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running voltage regulators in parallel

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New Member
can anyone advise on running or 4 of LM7805's in parallel. Do I need to increase the size of the capacitors or add something else in the circuit. I am in need of approx. 3 amps continious.


Brett Lemon


Active Member
It's not uncommon to add a current handling transistor or two (called a pass tranisistor). If I am not mistaken the datasheets for an LM317 describe the pass transistor application.


Active Member
The datasheet for the LM317 does show multiple regulators in parallel but with 0.5 ohm resistors on the "out" terminal of each regulator. The purpose of the resistor, as I understand it, is to equalize the flow of current across each regulator. I do not know if the value of the resistor is critical but make sure the power handling capability of the resistor is sufficient. A 0.5 ohm resistor carrying 1 amp would dissapate 0.5 watts so a 1/2 watt resistor is the minimum - 1 watt would be better in case one regulator provides a little more than 1 amp.

If you have the 7805s on hand and they have little monetary value then try paralleling them and see what happens with the resistors I've described (or without, maybe you'll get lucky).

If you have to pay money for parts I'd go with Mosfets circuit or purchase an LM338 which is a three terminal voltage regulator that will handle as much as 5 amps (with due regard for total power dissapation).

Dean Huster

Well-Known Member
Those output resistors ....

As Steve mentioned, you can put the regulators directly in parallel, inputs all connected together, grounds all connected together and each output connected to a low value resistor and then the other end of those resistors connected together as the output.

If you tried to connect the outputs of the regulators directly without the little series resistors, you'd have problems. If you have four regulators, each of them will try to set the output at a slightly different voltage, this variance being the normal manufacturing tolerance. The regulator that tried to set the output for the highest voltage (it might be only 0.1 volt above the others) will try it's best to supply all the current that the load needs by setting the output a tenth of a volt above the others. Meanwhile, the other four regulators see that the output voltage is too high and they'll throttle back and relax, trying to get the voltage to go lower ... and it won't.

Although simple, the disadvantage of using this method of series resistiors is that the regulatated supply becomes "softer", developing an internal resistance that won't allow the regulation to be as good as a single regulator. So, the circuits that use a hefty pass transistor around the regulator tend to work much better.

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