Now we’ll make some changes and additions to “El Cheapo BBU” that will allow us to use rechargeable “NiCad” cells to provide back-up protection and short-term (up to an hour or so) portable use of your ZX81/TS1000 machine. This approach uses your good old T/S wall-plug supply to power the computer WHILE KEEPING THE BATTERIES RECHARGED. This works fine for most machines, at a very reasonable cost. Both my machines have such circuits built-in “under the hood.” My 16K workhorse (my original ZX81) supports MEMOTEXT and CIF as well as the RAM pack, and has been “power-line proof” for well over a year, The other machine supports a 64K RAM and video driver/ reverse board. Both still have about 75 mA. leeway before overtaxing the wall supply. Both are on all the time though they have an ON-OFF switch (for crash recovery). These machines are the “prototypes” for this and future installments of THE CUSTOM T/S, so I’ll be referring to them now and again.
For larger systems (more stuff hanging off the edge connector) you may need to build a larger supply. This is a snap. Virtually every home micro magazine has published power supply circuits. But while you’re at it, why not include battery backup protection and allow portable operation with your supply? I’m amazed that there has been so little coverage of this, since it is a very simple addition, [Since this issue came out, almost EVERY mag has run such an article – I’m not saying we were copied, but we were one of the first to come out with such a circuit – ed.] The technique is by no means limited to the T/S machines, but the discussion here assumes the ZX81/TS1000/TS1500. [See SWN Vol, 2 for modifications for TS2068 -ed.]
So here we go. Finally you’ll be able to thumb your nose at those times when your trusty power line poops out.
RE-CHARGEABLE BBU
Here’s an expansion of “El Cheapo BBU” which uses readily available GE or Radio Shack rechargeable NiCd “AA” cells (rated 1.2-1.3 V. at 500 mA-Hr) to provide a battery backup which can also power your computer portably at minimum cost. In this case, however, the circuit needs at least 9.8 volts at the input to insure that the battery will remain at full charge during normal operation. With the power supply provided with the ZX81 or TS1000, this corresponds to a current demand of up to about 550 mA., or the basic machine plus RAM pack and perhaps one or two low-drain accessory boards. Before you go ahead and buy parts for this circuit, it would be wise to check the power supply voltage with your computer running and loaded with the accessories you’re using; if you get less than 9.8 volts you should consider the UPS option that follows.
Assuming that you get about 10.2 volts, as I do on my machines, this circuit will give you reliable and compact protection, running your computer for about an hour with the AC off and batteries fully charged. This is, in fact, the circuit for my former “BBU-1,” and I might add that as far as I know, none of these have ceased to function. As another side note, if you bought one of our BBUs and it HAS failed, do let me know; I will of course continue to provide support and service even though we’re no longer building them.
The circuit is almost identical to “El Cheapo;” the significant addition is resistor R1, which provides the charging current for the battery (7 NiCd “AA” cells in series). The value shown, limits the current to a safe level, and will fully recharge the battery (with computer off) in 14-16 hrs.
R4, zener diode Z1, and LED2 are an indicator circuit that shows power delivery to the computer. As battery voltage drops, this LED will grow dim, extinguishing near the point where the machine crashes (about 7 volts). Actually this circuit is slightly redundant, since you can tell when the machine is about to crash when the TV picture starts to wash out (lose contrast). So you may omit these if you wish.
The remaining parts (Q1, R2, R3, LED1) are an indicator circuit that shows when voltage is being delivered by the supply. More accurately, it is a “charging” indicator – LED1 only lights when current is flowing through R1 (and the battery is charging). We have to use this slightly more complex circuit as we can’t afford another diode at the input (for voltage reasons) to prevent the battery from lighting LED1 through R1.
Just about any PNP transistor will do for Q1, If you use a silicon transistor, LED1 will go out when battery charge current drops below about 10 mA. With germanium units the LED will be lit down to about 4 mA.
As with “El Cheapo BBU” BE SURE to switch OFF before connecting the output plug to the com puter, as a momentary short could blow D2.
Before using your unit after construction, you should first “cycle” the battery 3-4 times to break it in (when new, NiCd cells frequently show abnormally high terminal voltage). Alternately charge the battery (computer OFF) for 24 hours, then discharge it by powering the computer with AC off until LED2 grows dim or the computer crashes. Then give it a final 24 hour charge, and you’re all set you can now leave the computer on at all times. (Same goes for built-in BBU and UPS described below.)
Parts List
- B1 – 7 Nickel-Cadmium “AA” cells (.5 Ah) wired in series
- C1 – 1000 uF., 16 V., electrolytic capacitor
- D1, D2 – General purpose silicon rectifiers, 50 PIV, 2A (GE 531, R/S 276-1141 or eq.)
- J1 – 1/8″ phone jack
- P1 – 1/8″ phone plug
- LEDI – Red LED, any size/shape
- LED 2 – Green LED, any size/shape
- Q1 – General purpose PNP transistor, 2N4403, 2N3906, ete.
- R1 – 68 ohm, 1/2 W resistor (see note *)
- R2 – 180 ohm, 1/4 W Resistor
- R3 – 1200 ohm, 1/4 W.
- R4 – 240 ohm, 1/4 W.
- SW1 – SPST slide or toggle switch, 1A rating or greater
- Z1 – 1N5228 or 1N5229 or eq. (3.9-4.3 V, vd W. zener)
- Battery holder (if cells do not have solder tabs)
- Enclosure
- Zip cord (for output cable), wire
- Perf board
- R1 Note:
- 82 ohms ~ for .225 Ah. “2/3 AA” cells
- 68 ohms – for .5 Ah. “AA” cells
- 22 ohms – for 1.2 Ah. “sub-C” cells
- Consumer “C” and “D” cells (GE, Radio Shack) cells are “sub-C” (1.2 Ah.) in a larger case.
