Connecting two LM7805 regulators together

[b.james]

I have two of these but they are different brands . One puts out 5.1 Volts and the other 4.8.
I want to parellel them . Can anyone tell me of the dangers of this or would the difference be of no concern?.
How would the lower output one handle the input from the lower one ?

 

[MikeMl]

In paralleling them, as load current increases, the higher voltage one carries all of the load current until it goes into current limiting at around one Amp, at which point the voltage drops from 5.2V to 4.8V, and the load current can increase up to where the second goes into current limiting at about 2A...

 

[JimB]

I have two of these .... I want to parellel them....

Sorry, that is a very bad idea.

It would be much better to use a single regulator which is rated for the load current.

Or, maybe you could split the load so that part of the circuit was driven by one regulator and the other part by the second regulator.

Or, you could use one of your regulators in conjunction with an external pass transistor. Have a look here:
**broken link removed**
page 17 of this application note has suitable circuits.

JimB

 

[AnalogKid]

The 7805 is an old design, one of the first 3-terminal regulators. That's not automatically a bad thing, but it does mean that the part is not very friendly, and designed for specific uses that are relatively narrowly defined. Specifically, it does not current share well while maintaining decent regulation. If you add ballast resistors and an external opamp you can treat two of them like pass transistors, but that's a lot of work for something that can be done with a $1 part like an LM350.

ak

 

[Grossel]

A very simple way of enabling higher output current from a 78XX regulator is to use a PNP transistor.

 

[Tony Stewart]

A very simple way of enabling higher output current from a 78XX regulator is to use a PNP transistor.

Some 78x05 come in improved designs with lower dropout (<1V) and up to 5A limit.

**broken link removed**

All LDO's of this type are basically regulated Emitter Followers so they only pull up.
Poor man's Current sharing is possible by adding ESR to the higher voltage out to account for Verror at max I choosing small R such as 0.2 Ohm. At the expense of load regulation of source voltage... so choose a better design as suggested or active transistor bypass.

The improved LDO designs use MOSFETs instead of PNP.

If using large V drop, then compute W to design heatsink for 30'C max rise.

 

[granddad]

I would suggest the 4.8 v one is not 100% and may fail soon.... Just use LM 78S05 2A verson . don't forget heat sink.

 

[Tony Stewart]

I would suggest the 4.8 v one is not 100% and may fail soon.... Just use LM 78S05 2A verson . don't forget heat sink.

Higher output will always be active, Lower output in parallel will be floating. ( Like DTL logic High man wins)