# Reducing 24V to 5V for RPi PSU

I'm trying to power several devices off of a breadboard. For simplicity I want to power these all from the same PSU. The highest voltage amongst the devices is 24V. I was looking to use a 24V 2A (maybe 3A) DC PSU to power all of these.

I'm wondering how I could do this? I've seen these and wondering if they would achieve what i want (I realise I might have to build in a regulation circuit for the pi with this)

Can anyone help?

• There are many power supply questions on electronics.stackexchange.com -- you might want to browse them
– TomG
Commented Aug 23, 2014 at 21:43
• I use a UBEC (look on eBay, they are a few pounds). Generally they convert any voltage between 6V-26V to 5V (generally rated for 3-4 amps), E.g. ebay.co.uk/itm/201154872827
– joan
Commented Aug 23, 2014 at 21:47
• The unit you linked is 1 watt and unregulated. Pass on that item. You say the highest voltage among the devices is 24v but beware 24v is not a common DC voltage, it is a common AC voltage tho. For whatever it's worth... I would convert 5v to 3.3, but beyond that I would use the correct REGULATED power supply for the required voltage and simply bond all the negatives. Commented Aug 23, 2014 at 22:57
• One thing to remember, decreasing voltage increases heat. There is no efficient method. Commented Aug 23, 2014 at 23:17

While the suggested 7805 will work, it is also incredibly wasteful - assuming your Pi takes 1A input at 5V, the remaining 19V will be dissipated as heat - 19W of it, which would require quite the heatsink (and likely a fan as well).

Instead, I would suggest using a switching regulator, such as the TI lm2596 instead - it will require a couple more components, but will be much more efficient.

Here is a sample circuit from TI:

This will accept voltages from 7-35 in the input, so despite it showing 12 as the input, it will work fine for 24 as well, provided your Cin can handle 24v (assuming you need it - depending on your power supply, you may not even need the input capacitor).

If you are uncomfortable building it yourself, there are various versions pre-built available for sale on ebay and similar, although they may not be using components that can handle the full 3A that the chip supports.

• Thanks for this. I don't mind building it myself it's just the design of it I'm struggling with (Mechanical Engineer trying to remember his electronics) I've been wondering after thinking about the dissipated heat waste. Would it be better to step the Voltage up? So from 5V to 24V? That way I could use a 5V 2A DC PSU which would be perfect for the RPi, then step up the voltage to 24V for the other devices. Or would this present more problems?
– Tom
Commented Aug 25, 2014 at 15:35
• If you have a 24V power supply and you need the 24V, it would be easier to step down - that is what (newer + efficient) computer power supplies use (they convert the AC to 12V, then step it down to 5V and 3.3V) - also 24V at 1A is quite a bit more power than 5V at 2A, and the higher voltage allows you to get by with thinner wires since they need to carry less current. For comparison, look at the increased complexity from this design Commented Aug 25, 2014 at 15:39
• Ok then, I will need to use the 2A either way (Have tried powering the RPi on a 5V 2A plug, and also the 24V 1A, stepped down to 5V. on the 24V it would start up then reboot, even with the voltage at a constant 5V (monitor through a DMM) so I was thinking of getting a 24V 2A plug, to power it. Or would it be easier to try and build a circuit that would convert the AC? Then using that as an input I could separate it into the 5V 2A and 24V 1A DC from there.
– Tom
Commented Aug 25, 2014 at 15:47
• Something is wrong if the Pi is rebooting with 5V - the DMM may not be able to detect quick drops (that can cause the Pi to reboot) fast enough, can you explain the full circuit? note that if you are using a 24V 1A input and a direct regulator (7805), you also have a hard 1A 5V limit as well. To answer your question about converting the AC, there are "easy" ways to do it, and then there are more complex but efficient ways - the easy way would be to get a large transformer, a diode bridge, and a capacitor - that is what most wall-warts are - how clean do you want your 24V/2A to be? Commented Aug 25, 2014 at 15:58
• I'm currently using one of these Clean enough to not blow everything up preferably. Or I guess I could just use something like this Combined with one of these
– Tom
Commented Aug 25, 2014 at 16:03

Go get yourself a LM7805 (+5V DC regulator) Then read through this datasheet: http://www.fairchildsemi.com/ds/LM/LM7805.pdf and then scroll down to page 18, and look at Fig 6. Add higher valued capacitor(s) on C1, to prevent hysteresis, and (or) various parallel capacitors to C2, to filter out noise.

then read the rest of the application notes for fun. You can also put the Voltage regulators in parallel for increased current (max per package type TO-220 = 1A) yes, you could put a heat sink on that..

Alternatively, 3A (and up) power voltage regulators also exist. such as: LM350 http://www.ti.com/lit/ds/symlink/lm350-n.pdf

A word of advice: if you are powering something at 3 amps and you control circuit can only handle .75A (the PI), Isolate the power between the pi and the higher current sources to protect the PI from higher currents.

with an LM7805, you would be good to 35V.

Alternatively to heat-sinks (or in conjunction with) you may also try outputting PWM power at what ever frequency provides adequate stability in your circuit. The non-constant power output will produce less power and heat over time.

• Thanks for this - just what I'm looking for. I've got 4 devices that I want to run. The Pi (~1.5A), a mister (it's psu is rated at 24V 1A), and a couple of computer fans (both rated between 7 and 12V, not sure of the Ampage). I don't think with these I'd be in danger of damaging the pi except with the excess voltage. Oh and I'm using a PiFace but that is on the Pi so the ampage might be slightly higher.
– Tom
Commented Aug 24, 2014 at 16:00
• I tend to exert caution for all builds. The Pi has little to no circuit protection on board. Fans (inductive loads) can have a fly back effect. Specifics may vary depending on type. The Pi-Face should give you an increased measure of protection. Post back if you have any trouble or concerns.
– j0h
Commented Aug 24, 2014 at 20:19
• Isn't the 7805 a linear regulator? I thought people used switching circuits nowadays.
– joan
Commented Aug 25, 2014 at 7:54
• As stated above, do you think it might be better to step up the Voltage instead? That way the RPi would be protected by both PiFace, and the PSU.
– Tom
Commented Aug 25, 2014 at 15:36
• You may just want to use an old psu from a computer. Plenty of power and many voltages to choose from. I'm not sure about 24v @ 1.5A but there's a neg -12v line. -12 -> +12 = 24v. I power 12v relays, 12v fans, 5v fans and 3.3v PIs off one 230w psu. Commented Aug 26, 2014 at 1:00