Multiple Regulators in parallel?

Any problems with running more than one 7805-type regulator in parallel?

I'm currently running a VFD display that draws ~750mA from a 7805 regulator, I have a fairly large heatsink on it, and it still runs very hot(hot enough to burn me).This is a automotive application(average voltage drop of 8 V), where temps fluctuate quite a bit, I know once this project is enclosed and in the car, I will have heat problems.I was thinking I could increase reliability with dual 7805's, any thoughts, ideas, or suggestions? maybe swapping in a switching regulator?

For high output currents, switching regulators are recommended. But the problem with them is to construct torroid inductor which most of us find difficult. If you have access to LCR meter, switching regulator using LM3524/SG3524 will be a suitable and cheaper solution. Datasheets of both the ICs have sample 5V,1A reference schematics.
If you want high current series regulator use LM350/LM338 or similar ones.

Could you use an LM317 or fixed regulator in front of the '05 to take the voltage down partway? Just a thought.

stevez said:

Could you use an LM317 or fixed regulator in front of the '05 to take the voltage down partway? Just a thought.

Click to expand...

This is an easy way to at least "share" the power dissipation. A 5W zener diode of a couple of volts will do the trick also.

Extra 7805 current

The high power dissipation can be minimized by lowering the overhead voltage. In the case of an automotive application, you're stuck with the 12-14 volts of the system. But you can cascade regulators so that you preregulate the 12v system down to 8v and then feed that regulated 8v to the 7805. You'll still dissipate the same amount of power (or pretty much so), but you'll be spreading it across two different regulators.

Are you using one of those clip-on heat sinks or one of those finned sinks that mount with a screw? It's likely that you aren't getting good heat transfer. Make sure that you're using heat sink grease between the 7805 and the sink for good heat transfer. Rather than using one of those sinks, just mount the 7805 directly to the chassis or aluminum mini-box or whatever the project is, as long as it's metal and you still use the grease. It'll dissipate the heat a lot better than a fancy little finned sink and take up a heck of a lot less space.

Another option: the 7805K in a TO-3 package is rated for 1.5 amps vs. the 1.0 amps of the TO-220 package we normally see the 7805 in. It also can get rid of the heat better than the TO-220 case.

And finally, you can run several 7805s in parallel. The easiest way to do it is to make the inputs a common connection and the grounds a common connection. But don't connect the outputs directly together. The 7805 don't all regulate at exactly the same voltage and the one that regulates at the highest voltage will do all the work while the others loaf along and you'll don't have solved anything. Instead, put a small-value (e.g., 0.47 ohm, 1-watt) resistor in series with each output and THEN connect the other ends of those resistors in common as your final output. That small value of resistance will allow each regulator to work independently of the others and the current will be shared by all of the regulators fairly equally. The down side of this fix is that it makes the regulation a little bit "softer" (i.e., a higher internal resistance for the supply), but in most cases, won't be much of a problem.

Dean

awesome information Dean. You're the man!

While Dean's suggestion of paralleling regulators should work, I have never seen it used in any power supply I've came across.
They all had a stack of parallel transistors, driven by a single regulator to increase the regulator's current capacity.
Maybe its just too hard to get multiple regulators to share the current without upsetting one another.
The stack of parallel transistors all have an individual series resistor (~0.1Ohm/ 5-10W) at the output to make sure the slightly different transistor ratings are balanced out.

I have a 10-15V/ 40Amp power supply here that uses a single regulator driving 6 bypass transistors, all on a fan cooled, mighty big, heat sink.

Klaus said:

While Dean's suggestion of paralleling regulators should work, I have never seen it used in any power supply I've came across.
They all had a stack of parallel transistors, driven by a single regulator to increase the regulator's current capacity.
Maybe its just too hard to get multiple regulators to share the current without upsetting one another.
The stack of parallel transistors all have an individual series resistor (~0.1Ohm/ 5-10W) at the output to make sure the slightly different transistor ratings are balanced out.

I have a 10-15V/ 40Amp power supply here that uses a single regulator driving 6 bypass transistors, all on a fan cooled, mighty big, heat sink.

Click to expand...

Yes, there are current boosting circuits out there that can be added to the regulator, what that will do is move the majority of the power dissipation to another part (transistor(s)) and this might be an easier heat problem to solve..

Search on National Semi. website for "simple switcher" buck regulator. No need a big heatsink, just a few external components and online design help.

Thanks for the help guys.I forgot all about the National Semi SMS(switchers made simple) program, I DL'ed it quite some time ago, you just enter in your parameters and it designs a complete regulator circuit for you.Excellent software.