Introduction: 10VAC Adapter for LEGO RCX Mindstorms

By Technisch Speelgoed
About: I'm a social-worker, working with 12 - 23 year-olds. I used to be a printer. In 2018 I opened a small makerspace (www.imdib.nl) in my house, where I have lasercutters, 3d-printers, Arduino's, Mindstorms and ot…

LEGO used 9-12V AC power for some old mindstorms parts like:

  • RCX 1.0 smart brick
  • NXT 1.0 LiPo battery (the dark grey one)

I bought a second hand RCX set and an old model NXT LiPo battery. Both need 9-12V AC power. The problem is that AC/AC adapters are impossible to get. Everything uses AC/DC adapters. Even LEGO does't sell this part anymore.

As far as I know, a AC power supply is just the same as a DC supply without the rectifier. The output voltage might also differ a bit. (I am not a specialist so please correct me where I'm wrong)

Time to upgrade an adapter!

(Warning!!! working with electrical stuff is dangerous! Only do this when you are capable to do this safe! This hack might damage your adapter and LEGO parts, so only try this for your own risk when you have no other options)


You will need:

  • an AC/DC adapter
  • soldering materials
  • screwdriver
  • multimeter

Step 1: Select Your AC/DC Adapter to Hack

When you remove the rectifier from a DC adapter, the output voltage will probable become a little higher in AC than what it was in DC, so I selected a 9V DC adapter that also has a plug that will fit my LEGO parts.

It is also easiest to choose an adapter that is closed with screws. Some are glued closed. That would make it very difficult to open it without damage.

Be very careful with the next step!!!

  • Open the adapter
  • Get a power cable that is NOT plugged in!
  • Put the open adapter in the power cable that is still NOT plugged in!
  • Make sure that everything is plugged in safely and you don't need to touch it again.
  • Only now plug in the power. The adapter is life now so don't touch it!!!
  • Put your multimeter in the AC mode.
  • Measure the AC voltage on the adapter before the rectifier. (see picture) Be careful not to touch the adapter!
  • If the voltage somewhere in between 9 to 12 volt AC, than you are good.
  • Unplug the power.
  • Only when you are sure that the power is unplugged, you can take the adapter out.

Step 2: Make the Change

The change is very easy.

  • Desolder the cable from the pcb.
  • Solder the cable directly on the spool. Make sure that you do this on the low voltage side.
  • Close the adapter again.
  • Plug the adapter in.
  • Measure the output voltage.

Step 3: The Scary Part

Now comes the scary part of trying this on your expensive mindstorms parts.

  • Plug in your AC mindstorms part and hope it turns on.

Succes!

(Be careful! Newer LEGO mindstorms needs DC power!)

Step 4: To Finish It

You now have an AC adapter that says DC on the label. This is an accident waiting to happen!

  • So the last step is relabeling your adapter.

Make it clear for everyone that this is an AC/AC power supply and not suitable for anything else but your OLD mindstorms sets.

I'm not an electronics specialist so if I did something wrong, please teach me. For me this worked great.