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Topic: Combining the Ampere of two voltage regulator? (Read 4 times) previous topic - next topic


Follow the circuit posted that uses a PNP 'wrapped' around the regulator. It has short circuit protection, is a reprint from the National Semi Linear app notes book (1 of 3) and I would LOVE to get copies of those in digital format $$$... It works very well as I've built several dozen copies of it in the past 20 or so years... and I can tell you from personal experience that although later an that same app note AN??? it (the app note)
shows a version that used an NPN transistor as the pass transistor. The NPN requires either a variable regulator or a diode in the ground lead of the regulator to correct for the .7 V loss in the resultant emitter follower that results from adding the NPN transistor. The NPN version doesn't work as well and requires more parts. The single one I made was real problematical.. the current limiter was an issue... Use the PNP.

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I'm using linear regulator because it is easy to make and it is only a school project. So i will only use it once. I already have a big heatsink and thermal paste so generating heat is not a problem.

I think you under-estimate how hot a regulator will get passing 1.5A of current.  I'm too lazy to do the math, but if it's dropping any voltage whatsoever, it may be impractical to keep the die temperature cool enough to survive.  I built a 28v-to-15v regulator to supply 100mA or so of current, and the little 1"x2"x0.5" heatsinks got hot enough that I couldn't touch them comfortably.  You're asking for a lot more current than that.  Trust me -- that "1.5A" spec on the regulator is nothing short of wishful thinking.  There's no way you'll keep the case cool enough that the die won't damage itself at that kind of load.

That is unless the 1.5A spec is for a very brief, worst-case, transient load... and average current is really *significantly* less.


Do you really think you'll use that much power. The link the OP posted said bust up to 2 amps The Lm78 can handle 2.2 Amps in short burst.

These burst of the  SM5100B cellular module only last 4.6 mS


Jan 05, 2013, 08:23 pm Last Edit: Jan 05, 2013, 08:27 pm by Krupski Reason: 1

You don't want to do that there not going to do what you think. One will try to put out more the it should and basically cutoff the second one. See there not going to supply the same voltage.

I would do this use a PNP to get you more current

Here's a circuit that I use for more regulated power... the small resistor values cause the 78XX part to provide current limit (around 10 amps with these values) and since the regulator and pass transistor are on the same heatsink, the thermal protection of the 78XX is also provided.

Note the way the output is connected... this provides remote current sensing which compensates for drop along long wires if they are used.

Only disadvantage to this circuit is that a minimum load of a few milliamps is required because of the design of the 78XX regulator. If you use an LM-317 instead, then this is not a problem.

(edit to add): If you use a 79XX regulator and an NPN 2N3055 and reverse the plus and minus terminals, you get the same circuit.
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thank god we don't live in a simulator.

You don't really know that. (Sorry, off topic, but couldn't resist.  ;))

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