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A few questions on some voltage regulators

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bacterozoid

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See attached schematic.

I guess I'm basically just asking if the circuit will work. The power source is a DC power supply. The circuit should be able to output various voltages, as listed. I realize that each of the regulators is going to dictate how much current I can draw. For my purposes, I never expect to go nearly over spec.

One of my main concerns is what happens if I connect this up to a circuit and I happen to have a short or something? The DC adapter *should* limit the current draw, but I'm concerned that it still may allow too much current out and heat up and damage one of my regulators. Is this a concern and how should I go about limiting the current?

Just as a clarification, I don't plan on using this many voltages in any circuit - I'm just trying to build something I can use for a multitude of purposes.

Thanks!
 

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Most modern linear voltage regulators have internal protection circuitry that protects the regulator if it gets too hot or if too much current is being drawn, like a short circuit on their output. So just check the data sheets for your selected regulators and confirm that they are over temperature and over current protected, most do these days.

Other then that your main effort will be in deciding how big a heat sink you will need for each regulator. If you feed 15 volts to the 5 volt regulator and wish the regulator to be able to supply 1/2 amp then the regulator has to dissipate 5 watts of heat ((15-5) X .5), which is a lot and will shutdown on hi temp if a heaksink is not used.

Lefty
 
Assuming the voltage regulators will be more then 6" from the wall wart, you might consider input (0.1 uF) and output capacitors (1.0 uF) for your LM317's too (see: National Semiconductor datasheet, page 1). The latter is for improved ripple rejection.

John
 
Ok, so I'll need to toss some caps on the other 317s and just check the datasheet to see if there is temperature and current protection - if they don't I'll be back, but I bet they do. The only other trick is the heatsinks, which are easy enough to get on them.

Very good! This should go pretty smoothly then. Thanks!
 
The output of the LM317 won't go below 1.25V unless you take the lower resistor in the divider below 0V.

680R for R1 is too far too high, use 120R and make the pot proportionally lower.

The LM317 is thermally and over current protected so don't worry about fuses.
 
I was under the impression that as long as I kept R1 below 1k I should be alright - why should I use a lower value?
 
Where did you hear that?

It's total nonsense!

The LM317 isn't really an adjustable regulator, it's a fixed regulator with an output of 1.25V with Vadj pin being 0V. You can build adjustable regulators with the LM78xx series in exactly the same manner and the formula for the output voltage is exactly the same except you use the normal ourput voltage instead of 1.25.

Unfortunately the LM317 has a minimum load current of 10mA which means you need to account for it when selecting the value of R2, 1.25/120 = 10.4mA. If the minimum load is any less than 10mA then it's possible that the output voltage will be higher when the regulator is lighlty loaded. If you are sure that the minimum load on the regulator plus the current taken by R1 will equal at least 10mA then don't worry about it.
 
Be aware that the case of the LM317 is output, whereas the case of the other regulators is ground. An insulator is in order for the 317 if a common heat sink is used.
 
Use Ohm's law:
R = V/I
 
You show an LM2937 regulator with a negative output voltage of -5V. Impossible since it is a positive voltage regulator.
You show an LM2937 with an output of only 3.3V. Impossible unless ST Micro copies it and makes one for 3.3V.
 
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