Power Supply Loading and Switching question

[tombaker]

Hi All,
I'm building a 12VDC regulated power supply for my live keyboard rig. I need three regulated outs for a USB hub, headphone amp and audio interface. All of which are rated at 12VDC and 1amp or under.
So the plan is to get a laptop or similar power supply, 15VDC roughly 5amps and run it into this simple circuit. It offers diode protection on the 15VDC input and 3 x 7812 regulated outputs.
I am yet to ensure the polarity of the 12VDC outputs is accurate for each device but I'm aware of that issue.

To give me a little extra peace of mind I was going to build two of them in one box and then attach a switch on the input, in case one of the regulators fails during a gig. Both boards would be connected to the same DC output sockets, that way I'd only need to flick the switch and my gear would be back up and running for the remainder of the show, until I could track down why it may have failed.
Here's a picture of what I'm explaining.



https://www.dropbox.com/s/ugaldxwdvze3yms/Power%20supply%20regulator.png?dl=0

My question is, will connecting two boards to the same DC sockets create extra loading or problems that I have overlooked?
If the unused board isn't getting power will it greatly or destructively effect the other board?

Any help is appreciated.
Thanks in advance.

[MrStab]

while i do use voltage regulators from time to time, i don't have much in-depth knowledge to offer you, sorry bout that. but i DO know of a design concern regarding voltage regulators in parallel: apparently differences in tolerance across each device could cause issues with overheating and the likes, if one winds up working harder than the other. a solution i've read involves simply putting a low-value series resistor on the output of each regulator to account for this. i tried to google something definitive for you to read, but all the results are forum threads and you might find it better for your specific needs to sift through them yourself. maybe there's some preferred method mentioned on the datasheet? hope that's some help and at least goes toward giving you peace of mind!

[MrStab]

attach a switch on the input ... Both boards would be connected to the same DC output sockets

kinda forgot this when looking at your Dropbox link. if it were me, i'd use a DPDT to switch both input & output. or consider a diode or something to prevent current flowing into the other regulator's output. i'll stop talking now because this is pure conjecture. lol

[tombaker]

Thanks man,
I will have a closer read of the data sheet and thanks for the heads up about the parallel regulators. I certainly don't want to create overheating in the design phase otherwise it makes the idea of a central power supply pointless and unreliable.
I'll have a closer read through the threads. I did find something recently posted about regulated vs. unregulated power supplies and loading which is helpful but my main concern is with having two boards connected and the best way to switch between the two.
Thanks again

[mth5044]

I'd imagine a make-before-break switch would be useful here. I believe most of the toggles used around here are break-before-make, so you may have to some searching. When you flick the switch, you will lose power for a moment. Whatever devices that didn't fail would need to restart, and if they have a long start up or need to be reconfigured before use, you could be out the rest of the song. If you used a make before break, they wouldn't see a change in voltage, just a quick spike in amps which shouldn't be too bad, hopefully.

[tombaker]

good call, that would definitely be an advantage.
I always feel the need to have built in failsafe's when i can.

[Unlikekurt]

The first question i'd ask is why the redundancy protection?
My thought is that let's say that something causes the second regulator on pcb strip 1 to give up or short out, when you flip over to pcb 2 what are the odds that the same causal event isn't going to be revisited on the replacement regulator?
If you know in advance that you will be within the specifications of the allowable current for each of the individual regulated outputs, why anticipate them failing.  Linear regulators, at least in my experience, shy of some outside influence, aren't really prone to up and failing.

The other thing i would say is that according to the layout the regulators aren't exactly being run in parallel as suggested in an earlier response.  Yes, they are being supplied by the same supply node, but they aren't being run in parallel.

Also, you might want to slap some heat compound on the regulators and install them on vertical heatsinks.

[PRR]

The LM7812 is remarkably hard to kill, if done right.

Do NOT connect two power outputs (two LM7812 outputs) together directly. In this case it *may* work as long as both regulators are healthy. In other cases they fight each other and both suffer. If one fails, that's sure to impact the other.

I do not see the reverse protection diode. This is in the datasheet and is the *first* thing you should be doing if reliability is important.

The LM7812 is so reliable that the most-likely system failure is an assembly fault. Bad solder joint etc etC. Doubling-up more-than-doubles your opportunity to assemble a future problem into the build.

You still have the single-point-failure of the laptop supply.

What I am thinking: 12V DC *regulated* warts are not expensive. (Try Jameco.com for good ones, though prices vary.) If you have 3 loads, get 5 warts, and a 4-way AC power strip, all in a box. If one fails, you have #4 already powered-up and ready to plug in. If another fails, you plug #5 into AC power and rock-on. If you have more than two failures in one gig, pack up and go home, something is very wrong here.

[MrStab]

The other thing i would say is that according to the layout the regulators aren't exactly being run in parallel as suggested in an earlier response.  Yes, they are being supplied by the same supply node, but they aren't being run in parallel.

i wasn't suggesting that they were - the switch at the input makes that clear - i was nodding to an area where there may be common concerns. i did mention that i was out of my depth and verbalising rectally, though.

[tombaker]

The first question i'd ask is why the redundancy protection?

Redundancy protection in a live computer based midi controller setup is pretty critical to me and the rest of the band, especially on a few songs where I'm running track that feeds the drummer click and the front of house track while I'm also playing softsynths.

My thought is that let's say that something causes the second regulator on pcb strip 1 to give up or short out, when you flip over to pcb 2 what are the odds that the same causal event isn't going to be revisited on the replacement regulator?

I agree but the thinking is that it will hopefully last me until the set ends or the song ends to fail again and give me time to fix the setup.

PRR: That's not a bad idea except for when it comes to touring. I realise that I'd be running it all off of one wart and therein lies a deficiency in my redundancy idea, but it also sounds like quite a bulky setup for touring.

Will a diode with the cathode connected to the positive rail and the anode to ground not protect the input/output (which ever has the diode)? As polarity protection?
I looked at the 78XX datasheet and the 7812 fairchild datasheet but cannot find what you're referring to.

Thanks for the heads up about connecting outputs.

Perhaps a 12VDC wart with ample amperage daisy chained and a backup wart?

I just don't want to have to cart around 5 warts and a power board when the initial intention was to streamline the setup. But that may be the compromise for reliability.

[tombaker]

I should add, for the sake of understanding the situation, that I play guitar and keys in this band.
So I'm currently carting an amp, pedalboard, guitar, keyboard stand, two midi controllers, headphone amp, interface, usb hub, laptop and cables galore.
So I've designed a little briefcase to house my headphone amp, usb hub, interface and (eventually) power needs for the keyboard setup in an effort to reduce setup/packup time and make it easier to tour.

[ashcat_lt]

The steering diodes mentioned above will probably be your safest bet.  They go in series from the regulators to the jacks, basically in place of the wires you've got there now.  Use shottkey's to keep the voltage drop minimal.

[tombaker]

Awesome, thank you.