Appendix A
71
Power usage
The SPECTRA interface uses a sizeable amount of current, but the Spectrum power
supply is more than capable of handling this. However, if other peripherals are also
attached then care must be taken to ensure that the power supply’s current rating is
not exceeded.
The Spectrum power supply outputs a nominal 9V
dc
and is capable of delivering 1.4A.
The table below shows typical current usage for the SPECTRA interface and the range
of official Spectrum peripherals when in operation. To find the total current usage,
simply sum up the figures for the appropriate devices. The result should be less than
1.4A to ensure safe and reliable operation. Note that the current used by a device can
vary based upon its state, and so the figures shown may actually be lower than the
peak values and should therefore only be used as a guide. The Spectrum power
supply will generate heat when in use, the temperature of which will be related to the
amount of current drawn from it, and if more current is drawn than the unit is specified
to handle then it could heat up to a level where it becomes a fire hazard. Should the
summed current usage be close to the 1.4A rating of the power supply, it may be wise
to disconnect a peripheral or replace the power supply with a higher rated unit.
Typical current usage of Sinclair devices
The SPECTRA interface generates its own 5V supply rail from the raw 9V input from the
Spectrum power supply. This is done to reduce the loading placed on the Spectrum’s
internal 5V regulator and therefore to avoid additional heat being generated inside the
computer. However, the optional onboard ROM and ROM cartridge socket facilities are
powered from the Spectrum’s 5V supply rail. This is done to ensure that the current
draw of the SPECTRA interface remains reasonably constant irrespective of which
options are fitted. A DC-DC converter is used by the SPECTRA interface to generate its
5V supply rail, and this generates far less heat than the equivalent linear voltage
regulator (as used inside the Spectrum). By keeping the current draw restricted and
relatively constant the need for a heatsink is eliminated, and this helps keep the board
size to a minimum. Note that the DC-DC converter may still get hot and so care should
be taken not to touch it, or to prevent air circulation around it. The CPLD logic IC on the
SPECTRA interface is also likely to get hot and so should not be touched either.
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