Battery Free DRO display

DRO display units for 'x' and 'y' axis are of enormous benefit on the milling machine and are even better when freed of the vagaries of battery power.

It has to be said, my achievements in electronic engineering are non too impressive. My attempt at making a transistor radio in a sandwich box many years ago ended in disaster and all I achieved was the destruction of a perfectly good sandwich box, oh yes and a load of expensive burnt out components.

However, the unit described here not only performs brilliantly well it has been in constant use for 18 months. Credit where credit’s due, the unit was designed by model engineer and miniature boiler maker Sandy Campbell from Scotland. I thank Sandy for designing this unit and allowing me to share it with other model engineers.

The unit came about whilst I was trying to find a method of running the ‘x’ and ‘y’ remote display units on my  milling machine from mains supply rather than the two 1.5V batteries which the units are designed to take. The problem is that batteries often choose the most inconvenient time to expire – invariably in the middle of a tricky machining operation.

click on image for a larger view

An enquiry on HMEM (Home Model Engine Machinist) resulted in an offer of help from SandyC who kindly undertook to design a circuit, which would do the job. The outcome has been most successful and is a very welcome modification on the milling machine. Even when unintentionally left switched on overnight there have been no problems or signs of overheating.

If you are starting from scratch the remote digital display units are available from ArcEuro Trade at £30 each.  The fitting of the ‘x’ and ‘y’ scales is covered in a separate article under Machines & Tooling . You need two units for ‘x’ and ‘y’ displays.The essence of the problem is that you require dual output – 3v supply to power the display and 1.5v to power the vernier scale sensor.

Whilst the circuit schematic, as supplied by Sandy, provides full details of a suitable mains transformer and AC/DC rectification components, the good news is that you can avoid having to tinker with the dangers of mains electricity by using a readily available cheapo AC/DC power adaptor – the type used to run radios, MP3 players and similar gadgetry.  If using one of these units then components F1, L1, D1 can be eliminated from the circuit and, since there is already some amount of output capacitance, the value of C1 can be reduced to between 4700uF and 6800uF.

The DC output of the AC/DC substitute unit should be between 7v and 9v if choosing a fixed voltage unit, or, if a switchable unit is available it should be set within the above range. Do check with a multi-meter as these units often put out a higher voltage than indicated on the unit. The VA rating of the unit  should be at least 3VA.

Although the parts list shows all resistors to be 0.6w metal film types at 1% or better this really is only important for R1-R4 which are the voltage control resistors.

Warning – I must at this stage emphasise that neither Sandy or I, cannot accept any responsibility whatsoever for personal injury, loss of eyebrows or damage to your expensive equipment should you decide to construct one of these units.

One further word of  ‘elf and safety to anyone using these remote DRO display units, regardless of whether you are using batteries or mains derived power supply, it is important to remove the small cell battery in the vernier scale sensor unit.  My mentor, Bogs, advised me to replace the battery in the vernier scale with a .22uF value capacitor  soldered across the battery terminals. This modification immediately stabilised output readings. You do need to allow a minute or so for the capacitor to become charged when switching on.

I purchased all my components from Maplins at a cost of around £20.  Adjustable voltage AC/DC power adaptors are readily available and I found a couple in original packaging at just £4.00 each at a local car boot.

Copper stripboard proved ideal for the build. All the components were slotted loosely into position before soldering on the reverse side. Mini-din plugs and chassis sockets provide neat connections from unit to DRO displays. The final connection using ‘choc-bloc’ connectors which slip conveniently into the battery compartment in the DRO.