19v adapter to 16v?

[spleenharvester]

Re: 19v adapter to 16v?

Also, for anyone reading, just fired up a +16v IBM X31 with a +19V adapter. So clearly not much of a problem!

[sam_sam_sam]

Re: 19v adapter to 16v?

Look for a part that has "431" written on it. These are commonly used in smps feedback circuits. They are adjustable voltage shunts. The voltage is set by two external resistors. To change the output voltage, you simply change the resistors to the right values.

What is the part # so I can find a data sheet thanks

[mariushm]

Re: 19v adapter to 16v?

It's TL431 ... a "jellybean" component, thousands of versions from lots of manufacturers.

http://www.digikey.com/product-searc...keywords=tl431

[ben7]

Re: 19v adapter to 16v?

It's TL431 ... a "jellybean" component, thousands of versions from lots of manufacturers.

http://www.digikey.com/product-searc...keywords=tl431

Yeah, it is also commonly called the 'KA431' or the 'LM431'.

[tom66]

Re: 19v adapter to 16v?

"TL" stands for Texas (Instruments) Linear
"LM" stands for Linear Monolithic

The first two characters are usually manufacturer specific.

[ben7]

Re: 19v adapter to 16v?

"TL" stands for Texas (Instruments) Linear
"LM" stands for Linear Monolithic

The first two characters are usually manufacturer specific.

I think 'KA' is used on the Fairchild made 431's...

[Behemot]

Re: 19v adapter to 16v?

And KIA431 is for KIA obviously

[sam_sam_sam]

Re: 19v adapter to 16v?

Thanks for the info

[kapurk]

Re: 19v adapter to 16v?

ben7 has the right answer. Almost all Laptop adapters use a TL431 or similar voltage regulator which uses 2 resistors as a voltage divider to sense the output voltage. see the attached pdf file. R13 and R14 are the two resistors. The values of the 2 resistors are not critical; what is important is the ratio. The ratio should be such that for the required output the voltage at pin 1 of TL431 is 2.5. The formula for calculating R13 and R14 is as follows:
R13 = R14(V-2.5)/2.5 where V is the required output voltage
If you fix the output as 16V, then it simplifies to:
R13 = R14 *13.5/2.5 OR
R13 = R14 * 5.4
So if R13 is 10k then you need 54k for R13.
you can try different values for R14 to get an easily available value for both so long as R13 stays in the range 10k to 15k. Or better still replace both with a 100k preset and use it to set the output to any voltage you want!

[mariushm]

Re: 19v adapter to 16v?

TL431 is used as a voltage reference, not as a voltage regulator... it can be adjusted to output a certain voltage based on the resistors used.

Then, it depends, a classic method is to use an operational amplifier or a comparator to compare the output voltage with the voltage reference and adjust the pwm frequency accordingly, to raise or reduce the output voltage.

All that resistor math is pointless, there's no unique formula, each power supply can be different (give it a different input voltage, adjust the reference for a particular output voltage etc). It's enough to read the datasheet of such a *431 IC to understand how it works.

[kapurk]

Re: 19v adapter to 16v?

Whoa man! Yes, I agree that the TL431 is a voltage reference not a regulator - that was a slip. However, that resistor math will work with almost all adapters that use the TL431 in this configuration. I hope you saw the attached PDF file. Just to let you know, I am a manufacturer of laptop adapters. Almost all adapters these days use a very similar output circuit design with a TL431 or similar voltage reference for output voltage control. The PWM part of the circuit, does have a lot of variations but the overall design concept is still common.
I noticed someone mentioning an undervoltage cutoff coming into play. That is a possibility. Tweaking the voltage dividers may not work beyond a stage because the PWM circuit may become unstable or stop working altogether.