# Combine two 12V power supplies for 24V

#### Fred3000

Joined Sep 16, 2020
13
I bought a 12V 10A power supply for a MIDI solenoid percussion project.

I was thinking experimenting with 24V for a few larger solenoids. The 12V power supply doesn't have a fan, but the 24V versions I've looked at do have fans. Since this needs to be quiet for use in a recording studio, I was wondering if I could buy a second 12V unit and combine them for 24V. The 24V will go to mosfets powering the solenoids.

I've watched a few videos and read up on this but want to check out the right way to do this.

Can someone please advise me on the right way to wire two of these power supplies to supply 24V? Thanks

#### MrChips

Joined Oct 2, 2009
26,790
The key question here is: "Is the power supply floating?"

If the power supply units (PSU) are floating, i.e. isolated from earth ground, then yes, you can wire them in series. -V of PSU #1 to +V of PSU #2.

#### Fred3000

Joined Sep 16, 2020
13
Thanks panic and MrChips.

It's good to know there's fanless 24v units, but the Rhino one lists for $120 and I'm just in the testing stage. It could be that 12v alone is enough for what I'm doing. The 12v supply I have was$15.95 on eBay, I don't want to shell out too much right now.

The power supply I have is plugged into a standard house outlet so the ground is not floating.

#### AlbertHall

Joined Jun 4, 2014
12,116
The power supply I have is plugged into a standard house outlet so the ground is not floating.
It probably is floating. Check for continuity between the two output terminals and the earth terminal. If there is no continuity then it is OK to connect them in series.
BUT you should add a reverse diode across each output, at least 10A rating, to avoid one supply backfeeding the other under fault conditions.

#### Fred3000

Joined Sep 16, 2020
13
Thanks AlbertHall. With the unit unplugged there is no continuity between any of the 4 output screw terminals and the white (neutral) or green (ground) wire terminals of the 120vac ac input cable.

A quick search for "10A diode" came up with some "10A10 1000V rectifier Diodes 10A" is that what I should be using across each output?

And I was planning on using something like Schottky 1N5821 diodes across the solenoid wires, would the 10A diodes across the power supply's output make those not needed? The solenoids I'm using are mostly 1A but I do have one that is advertised as being 8A but who knows if my intermittent use will come anywhere close to that.

Thanks

#### Fred3000

Joined Sep 16, 2020
13
Thanks AlbertHall. With the unit unplugged there is no continuity between any of the 4 output screw terminals and the white (neutral) or green (ground) wire terminals of the 120vac ac input cable.

A quick search for "10A diode" came up with some "10A10 1000V rectifier Diodes 10A" is that what I should be using across each output?

And I was planning on using something like Schottky 1N5821 diodes across the solenoid wires, would the 10A diodes across the power supply's output make those not needed? The solenoids I'm using are mostly 1A but I do have one that is advertised as being 8A but who knows if my intermittent use will come anywhere close to that.

If it matters, I'm using an Arduino Leonardo to change a MIDI signal into solenoid triggers. The Arduino will send a trigger signal to an IRF52O mosfet module that the solenoid and power supply are hooked up to.

Thanks

Joined Jul 18, 2013
25,626
The power supply I have is plugged into a standard house outlet so the ground is not floating.
The ground is for the metallic chassis and case, all the ones I have come across the SMPS is isolated and earth is not referenced to the DC side.
Max.

#### AlbertHall

Joined Jun 4, 2014
12,116
And I was planning on using something like Schottky 1N5821 diodes across the solenoid wires, would the 10A diodes across the power supply's output make those not needed?
Use all of them. They provide protection for different things.

#### Fred3000

Joined Sep 16, 2020
13
Thanks Max and AlbertHall.

Does this look ok?:

The power supplies are 12VDC 10A and each will have a 3-wire power cable plugged into a 120V outlet. The solenoid is rated at 8A.

I found 10 Pieces Schottky Barrier Diode 15A for $7.95 online so could I use them for all three diodes? Should the diode on the solenoid be placed as close to the solenoid as possible or does it not matter? I'm paying more to get the parts from US sellers so I'll get them within a week. Thanks! #### AlbertHall Joined Jun 4, 2014 12,116 Yes, but presumably there switch of some sort between the power supplies and the solenoid? Thread Starter #### Fred3000 Joined Sep 16, 2020 13 Yes, but presumably there switch of some sort between the power supplies and the solenoid? Yes, I simplified it, sorry about that. The combined outputs of the two power supplies go to an IRF520 mosfet module like this: The mosfet will send the 24V to the solenoid when it receives a trigger from an Arduino Leonardo. The Arduino is processing MIDI data that is coming from my MacBook. Thanks #### AlbertHall Joined Jun 4, 2014 12,116 The IRF520 is rated at only 9.2A and that would be with a *&^%$£ big heatsink. You're gonna need a bigger FET than that.

Joined Jul 18, 2013
25,626
Thanks Max and AlbertHall.
Should the diode on the solenoid be placed as close to the solenoid as possible or does it not matter?
Yes, preferably directly across the coil !.
Max.

#### Fred3000

Joined Sep 16, 2020
13
Thanks for your replies. I will be using the largest heatsinks I can fit on the mosfets.

I just looked up the an IRF520 schematic and there is a 1N4004 diode across the output voltages. The schematic I found stated a max 2A load, even worse.

Apart from my one 8A (or so claimed) solenoid, the other solenoids I'm using are the $20 Large Adafruit 1A units. I might not use the 8A solenoid especially if it requires too much special consideration. It's not that much better than the 1A solenoids. I know the IRF520's are not revered by those in the know. I'm using them because I started this project by copying a similar project and that was the mosfet used. It does seem to work fine in this particular use so I'm not sure why I shouldn't keep using them or if there would be an advantage to a different choice. I like the convenience of a module for this project. Should I use another diode right across the solenoid or is the one in the IRF520 sufficient? I'm guessing the problem is that the wire stores some current and that's why you want the diode right at the solenoid. #### AlbertHall Joined Jun 4, 2014 12,116 The IRF520 might be OK at 1A, but check how hot they get. If you have any significant length of wire to the solenoid then I would want a diode right at the solenoid. Thread Starter #### Fred3000 Joined Sep 16, 2020 13 Thanks. I'm trying to keep the wires from the mosfets to the solenoids down to 4ft max. There's an adjustment screw on the power supply... if I'm just running one power supply, not combining two, is it ok to max out the voltage, I think it might put out 12.5V or so. Is it ok to run it that way all the time, it won't lessen the life of it? The reason I ask is because it would be the simplest if I could just stay with 12V and .5 V more might make a difference. #### AlbertHall Joined Jun 4, 2014 12,116 If it is adjustable to 12.5V then that should be fine. Thread Starter #### Fred3000 Joined Sep 16, 2020 13 Thanks, I really appreciate everybody's help on this. Thread Starter #### Fred3000 Joined Sep 16, 2020 13 The IRF520 is rated at only 9.2A and that would be with a *&^%$£ big heatsink. You're gonna need a bigger FET than that.
There's a mosfet module sometimes described as 15A 400W Mosfet AOD4184A. I found a video showing an Arduino controlling a motor. Could I use that in place of an IRL520 module for my 8A solenoid and is there any reason I couldn't use them for all the solenoids and not use IRL520 modules at all?
Would they likely need heatsinks?

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