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Try to use an old laptop charger to construct a 3.3 V power supply

Thu May 25, 2017 4:23 am

I'm trying to construct a 3.3 V power supply using an old laptop charger as an input voltage of 6.4 V. I am using an LM317 regulator and two external resistors to provide an output voltage of 3.3V. R1 is fixed while R2 is a 10k pot that can be finely adjusted to within an ohm or so. When no load is connected, I get my output to be what I want, and I can adjust the potentiometer to adjust my voltage. I used the formula Vout = 1.25 V * (1 + R2/R1), which is in the LM317 datasheet.

However, I've come to realize that when low resistance loads are connected to the output, my output voltage drops considerably. I tested a 14 Ohm resistor by connecting it between the output and ground, and the voltage across it ended up being 0.27 V. This does not happen with large resistances.

I've looked through some old posts and I found that the resistance for R1 is highly recommended to be 120 Ohms. However, I found a website (link below) that essentially says anywhere from 100 to 1000 Ohms is fine, and because I was getting my desired voltages without a load connected, it seemed that this shouldn't be an issue. However, I have little experience with this circuit element so I'm not sure.
 
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Re: Try to use an old laptop charger to construct a 3.3 V power supply

Thu May 25, 2017 7:20 am

A few thoughts...

- The data sheet for the LM317 has quite a few different circuits in it. It would be helpful if you could confirm the specific one you are using. Are you using the simplest one (just the LM317, R1, R2, and maybe a couple of bypass caps)?

- You're operating very close to the dropout voltage limit of the LM317, as it requires at least 3V margin between input and output. It *should* be OK, but if the laptop supply is failing and its voltage is lower than expected that could cause problems. Have you checked the output voltage of the laptop supply under load?

- What *is* your R1 value? The value of R1 is a tradeoff. Lower values minimize the voltage error caused by bias current on the Vadj pin, but also increase power consumption of the circuit. Since you're just adjusting the pot until the output voltage is right, and the offset is small in any case (typically a few 10s of millivolts), this should not be a problem unless R1 is far above the recommended range.

- LM317 is a linear regulator, which means it is very inefficient under load. With low resistance loads it will need to dissipate a significant amount of waste heat. Did you provide any heatsinking? If not, it may be going into thermal limiting when you connect your 14 ohm load.
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Re: Try to use an old laptop charger to construct a 3.3 V power supply

Thu May 25, 2017 7:27 am

I assume you are using a circuit something like this.

Image

First, make sure you have the resistors in the correct place. The low, fixed value resistor (R1 in the schematic) is connected between Vout and ADJ. The variable resistor is connected between ADJ and GND (R2 in the schematic). Looking at a few schematics in Google images, several have R1 and R2 reversed in labeling and then reference the same formula you did. To get 3.3V out, you want R2 to be about 394 ohm.

The other item to consider is that the LM317 needs something around 3V difference between input and output, in order to stay within regulation. a 6.4V input is just barely within the margins for a 3.3V output. If the laptop power supply is slightly low, or your output voltage is closer to 3.4V, then the LM317 may be dropping out of regulation.

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Re: Try to use an old laptop charger to construct a 3.3 V power supply

Thu May 25, 2017 7:52 am

Instead of bodging on a LM317 I think you should modify the op-amp comparator circuit in the power brick. Its already so close to the required voltage; and this way you would be able to tap much more power than what the LM317 could provide.

I found myself in a similar situation a few months ago when I needed to take an old 19V brick down to 12V to power a hobby charger that needed a power supply purchased separately.

This is the site I used as reference:

https://hackaday.io/project/3469-modify ... wer-supply

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