Using 7805s in parallel

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Teedoubleudee
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Using 7805s in parallel

Postby Teedoubleudee » Tue Mar 28, 2017 3:33 pm

I have got an ALTAI P001C bench power supply unit that can deliver up to 3-5 amps at 13.8volts DC. I want to drive various logic circuits around the layout at 5v DC using L7805 regulators. Because the 7805 can only deliver a max. current of 1.5 amps I was thinking of connecting 3x 7805 to the 13.8 v output and spread the load between the circuits. However it occurred to me that it might be possible to connect the outputs of the regulators together (ie in parallel) to provide a combined max. current of 4.5 amps. Is this possible or should I stick with the original idea of separate outputs?

Also, I already use some of the small T022 heat sinks but should I get myself 1 big heat sink to bolt them all on to?

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Bufferstop
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Re: Using 7805s in parallel

Postby Bufferstop » Tue Mar 28, 2017 4:45 pm

Before you think of connecting them together what are you going to do with the 75% (assuming my internal calculator still works) or so of each ones output that will have to be dissipated as heat. Better to look for a closer match to the required voltage.
You are looking at a 5volt requirement in the sort of range delivered by a PC's power supply. In fact an old PC case with its PSU in place might make a good start meeting your needs.
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TimberSurf
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Re: Using 7805s in parallel

Postby TimberSurf » Tue Mar 28, 2017 5:11 pm

As Bufferstop says, inefficiency of loosing all that voltage will use more current than you think! I am not sure, but I think they might fight each other (one will take the load and others will limit to nothing). Normal design is to use one LM7805 and feed to an emitter follower transistor (say 2N3055) as a larger amp carrier. That's what your Altai PSU uses! Either buy a couple of 5v 2A wall plugs and keep supplies separate or one big one and fuse and separate smaller circuits. Or if you must use the Altai, look up step-down Buck Converter, they are cheap and 95% efficient!
Why use 5V? I have a common layout supply of 12v (10A open frame PSU feeds 10 x 1A fuses)
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Suzie
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Re: Using 7805s in parallel

Postby Suzie » Tue Mar 28, 2017 6:59 pm

40W of heat is a lot to lose somewhere, if you need that much current it is well worth thinking about a modern switching PSU rather than a lot of work.

If you can get by with a mere 4A Rapid have this:-

https://www.rapidonline.com/ideal-power ... 30w-564995

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It will probably cost less than the parts you need to make a linear PSU, and the electricity saving will pay for it in no time.

Suzie x

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Teedoubleudee
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Re: Using 7805s in parallel

Postby Teedoubleudee » Tue Mar 28, 2017 7:16 pm

OK guys, thanks for your inputs. I just happened to have that Altai still in it's box doing nothing (can't even remember what I bought it for!) so thought I'll get some use out of it. Not to worry, like you say there are other options.

Why 5v? Because I use a lot of microprocessors, TTL logic and servos. I don't have a spare PC psu.

I currently have a 9v bus running round the layout providing the lighting. So far I've been using some of that and dropping it down with 7805s. I also have a need to drive 3volt and 12volt motors. My layout is DCC so that's another bus but I'm reserving that for track and DCC accessories. I've seen 5v 5A PSUs on ebay for £7.99 delivered (from UK) so I'll get one of them. Thanks again, appreciate your input.

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Re: Using 7805s in parallel

Postby bnb » Tue Mar 28, 2017 8:14 pm

Why not use a by pass transistor to increase the current out of the 7805. There are a number of circuits on the web and basically all they need are a few resistors, couple of capacitors and a power transistor.
bnb
increasing regulator current.jpg
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Suzie
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Re: Using 7805s in parallel

Postby Suzie » Wed Mar 29, 2017 8:50 am

The bypass transistor will work (it is probably what is in the existing power supply, just with a 7812 instead of a 7805!) but will need a big heatsink that will cost more than an efficient PSU.

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Teedoubleudee
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Re: Using 7805s in parallel

Postby Teedoubleudee » Wed Mar 29, 2017 9:16 am

I agree it's a humungus heat sink on the back of the Altai. So decided to put it away for another day and get one of http://www.ebay.co.uk/itm/DC-5V-12V-24V-36V-48V-Universal-Regulated-Switching-Power-Supply-LED-CCTV-PSU-/151886747221?var=&hash=item235d27d655:m:mQZovYXghnw8y48gBQEuuLw

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TimberSurf
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Re: Using 7805s in parallel

Postby TimberSurf » Wed Mar 29, 2017 10:25 am

Perfic!

You didn't say what amperage?

Don't forget to divide the circuits and fuse appropriately, else your wiring needs to be capable of the full PSU amperage and all devices will draw it, on short circuit!

Here is mine
10 Way  fused 12v 10A splitter.jpg
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Teedoubleudee
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Re: Using 7805s in parallel

Postby Teedoubleudee » Wed Mar 29, 2017 2:14 pm

Hi Timbersurf, Was thinking of getting the 5v 5A as this was what the Altai was capable of.

I like your splitter box, is that home made or available to buy (just had a look and couldn't see anything like it on eBay)? I guess I could make do with a 5x 1amp output.

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TimberSurf
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Re: Using 7805s in parallel

Postby TimberSurf » Wed Mar 29, 2017 6:15 pm

Home made, just a bit of Veroboard with 20mm fuse holders and an LED and resistor fed from a common bus. LED goes out if the fuse blows! Fancy 20 way connector with 2 way plugs gives me lots of swapabilty!
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Teedoubleudee
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Re: Using 7805s in parallel

Postby Teedoubleudee » Wed Mar 29, 2017 6:59 pm

I like it. Hope you don't mind me copying. I've ordered the psu, some fuse holders and fuses already got the rest. Thanks again.

Gordon H
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Re: Using 7805s in parallel

Postby Gordon H » Wed Mar 29, 2017 11:07 pm

Teedoubleudee wrote:Was thinking of getting the 5v 5A as this was what the Altai was capable of.

If you intend to continue with this approach, it would be easier to use an LM338T instead of multiple 7805s as it provides up to 5A @5V in a single TO-220 device. However, the requirement for extensive heatsinking still applies.

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Teedoubleudee
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Re: Using 7805s in parallel

Postby Teedoubleudee » Thu Mar 30, 2017 7:27 am

Gordon H wrote:
Teedoubleudee wrote:Was thinking of getting the 5v 5A as this was what the Altai was capable of.

If you intend to continue with this approach, it would be easier to use an LM338T instead of multiple 7805s as it provides up to 5A @5V in a single TO-220 device. However, the requirement for extensive heatsinking still applies.

Hi Gordon, as per my previous post I have already made purchases to follow the same approach as Timbersurf and ditch the Altai.

Though one thought has just occurred to me: Does the Altai employ a voltage regulator IC which could be swapped for a 5volt one? Everything else is there including the power transistor and huge heat sink.

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TimberSurf
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Re: Using 7805s in parallel

Postby TimberSurf » Thu Mar 30, 2017 7:55 am

Even if the Altai uses a variable voltage regulator, there is still the issue of dissipating the excessive voltage as the design will be based on a few volts (16Vin - 13.8v = 2.2v drop vs 16Vin - 5v = 11v drop) and a given size of heat sink, 5 times the volt drop will generate 5 times the heat and over temp the output transistor! Just change the transformer for a 6VAC (£20) and it will work! No? OK, stick with the Plan A at £7, then :D .
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