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Voltage Regulators : Doubling Up ??

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Junior Member level 1
Oct 13, 2001
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Can somebody tell me if its possible to use two voltage regulators in place of one, if the one you want is awkward to get ??

EG Running them in parallel with each other

I need an LM350T for a motorcycle battery circuit, but Maplin in UK don't stock them, but, they do have the LM317T, which is the same but only handles upto 1.5A, where the LM350T can handle upto 3A

Soooo, if I connect 2 LM317T's in parallel, will this be technically the same as using 1 LM350T

Thanks for any help, then I can get started on this damn battery charger

The circuit I'm using is better than perfect for charging motorcycle batteries, and can handle 6v and 12v batteries, with fully automatic float. To buy one would cost at least £30, which I'm too tight to spend, but would rather build my own, as I have a lovely metal cased 1970's constant current 12v battery charger, which would be an ideal donor case and transformer

It works as I've used it to charge car batteries in the past, but all there is internally is a small bridge rectifier and a huge metal finned type thing clamped to the case (can't remember what it is, but I've seen them is old equipment), so all I'm using is the transformer

battery treatment

Lead acid batteries last longer if they are charged and discharged at low currents. Your battery will last longer with the 1.5 A charge compared to the 3 A charge if you have the time available for the slow charge.

I have a circuit somewhere that shows you how to increase a regulators rating by using a single transistor, I'll dig it out

Also, the reason I assume a 3a regulator is used, is so as not to stress the 1.5a one, I don't mind waiting longer, I'd prefer it, motorbike batteries prefer a small charge rate anyway, especially as this circuit features automatic float after charge there is no worries about overcharging it

But, back to the question, if I really, really wanted to, can I use two regulators in a simple parallel setup ? Well, side by side on a big heatsink, or bolted to the case, with mica insulators of course, and having them linked together to create the parallel arragement


Yes you can use a high current transistor, or as you sugested use two LM317 All you need to do is use a low value resistor in the out put line this will back bias any differances in the regulators . Elector published a power supplY using 10 of these regulators all biased in this fashion


it is ok to use 2 or more voltage regs in parallel look at the lm7805 ect the old c64 psu had 2 lm7805's some times with a 1R and some times not? take a good look at the data sheets or look at the old 2n3055 type psu's that can give 5A with one device it's all down to cost!

It is not a good idea to put 2 linear regulators simply in parrallel. One of the two will always regulate to a slightly higher voltage en therefore will dissipate most of the power. It is better to use an external power transistor or a single regulator that can handle the current. A few manufacturers that make these regulators are linear technology, ON (formerly motorola) and national semiconductor and maybe fairchild. :idea:

its no problem using two fix-regulators (or even adjustable ones [LM317/337/...]) in parallel. You must only add a series resistor on each regulator output to compensate the voltage deviations of each regulator. In the case of e.g LM7805 @ 500mA a 0R33 Resistor will compensate the device deviations (estimate the power of the resistors in high current applications via P=I²*R).

I built a 1.2 to 30V / 10A adjustable supply using 12 * LM317 (cooled) and an little opamp circuit to control the voltage of all devives simultaneously.

Consider section 4 of the book, which KaSpeR give you a link ->


Presently, most monolithic IC voltage regulators that are available have output current capabilities from
100 mA to 3.0 A. If greater current capability is required, or if the IC regulator does not possess sufficient
safe–operating–area (SOA), the addition of an external series pass element is necessary.
In this section, configurations, specifications and current limit techniques for external series pass
elements will be considered. For illustrative purposes, pass elements for only positive regulator types will
be discussed. However, the same considerations apply for pass elements used with negative regulators.

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