Onward to replacing our good old wall-plug power supply with something more substantial. You have several ways to go, and I’ll try to touch on some of them here. BE CAREFUL as in this project we’ll be dealing with “raw electricity.” Not to scare you off, but “follow the rules” about insulating connections, observing polarities, and just generally not being careless. Don’t blame me if your “smoke test” actually fills the room with nasty PVC smoke. The stuff we’re doing here is really so simple that I can’t foresee any major problems, but if you are having trouble, do write or call and I’ll do what I can to get you back on the rails.
Perhaps the simplest approach is to get a 12V car battery and a trickle-charger to go with it (1 or 2 amp. jobbies, I recently got a 2 amp. “MityMite” from a discount auto parts for $14.97). Oh, yeah, you say. Then put a string of diodes in series to drop the voltage to the computer. While this works, it is DEFINITELY NOT (!!) RECOMMENDED. The first time you short the output you’ll have a string of “fry-odes” (short circuits) instead, and the system sees over 13 volts, causing regulator overheating and possible RAM failure; goodbye TS! The alternative is almost as simple, and costs about a buck more than 6 good diodes. This is a miracle device called the fixed 3-terminal regulator. They are available in a “7800” series (7805 – 5 volts, 7808 – 8 volts, etc.) which provide about an ampere of regulated output, and in an “LM340T” series (LM340T5 – 5V, etc.) which provide up to about 1.5 Amp. Both are in the popular “power tab” package, but LM340K is a TO-3 equivalent (diamond-shaped two hole package) also available. An similar adjustable unit is the LM317, another is the LM350 which allows up to 3 amps. These are all accurate and dependable, best of all they’re protected against excessive current AND temperature, so are output short proof. Virtually indestructible. Such a device (7805) provides the regulated 5V inside your machine. As a side-note, many borderline machines benefit by substituting an LM340T5 for the 7805. If we preregulate the 9V in, we’ll have “double protection” against crash-producing glitches.
How these are used will be clear by the time we’re through, but first let’s consider some other alternatives. You could replace the car battery with a motorcycle battery, or better yet a 12V “gel-cell” of 4 – 12 amp-hr rating. Add a charge limit resistor, filter capacitor, a couple diodes and indicators, and you have a UPS. The circuit shown is a general one, usable with transformers of 12 VAC 13 amp (or 25.2 VAC center-tapped as in most car battery chargers. Takes two less diodes. See diagram.)
Another side-note about car-battery chargers; they usually have a circuit breaker to prevent burnout due to shorts, and already come with two hefty diodes (4 in untapped 12.6 VAC units) and a case. There’s even room for ten “sub-C” Nickel Cadmium’s for protection up to an hour (at 1 amp. – up to 2 hours for basic machines + RAM.)
Or you may prefer to start from your choice of transformer. In choosing a transformer, you trade off between weight/size/cost and power. If you’re powering a machine + RAM only, 12.6 VAC 1A is more than enough. To be on the safe side choose 12.6 VAC 2A. If you’re REALLY power hungry get a 4A unit and use it with an LM317K or LM350. You will also have to choose diodes and regulator to handle the current.
But a graphic is worth 0400h words, so let’s look over the schematic and other scratchings. There are four main elements; transformer/ rectifier, filter (C1), battery charge/switching (R1, D5-7) and regulator, The first section might be a car battery charger, the filter capacitor should be 2000 uFd. or so for .8 A. output, 4700 uFd. for 1.2 A or more. The idea is to get about 17 (no less than 16) volts raw DC at TP1 under full load, increasing C1 might bring it up to spec, if not you’re overtaxing the transformer or have a baddie.
The charging circuit you’ve already seen. The D5/R1 combo is for NiCd cells; for small sealed lead acid (gel-cell) add a 15V, 5W or up zener diode between TP2 and ground (battery minus). This prevents overcharge damage with this type of battery.
That leaves the regulator, which is the easiest part. If you’re running from a car battery, this and the switch is all you need; use the charger as-is to keep the battery at full charge (battery itself provides ripple filtration, no need for C1). Schematic tells all. Small capacitors are recommended at input & output, .1 – 1 uFd. disc ceramic or tantalum. Note the alternate circuit most machines work fine (and with less internal heating) at 8V, but some RAMs are touchy about this so you may want to trim it up to 9.5 volts or so. Experiment with R5 to get the voltage you want.
