my simple UPS for Prusa Mini (Build/thougths)  

my simple UPS for Prusa Mini (Build/thougths)

Hi there.

I would like to share with you my simple uniterruptable power supply build. For me, this simple setup is good enough. Maybe I will add some features in the future.

Since I had two 12V Batteries laying around, I decided to give them some purpose in the form of an UPS. The printer runs on 24V which makes it fairly straight forward with 2 car batteries in series.

Basic principle:

batteries and original PSU connect to a circuit board which connects the battery to the output if PSU voltage goes too low (power fail).

How does it work:

Some diodes separate the different power sources and automatically connect the one with higher voltage to the output. Unfortunately, battery voltage is always  above 24 V (if it's healthy). This would connect the battery all the time. To mitigate this problem, a MOSFET switches the battery only if the PSU output is dead. The MOSFET is controlled passively via some carefully chosen resistors.

What are advantages:

• Car batteries have plenty of power. It can power a 3D Printer easily and for quite some time (depending on the battery of course)
• It is not "smart" and made by just some basic components
• really cheap (if you have the batteries..)

What are the drawbacks?:

• Batteries are not automatically charged. DO NOT ATTEMPT TO CHARGE THEM WHILE CONNECTED. DISCONNECT BEFORE.
  • Since this is only for emergencies, I don't consider charging the batteries every now and then a big deal.
• It is not "smart". No battery monitoring etc.
• the circuit introduces some additional losses (diode conduction losses) even when battery is not needed.
  • This is totally negligible with proper diodes
• Some people turn their printer on with a smart plug at the 230V side. This is not compatible with this UPS because the battery would provide power instead.

Schematic:

Choice of components (example parts later):

• Batteries: Lead Acid car batteries. The smaller ones used in most consumer UPS would be a good and cheap alternative as well.
• Diodes: Use Schottky power diodes. This is important to keep losses low. Use a TO220 package (or similiar) for good heat dissipation.
• MOSFET: has to be a p-channel power MOSFET. It should not be a dedicated logic level FET or it will result in unintended behaviour (such as battery always connected or a toasty MOSFET).
• Resistors: doesn't matter, just use the correct values.
• Cables: I used 1,5 mm² which is enough. Wouldn't go lower though. When soldering, do it properly.
• Connectors: You will need the connectors for the PSU and the mini (part numbers later). For connecting battery cables it's up to you, just make sure it's fine with at least 6 A.
• Heatsink: Diodes will get warm at full load. MOSFET might get warm too. Get some heatsinks (small ones work), there are many available for TO220 components. Or do it like I did: bolt it to some aluminium.
  • Keep in mind: if you directly connect the diodes to a heatsink, the heatsink will be at 24V, so it should touch nothing else. The diodes can share the same heatsink because their connectors share the same potential anyway. The MOSFET however needs to be on it's own heatsink or mounted isolated (non conductive heat pad and plastic screw).
• Fuse: I put a fuse in the battery path to be safe (not included in the schematic). I recommend a  6,5 - 10A fuse. It's just in case if for some freak reason the battery shorts. You can directly attach it in the battery cable if you like (automotive inline fuse)

Example parts:

• MOSFET: IRF4905
• Schottky Diodes: SBT1030
• Capacitor: Some Elko rated for 30V or higher, 100uF or more (not that important).
• Connector for Mini and PSU:
  • KPJX-4S and KPPX-4P
  • or
  • MDP-402-4P and MDJ-401-4P-E
  • tip: search for the kycon assembly video for the plug
• Resistors: no special resistors needed, use values from schematic
• LED: optional for indicating when battery is active. Use whatever standard LED you like (I chose red).

Everything assembled:

Battery voltages vs Prusa Mini:

Since fully charged batteries will give you more than 24 V, your printer will be connected to more than the originally intended 24 V. I double checked the schematic and datasheets of all the components involved in the printers electronics. There won't be a problem if you stay below 28V. In practice, the batteries will settle around 25-26 V. Since you are not supposed to charge them in circuit anyway, there is no problem to expect.

When to build this and when not:

This build only makes sense with 2 car batteries (or similiar voltage battery setup). If you want to use only one battery and combine it with a DCDC boost converter most of the circuit becomes useless and could be replaced by a single diode. I chose this way because most cheap DCDC modules are not suited for above 3 A and I had the batteries anyway.

Want to include your RaspberryPi?

Connect a DCDC to the 24V output of the UPS. Set DCDC to 5V output. Connect the positive output via a diode (this can be a smaller shottky diode) to the 5V pin with the cathode facing 5V pin of raspberry. You can leave out the diode if you decide to power the raspberry by this supply all the time.

Safety:

DO NOT ATTEMPT TO CHARGE BATTERIES WHILE CONNECTED. DISCONNECT BEFORE or you will possibly short a battery and fuck shit up.

In theory this circuit is safe if you do everything correctly and don't interfere with it (like charging, see above). The diodes isolate the power sources so there is no accidental charging or backfeeding or whatever. Car batteries can produce gigantic currents so always be wary when working with them (reminder: never work around big batteries wearing a metal ring). Of course I don't take any responsibility for anything bad that might happen.

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Hope someones finds this interesting. If you have questions, don't be shy.