Portable Power Supply
Recently I acquired a second-hand copy of the RSGB Publication, Practical Projects, Edited by Dr George Brown, M5ACN - First published in 2002.
Within the pages, I found this project.
As I had been given a large 12 volt sealed lead-acid battery, from my brother in law, who had just bought new ones for his mobility scooter, I already had the most expensive part.
I thought I would see what other components I could find in my very large junk Box. My search revealed that I had everything needed to proceed. I even had a small Chinese made digital volt/amp meter which could be added to give the battery status.
Like the author of this project, I also enjoy operating portable and am interested in operating QRP. I have a caravan in which I take my holidays so a portable power supply/power pack/UPS would be really useful in that respect also.
I made a box large enough for the battery, toroidal transformer and circuitry. This I wanted to keep as compact as possible and yet have room for the controls and added digital volt/amp meter.
I had a good supply of old computer side panels of an appropriate gage and so set about fabricating the box and lid using this and pop rivets to hold it together.
Some angled aluminium was used to finish off the sides of the lid and a piece of 6mm MDF board inside to strengthen where the handle and controls would be placed.
A sturdy aluminium door handle obtained at a local boot sale for 50p fitted nicely to the lid of the box, giving me a strong means of carrying the unit.
Placement of the 2N3055 power transistor took advantage of the aluminium angle strip on the side of the lid in addition to the cooling offered by a small heatsink I had on hand.
It was hoped that this would prove more than adequate to dissipate any heat generated from float charging the battery.
After marking out where I intended the digital meter to go, I drilled a series of holes as close as possible inside the outline I had marked out.
It was a simple task using small tin snips to cut the small linking bits of metal between the holes.
Then with a file smoothed out the jagged edges up to the outline while constantly checking the meter for a good fit.
All fittings, switches, fuses, sockets and additional parts installed into the box and ready to be wired up.
SAFETY FIRST
All mains wiring was enclosed with thick plastic shielding to prevent accidental touching of the mains voltage. The case was also earthed with safety in mind.
The above diagram shows what changes I made. Switch S1a & S1b had a built in Neon bulb which caused the switch to glow red when the power was turned on, therefore I felt I could leave out the parts of the circuit shown in green. If I needed to know whether the rest of the charger was active, all I needed to do was loosen F2 and check the meter for output. This saved me having to drill three more holes to fit LEDs I could manage without.
As I already had the meter, I felt it was a worthwhile addition, giving me a clear indication of the batteries status and current being drawn by the equipment.
A strip of tag board was secured to the MDF board on the underside of the lid, with a small piece of Veroboard to hold the LM723 IC. Due to my impatience to complete this project, the wiring was perhaps, not as neat as it could have been, however, all worked satisfactorily and the finished project looked good.
Above: The original circuit. Below: The finished unit with the power plug and glowing power switch visible on the back.
