question and answer at this point because I don't know of a way to explain it any better than I have.

Tony
 
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