Parallel Power Supply

[Rigolon]

Hi everyone,

I'm working on two automatic sliding doors. My project is to create some new interactions between them and add some other functions. Each door has it's own system and each system provides a power supply to power some peripherals, as the radars and access control. But each is limited to up 500mA.

I added a few more sensors and created a board to make the interactions. Even though it doesn't go up to 500mA with everything installed, it gets really close (around 450mA).

I was thinking of distributing this current over these two "PSU" by using them in parallel, to give me more room to work. I've read that to use PSU in parallel is a bit more complicated, since the ideal case is for them to have the same voltage, which is not always the case with the doors, specially when they are powered on the batteries. And the motive they should be the same voltage it's so the current won't flow from one PSU to another.

If that is the case, I thought I could use a couple of diodes, one after each "PSU" so the current only flows to my board and the peripherals. Would that work?

Edit: Although it's not exactly the same voltage, they are really close. One has about 29V and the other has 30V.

[SebG]

Hi
Using diodes to OR the two supplies still won't split the current load between them. The 30V PSU will supply all the current through the diode which will result in a 0.7V drop (or something) thus 29.3V which is still higher then 29V - 0.7V so all current will come from the 30V PSU unless it's stressed to the point where the voltage is reduced.  One option is to put both PSU in series to create 59V and use a step-down DC/DC converter to regulate the power.  Or you can power half the peripherals using one PSU and the others with the other PSU and keep the Grounds connected together.

If the current supplied is more or less constant then you can balance out the load from both supplies using resistors.  A slightly larger resistor on the 30V PSU and a lower on the 29V PSU. Although if the current varies a lot then the balance will not be equal.

[Rigolon]

Thx SebG,

Unfortunately the current supplied is not very constant, I guess I can change some things so they would be more constant. But I will leave this to the future.

Powering half of the peripherals with each PSU and connecting ground was what I did. I will continue with that, having them in series will just make my project more complex than it needs to be.

For those who comes to this post with some curiosity, I also found this article: https://www.digikey.com/en/articles/techzone/2016/sep/properly-configure-parallel-power-supplies

[MrAl]

Hi everyone,

I'm working on two automatic sliding doors. My project is to create some new interactions between them and add some other functions. Each door has it's own system and each system provides a power supply to power some peripherals, as the radars and access control. But each is limited to up 500mA.

I added a few more sensors and created a board to make the interactions. Even though it doesn't go up to 500mA with everything installed, it gets really close (around 450mA).

I was thinking of distributing this current over these two "PSU" by using them in parallel, to give me more room to work. I've read that to use PSU in parallel is a bit more complicated, since the ideal case is for them to have the same voltage, which is not always the case with the doors, specially when they are powered on the batteries. And the motive they should be the same voltage it's so the current won't flow from one PSU to another.

If that is the case, I thought I could use a couple of diodes, one after each "PSU" so the current only flows to my board and the peripherals. Would that work?

Edit: Although it's not exactly the same voltage, they are really close. One has about 29V and the other has 30V.

Hello there,

Placing two power supplies in parallel is not that simple as i think you are finding out.

The voltage has to be the same, and the internal impedance does too.  If one of these is not met, one power supply supplies more current than the other.

To get this to happen with two similar power supplies, sometimes a small series resistor is added to each power supply so as to try to balance the impedances of the two so they both put out almost the same current into a single load.  The power supplies have to be similar though, and have t he same voltage.  If the voltages are not the same, this is also going to cause a problem unless the two just happen to load in a way that allows them both to supply half of the total power.  That takes a bit of luck and we dont usually depend on that.

Taking a step back though, we might note that the current is only 500ma per system.  That's very low in the power supply realm.  You can get 1 amp wall warts that are regulated for less than 10 dollars these days and they are fairly well regulated.  If this was 500 amps or even 50 amps or geeze even just 5 amps it would be a whole different story, but with 500ma load current per system, you can easily obtain two wall warts that would easily handle both of them.  You can even get 2 amp regulated wall warts that can do both systems.

Did you happen to mention the voltage yet?
Common wall wart voltages are 5v, 6v, 9v, 12v, 15v, 18v, 19v, 20v, 22v, 24v, but other voltages are possible too.  Take a look on Amazon, they even have wall warts that have a little switch on them you can change to get one of those voltages above, and can change the switch to get another voltage.  Current level varies with the model yhou purchase, from 1 amp up to maybe 5 amps.  Prices maybe 8 dollars to 20 dollars USD.

So that's probably your best bet.

[Rigolon]

MrAl,

Thanks for the reply.

My peripherals work with 24V, the wall wart is a great idea. I guess I will go with that if I really need to go further than the 500mA.
But my first idea was to use these systems, mainly because it's easier to use them for several reasons. Not only that, but my project uses the ground from these "PSU" since the commands to open or block the doors (for example) depends on those grounds. Basically my board is a programmed circuit that use some relays when certain conditions happens.

And since I have to get the grounds, it's easier to also connect the positive on the board and concentrate all the wires on a single place. Well, easier as long as I don't need more than 500mA. 

[gigabyte091]

Why don't you use one of those open frame power supply's, like meanwell ?

You can get something like 24V 3A PSU for 10 or 15 dollars.

[RobBarter]

You may want to have a look at this site.  Although at higher current, and voltages, he is doing basically what you are looking for.

http://www.w6pql.com/a_big_power_supply.htm

[Rigolon]

Why don't you use one of those open frame power supply's, like meanwell ?

I already have 2 automatic doors installed, like those we find in supermarkets or shopping malls. So each of them has it's own system to run the automation and each has a supply to connect the radars and/or access control devices. Each door has a pair of radars (inside and outside) that consumes less than 100mA, so I have 400mA on each system to use, a total of 800mA.

In my project I'm creating new functions that these doors don't have, e.g: When door 1 is open the second stay closed or knowing which door opened and the direction the person is going, and a few more things using other sensors and devices. For now i added a consume of 350mA, wich I can distribute between these two systems.

Since those supplies are already there is way easier and logical (for me) to use them than having to buy/use other power supply.

What I didn't know was that using parallel power supplies is that much complex, and that was my initial question. Because in my mind the major problem was that current would flow from one supply to another so using diodes would prevent that. But now I know that there are way more things that happens when using them in parallel.

[Rigolon]

RobBarter,

This article is actually using schottky diodes after the psu as I intended.

A pair of 30 amp 12v supplies provide the base output (13.8v @ 60 amps), and have their currents summed with high current schottky steering diodes to keep them from bothering one another. The diodes also do a good job of providing current sharing, and the returns of the two units are tied together and routed to chassis ground through a current metering shunt, where the total current from all outputs is measured.

So I was thinking on using 27V zener diodes with 1% tolerance to get my voltages close enough to each other. I will try this as soon as possible and see if it works.

[Seekonk]

You could use diodes in place of the dropping resistor. Mix and match the .6V and 25 drop to get close. That would get you close enough to share some.  Likely these are not regulated and internal resistance will do the finishing balance.  I don't think it is that critical.  I parallel several buck converters to get more current with no problem.