Condensing number of adapters

Thread Starter

mike_canada

Joined Feb 21, 2020
56
I currently made a few identical circuits and they all successfully work on a 6VDC 900mA DC unregulated adapter. I use internal regulation via LM2940 IC since the rest of the parts use 5V. The easy way out is to buy a power adapter for each board, but I was wondering on the easiest way to condense the number of power adapters to 1 for all the boards.

If each board consumed 900mA (as worst case) and I wanted to daisy-chain say 5 boards instead of 2, then would I need an adapter capable of delivering 7 to 35V (or whatever the maximum LM2940 can handle with no heatsink without overheating), and would that adapter need to be able to supply 4.5A output? or did I get my math wrong?

For reference, I added a picture below to help you understand the setup I have (left) and the setup I'm trying to achieve (right).

wall.png
 

dl324

Joined Mar 30, 2015
14,484
If you use one adapter for 5 devices, you'd connect them all in parallel with the adapter. Current required would be the sum of the maximum current for each device.
 
You are thinking correctly with the 4.5A, but may want to go a little over to add a little wiggle room.

You could also use something like a VX07805-1000 switch mode instead of a linear regulator and get away with a higher voltage, but less current transformer (kind of depends on how the costs work out, and if you can fit them) Say 24V @ 1.5A should be enough, but allow for 70% efficiency if need be. The datasheet claims 93% with 24V input.
 

jpanhalt

Joined Jan 18, 2008
11,088
If each board consumed 900mA (as worst case) and I wanted to daisy-chain say 5 boards instead of 2, then would I need an adapter capable of delivering 7 to 35V (or whatever the maximum LM2940 can handle with no heatsink without overheating), and would that adapter need to be able to supply 4.5A output? or did I get my math wrong?
Do not connect the boards in series. They should be in a parallel. Voltage then stays the same at about 6 to 7 volts which is reduced to 5 V with your regulator. If you want to use only one regulator, it will need to supply the combined current and will probably get hot when supplying about 5 A. I would use a separate regulator on each board.
 

MrChips

Joined Oct 2, 2009
26,143
With the proliferation of low voltage DC powered household electronics one would think that it would make sense to have all new homes wired for 12VDC or 24VDC as well as the usual AC.
 

Tonyr1084

Joined Sep 24, 2015
6,894
If all the devices hooked up to one supply then they all should be the same voltage. Unless each device has its own regulator on it. But assuming all the boards operate on 5V, then you certainly don't multiply the voltage, you multiply the current. If five boards, each using 1/2 amp then you'd need a supply capable of delivering 2 1/2 amps (2.5A or 2500mA)(they're the same thing). And as @geekoftheweek said, you want to give yourself some head room (meaning more power available than what you expect the worst case use to be)
 

dl324

Joined Mar 30, 2015
14,484
All boards each have their own LM2940 regulator in it.
The correct way to power them is to place them all in parallel and use a single 6V unregulated adapter.

Consider this analogy for house wiring. If you have 5 light fixtures on the same circuit, each containing one bulb, you wouldn't wire them all in series and need 5 times line voltage to light them. You'd place them all in parallel across the line.
 

Thread Starter

mike_canada

Joined Feb 21, 2020
56
Say I'm powering 5 boards, each requiring 900mA max. So I'd need 4.5A at 6v. Sadly, I can't find such an adapter. Would an 18V adapter at 1.5A work as well if I added some sort of heat sinking to the regulators?
 

Tonyr1084

Joined Sep 24, 2015
6,894
Say I'm powering 5 boards, each requiring 900mA max. So I'd need 4.5A at 6v. Sadly, I can't find such an adapter. Would an 18V adapter at 1.5A work as well if I added some sort of heat sinking to the regulators?
ABSOLUTELY NOT! If the boards run on 6 volts, then get or build a 6 volt power supply with sufficient amperage to run all the boards at once.

And no, if you're thinking about putting the loads in series - that's a non-starter as well. One board will draw all the power and the voltage will kill it. Then the next most demanding board will take all the current and again, the voltage will kill it. From here the cascade only accelerates to approaching the speed of light. The third board will fail; then the fourth, then the last.

If boards run on different voltages, choose the highest voltage board and get or build a power supply for that board. Other boards that require a lower voltage can have an on-board regulator. But the power supply over all must have sufficient amperage to run the whole show. Always engineer for 133% to 150% of what's required. If we're talking very low amperages then go for 200% (Twice the capabilities over the needed capacity (amperage)).
 
Say I'm powering 5 boards, each requiring 900mA max. So I'd need 4.5A at 6v. Sadly, I can't find such an adapter. Would an 18V adapter at 1.5A work as well if I added some sort of heat sinking to the regulators?
With the LM2904 it won't work out. The reason being if you are powering with 18V and pulling 900mA at 5V the other 13V is dissipated as heat at 900mA. In other words each regulator is actually consuming 900mA at the input voltage (18V in this case).

I had mentioned the VX07805-1000 earlier which would help alleviate the issue since it would actually work out to around 250mA of input at 24V to make 900mA at 5V assuming 75% efficiency. They claim to be better, but I don't know the real world numbers or if my thought process is right... still may be worth taking a look at.

Otherwise you will have to find a 6V 4.5A+ power supply. The lower you keep the input voltage the less heat you will have to deal with.

I currently made a few identical circuits and they all successfully work on a 6VDC 900mA DC unregulated adapter. I use internal regulation via LM2940 IC since the rest of the parts use 5V. The easy way out is to buy a power adapter for each board, but I was wondering on the easiest way to condense the number of power adapters to 1 for all the boards.
I don't mean to be rude, but I would like everyone to re read this paragraph. The key words being identical circuits. I have a feeling the OP knows how to connect the boards and needs no advice there, it states 5V which I may be wrong to assume is the only real voltage being worked with, and just wants ideas on what will work for a power supply. 6V is the minimum for a LM2904, and 26V is the max.
 
Assuming the end goal is purely 5V you could eliminate the regulators and...

Does it have to be a wall adapter or can it be a different type? Digikey lists several 5V 10A power supplies that will need to be enclosed https://www.digikey.com/products/en/power-supplies-external-internal-off-board/ac-dc-converters/133?k=ac+-+dc&k=&pkeyword=ac+-+dc&sv=0&pv1120=72441&sf=1&FV=1525|249171,1989|0,-8|133&quantity=&ColumnSort=1000011&page=1&stock=1&pageSize=25

There are also wall adapters with similar specs.
https://www.digikey.com/products/en/power-supplies-external-internal-off-board/ac-dc-desktop-wall-adapters/130?k=wall+adapter&k=&pkeyword=wall+adapter&sv=0&pv1120=247706&pv1120=266261&pv1120=293724&pv1120=72441&sf=1&FV=-8|130,48|249171,1989|0&quantity=&ColumnSort=0&page=1&stock=1&pageSize=25

Edit...

You could also keep the LM2904 and go with a 7.5V adapter... https://www.digikey.com/products/en...tity=&ColumnSort=0&page=1&stock=1&pageSize=25

Keep in mind I didn't filter for input plug configuration. You'll have to check to make sure you can plug it in to your outlets.
 
Last edited:

dendad

Joined Feb 20, 2016
4,140
I think it would be worth replacing the LM2940 regs with a switch mode one. Something like these...

3A buck converter.jpg
You can get adjustable and fixed versions.

Then just power them all from an old 19V laptop supply as they are generally easily available.

Or even add these set to 6.5V out then feed the LM2940 regs.
 

Thread Starter

mike_canada

Joined Feb 21, 2020
56
Yes I'll be connecting the power to each board in parallel. I knew that but I guess my diagram was unclear. I guess I'll have to order a special adapter then. I figure 6VDC for the adapter is reasonable because the LM2940 uses some voltage (0.5ish) for its processing then the rest of the circuitry runs well from about 4 to 5.5VDC.
 

Thread Starter

mike_canada

Joined Feb 21, 2020
56
I made complete circuits that contain the LM2940 regulators which I don't want to remove. I use a 6V adapter because a 5V adapter will likely make the circuit have insufficient voltage in some places, simply because the LM2940 deducts at least 0.5V for its on regulation purposes, so with a 5V adapter, I'll be lucky if I get 4.5V. Where as if I use a 6V adapter, the regulator will be happier because then 0.5V or more is deducted for its internal regulation and then I can be more guaranteed a 5V output.
 
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