Battery Eliminator Circuit

A 5V regulator allows you to power your receiver straight from the 12V or 24V batteries.

The heart of this regulator is a special regulator chip with three legs:- Input, Output and Common

These chips come in all different varieties and the ones we are interested in are the:-

• 5 Volt positive regulator..........7805
• 12 Volt positive regulator........7812

These regulators depending on manufacturer and type have a maximum input voltage of between 20 and 40 Volts. You can buy them readily in the UK from:-

• Maplin Electronics
• Farnell
• Radio Spares

So that our circuit will work with all types of regulator we shall do it in two stages. First regulate down to 12 Volts then regulate that down to 5 Volts.

This has the added bonus of providing the robot with a 12 volt supply as well as eliminating the chance of any interference on the supply rails to the receiver.

If you want to run the circuit from a 12 Volt supply then simply leave the first stage of regulation out.

Note that the capacitors are mounted close to the pins of the regulators. This is to reduce the chances of the regulators oscillating at high frequencies

OK so now to start you will need:-

• Vero Board at least 15x15
• 1x 5 Volt positive regulator and mounting kit..........7805
• 1x 12 Volt positive regulator and mounting kit........7812
• 4x 0.1 microFarad capacitors (50 volts or greater)
• 2x 100 microFarad capacitors (16 volts or greater)
• 5x Vero pins
• Tinned copper wire
• Soldering iron & Solder
• Wire cutters & pliers

First cut the Vero board to size, then clean the underside with a scouring pad. Don't miss this bit out as it makes soldering much easier later on.

File the edges of the board clean, this makes for a much nicer looking job.

Carefully bend the legs of the regulators and fit them to the board.

Solder the regulators in taking care not to leave any solder bridges between the tracks.

Next add the capacitors, tinned copper wire links and Vero pins.

Build up the tracks with solder (Not too much) so that they can handle the current..

The tabs of the two regulators are internally connected to 0 Volts, it is a very good idea to electrically isolate them from the metalwork with a mounting kit (Mica Washer and plastic ferrule), thus reducing interference.

The regulators must be bolted down to a heatsink to prevent them overheating. The heatsink can be the metal box you house your receiver in.

Make the noise rejection coils with a minimum of 10 turns through a ferrite former.

Specifications

The maximum input Voltage depends on the specific chips used


• LM7812CT ............. 27 Volts
• MC7812CT ............. 35 Volts
• UA7812CKC .......... 30 Volts
• L7812CP ................ 35 Volts
• L7812CV ................ 35 Volts
• MC78T12CT .......... 35 Volts

The minimum input voltage is about 15 Volts


• LM7812CT ............. 14.5 Volts
• MC7812CT ............. 15 Volts
• UA7812CKC .......... 14.5 Volts
• L7812CP ................ 15 Volts
• L7812CV ................ 15 Volts
• MC78T12CT .......... 15 Volts

The maximum Current also depends on the chips used


• LM7812CT .............. 1 Amp
• MC7812CT ............. 1 Amp
• UA7812CKC .......... 1.5 Amps
• L7812CP ................ 1 Amp
• L7812CV ................ 1 Amp
• MC78T12CT .......... 3 Amps