Hi

recently i was playing with some regulators and i wanted to parallel 3-4 of them so i can raise the current.. no specific reason, i was just playing...

now.. from what i know i can't parallel many of them, but i don't know why.. can somebody explain to me the reason?

thanks.

 

Regulators don't like being paralleled because they all have slightly different output voltages. The one with the highest voltage will power the circuit at low currents, at higher currents its voltage will start to drop and one of the lower voltage regulators will kick in.

This process can result in instability, I've never tried it myself but I imagine that the voltage could drift all over the place and there's a risk of oscillation.

Just out of interest, how many did you parallel before you started having problems?

What sort of problems did you have?

Are you sure that your power sorce can handle the current?

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hmmm you're right! as far as i recall somebody told me that oscillation is the main concern when you parallel many of them... thanks!

in my case i tried with 3 and there was no problem... probably they matched ok?

 

What current did you try to draw from them?

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Regulators can be somewhat successfully paralleled if you add a small-value resistor (of a power rating that can handle the power dissipation at full current, in this case 100mA), maybe less than a few ohms. The result is that there will be a small voltage drop among the regulator outputs and all will do better at sharing the load and won't fight and get frustrated. The down side is that you end up making the overall regulation a bit softer, in that you've increased the source impedance and the percent of regulation will increase. To avoid oscillation in this configuation, try a 0.1µF cap to ground at EACH regulator output (between output and resistor) as will as the final output (common connection of all resistors).

Once you have to parallel more than two 78L05s, you might as well give up the space and use a 7805.

Dean

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Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

 

Did you scope it to make sure there were no instabilities? I've seen 78xx series regulators with >1Mhz oscillations even though a multimeter showed the correct output voltage.

 

Why not use a transistor, or several of them to "up the current"?
an old psu I made a good few years ago used an LM317 (adjustable voltage regulator) but low current I think it was 1A max? not sure it was many years ago
and a bank of 2n2222's to bring the current up to around 20A

I've lost the circuit many years ago, but the attached one gives you a basic idea to work from

Attached Images

circuit.png (11.6 KB, 41 views)

 

What was the board size of that device?
It takes a bank of at least 34pcs. 2N2222 to get 20A out of the circuit.

 

Did you use SMT versions you save space?

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I think It is the better when we need to make a power. The first you need datasheet of IC that you will use

 

I'm not with you ;what are you refering to?

Welcome to the forum by the way.:0

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