ZX Spectrum and add-ons power measurements

The ZX Spectrum has an internal 5V voltage regulator.
With the ZX Spectrum 16/48K that is a 7805 which can supply 1A.
With the ZX Spectrum 128K toastrack and +2 that is a 78S05 that can supply 2A at max.

When nearing the limit of these voltage regulators, the heatsinks will become very hot.
Another cause for a heat is when an original ZX Power Supply is used, which supplies about 11V when connected to the ZX Spectrum: the 7805 needs to ‘work harder’ to output a stable 5V, compared to when a modern 9V switching power supply is used.

When the 7805 becomes too hot due to reaching the current limit, or when the 7805 needs to handle too much current combined with a higher input voltage (original ZX Power Supply), the 7805 will shut down.
When the current limit is exceeded, the 7805 may be damaged, and there is a chance that the ZX Spectrum itself is damaged as well

So it’s important to stay within the maximum current specifications.

Hence I made this list of machines and add-ons to know what is reasonable to combine and what not.

So to check the load on the 5V regulator, add up the numbers in the ‘@5V’-column for the devices you are using.
Examples of checking the load on the 5V regulator for some common combinations:

  • ZX Spectrum 48K + ZX-HD + DivMMC EnJOY!: 0.66 A @5V
  • ZX Spectrum 128K toastrack + ZX-HD + ZX-AY  + DivMMC EnJOY! *mini*: 1.08 A @5V
    • The 78S05 can handle 2 A max, so no problem
  • ZX Spectrum 48K + ZX-HD + PlusDlite: 0.81 A @5V
    • About the max of the 1A the 7805 can supply


To check the power consumption on the 9V power supply, add up the numbers in the ‘Power’-column.
Please always add about 10% to this number, since the 7805 regulator is not very efficient, causing a loss in energy that it does consume, which will be output as heat.

An original 1.4 A ZX Power Supply can handle just over 12W of power.
An original 1.8 A (toastrack) ZX Power Supply can handle about 16W.

A modern switching 9V 1.5A power supply does about 13W.
A 2A version (the ones we usually offer in the webshop) does about 18W max.