High current voltage regulator

Svejk

Senior Member
Hi there,

I have an application that require about 3A of 9V regulated voltage. Can this be done with 3 x LM7809 connected in parallel? Any precautions?

Thanks,
S
 

Andrew Cowan

Senior Member
But also the last part of post 7.

Look at the datasheet for the 7809 - you can increase the output current using a transistor. Problem solved.

A
 
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westaust55

Moderator
But also the last part of post 5.

Look at the datasheet for the 7809 - you can increase the output current using a transistor. Problem solved.

A
Think that should be the last part of post 7.

- - - - - - - - - - - - - - - - - - - - - - - -

3 Amps from three 7809's? that would have all three regulators operating at maximum curent and require good heat sinking to dissipate the heat (wasted power).

The 78xx series regulator datasheets show a circuit using a parallel transistor as a series pass device for greater current capacity - still have to consider the heat dissipation at 3 Amps.

A better solution if you can locate and afford one (they can be 10 time the cost of a 7809 per amp) is a switching type regulator which are far more efficient.
There are 1 Amp devices: http://pdf1.alldatasheet.com/datasheet-pdf/view/238742/CUI/V7809-1000.html
typiclaly these switch-mode regulators can be paralleled but if you do more searching you may find a 3 Amp or 5 Amp version.
 
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fernando_g

Senior Member
Hi there,

I have an application that require about 3A of 9V regulated voltage. Can this be done with 3 x LM7809 connected in parallel? Any precautions?

Thanks,
S
Other than the explanations provided by other forum members.....
What is your source voltage? 12V?, 24V? If you don't know, you need to measure it.

Remember that voltage difference between your regulator's input and output multiplied by the current will be the watts produced as heat that will require dissipation.

You could find out that your heatsink requirements to be actually quite large, and may even require a fan.
 

Svejk

Senior Member
Thank you for your responses.

I've found a LM350k sorting some PCB in my "lab" (also known as garage :)) and that solved the problem.

S
 
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BeanieBots

Moderator
Nice find cvrwy.
A bit large for "drop-in" replacement but would be fine in many cases and quite a good price for such a versatile device.
 

westaust55

Moderator
I concur with Beaniebots = Nice find cvrwy :).

I have previously seen and mentioned the fixed 5V switching supplies from Dimension Engineering but had not spied their variable voltage version. :rolleyes:

PCB area wise not much different in area to a 78xx with a good small heatsink but they (including the fixed voltage versions) are considerably higher.
 

hippy

Technical Support
Staff member
It's always worth remembering that what datasheets and application notes do not show can be just as important as what they do show.

In the case of simply paralleling up 780X regulators to get extra current capability, it would be logical that if this were possible the datasheets would say so and give an example of such use, especially when they describe other means to increase the current capability. The absence therefore suggests that this is not a suitable configuration or, if it could work, the manufacturer is not prepared to guarantee or recommend such operation ( eg, the caveats for such use are too numerous to make it worthwhile ).

Were a manufacturer being protectionist, hoping to sell a more capable, higher priced component, for common issues such as higher current regulating there would probably be some information on the Web saying it wasn't necessary and demonstrating the simpler, cheaper alternative.

So it's a pretty good bet that if the manufacturer doesn't say it can be done, no one on the web shows it can be, then it's likely not possible. That's not necessarily a reason not to try though if done in a proper and safe environment and you are prepared to accept damage or destruction of the components during research. It's always a good idea to ask what might happen before trying though, don't just dive in and win a Darwin Award.
 

BeanieBots

Moderator
Absolutely hippy.
It CAN be done but you are spot on with the caveats. In particular, the final spec depends on how it's done.
What CANNOT be done, is simple pin-pin paralleling.
It won't explode, it will simply have the same spec as a single regulator which simply renders it a pointless excersise.
 

westaust55

Moderator
It's always worth remembering that what datasheets and application notes do not show can be just as important as what they do show.
:
:
:
It's always a good idea to ask what might happen before trying though, don't just dive in and win a Darwin Award.
Receipt of a Darwin Award would suggest that the rest of us might never know how the project transpired :eek:
 

Ralpht

New Member
Quite a long time ago I remember seeing 3 X 7805's connected in parallel, in a commercial power supply. Can't remember what it suppled power to but the thing I noticed about it was how the regulator outputs were connected together.

Each output had a 0.5ohm 5W resistor then a diode, I think it was an IN4004 but not sure, in series. The idea was to try to equalize the currents from each regulator and to also isolate each regulator output from the other. Each regulator contributed to supplying its share of current and they didn't affect each other because they were isolated from each other via the diodes.

I thought it interesting at the time and this thread reminded me of it.

Personally, I would just get a bigger regulator or use a transistor to take most of the load off the 7809.

I might just rig up an experiment as per the above with the diodes and see what happens. Expect it to work OK.
 

BeanieBots

Moderator
That arrangement will be fine as far as the regulators are concerned.
However, what output voltage and regulation would you expect from it.
Pretty naff is how I'd describe it. ~4v +/- 1v depending on load, hardly "regulated".
 

Ralpht

New Member
bb -

Yep, about what I think will happen.

Regulatinon shot and voltage down by the diode voltage drop.
Like I said earlier, can't remember what it was driving, but it couldn't have been too fussy about it's power requirements.

It'll work but I can't see the point of it considering that a decent transistor will happily supply any extra current the regulator itself couldn't handle. Correctly heat sinked of course.

It takes all kinds I suppose....
 

Andrew Cowan

Senior Member
When I connct three 7809s in parallel, the output is 9V exactly. It can provide up to 3.1A. If one was oing all the work, it would ge hotter, thus automaatically let less current trough, thus reducing the amount of work. I find they all share the work, and stay the same temperature.

A
 

hippy

Technical Support
Staff member
It's not clear to me how diode-mixed 78XX regulators ( post #14 ) would work to increase the current load. Any regulator giving a lower voltage than another would not supply any current as the diode would be reverse biased.

Perhaps the implemented circuit was more complicated ? Or maybe it relies on a single regulator providing more than rated current which it probably will for a while before it shuts itself down on thermal overload, then when it 'fails' diode mixing allows another to take over and this cycles around, supply switches between regulators ?

In a product a previous company made with a simple multi-rail supply (+12V, +5V, 0V, -5V, -12V ) and just one 78XX/79XX regulator per rail, some regulators would latch-up and not deliver if rails came up in a certain manner. I don't recall the details or of the fix but it did involve diodes. I would therefore be expecting similar issues with paralleled regulators but I could be wrong.
 

BeanieBots

Moderator
@Hippy, that can happen if the regulator ever sees a higher voltage on it's output than its input. That can often happen in multi-rail supplies when powered up with the loads connected. You will often see diodes between input and output to help prevent it.

@Andrew, you may have "got away with it", but it cannot be gauranteed to work.
As hippy so rightly points out, if it was an acceptable method, the app notes would show you how to do it.

That's the difference between a "Hobby botch using what I've got" and a "proper design". Do it, try it, but expect tears at some point.

TIP:
Read the datasheet. What the regulator output impedance?
What is the regulator voltage tollerance?
What are the consequences?

I drove down the road the other day without my seat-belt on and "got away with it". Does that make it an OK thing to do?
 
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