Proof of concept and efficiency test

Today I tested whether it was possible to charge a LEGO train from beneath the tracks. At least, I checked whether the receiver coil would be positioned correctly with reference to the transmitter coil. To see whether the power transfer was working I tested with a PNP power transistor dissipating...

Attaching the coil to the train

First of all: the guage of LEGO rails is 37.8mm, and the Wuerth receiver coil is 37mm wide: a match meant to be!!! To get a connection between the RX coil and the TX coil they should be placed within 10mm of each other. The TX coil is placed under the rails, so the thickness of the rails + distance to the bottom of the train should be minimized.

Receiver coil attached to the train. Blue plates are placed to lower the coil to the track.

Transmitter coil under the tracks, I had to remove the plastic covering of the coil to get the receive and transmit coils close enough.

Close-up of transmitter / receiver combination

Testing time!

With the rx coil lowered a bit, and the tx coil close to the rails, a connection between receiver and transmitter could be made. Alignment is quite critical, but since the train rides on rails and the coils can be fixed in the 'end product' I only have to think about how to get the train on the right position on the rails. The 'beep' from the transmitter that indicates the start of a connection is very helpful!

What can we see here?

• Train positioned above transmitter, receiver is active (green LED)
• Load attached (old PNP TO-3 transistor connected to 5V output, potmeter between base and collector controls current drawn from output)
• multimeter shows >700mA output  current, input is 19V / 250mA.

Conclusions

Hooray! The proof of principle is working! Even though the positioning is a bit finnicky, once a connection has been made the full output range can be used. I managed to pull a maximum of around 1200mA from the output, and efficiency was between 75 and 85%. I think I'm going to modify a piece of rails to get the Tx coil closer tot the surface. That should improve efficiency and reliability.