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.

Current at 5V
ZX Spectrum 48K 0.40 A (max 1 A)
ZX Spectrum 128K toastrack / +2 0.75 A (max 2A)
Harlequin rev G  0.14 A
Harlequin rev 2D 0.16 A
ZX-HD with Raspberry Pi Zero 0.14 A
ZX-AY 0.10 A
DivMMC EnJOY! *mini* 0.08 A
DivMMC EnJOY! with GAL 0.12 A
DivMMC EnJOY! with joy CPLD 0.10 A
DivIDE 0.21 A
Kempston Joystick interface 0.06 A
K-Mouse Turbo (2008) 0.12 A
PlusDlite 0.27 A
Interface 1 + Microdrive 0.05 A
Interface 2 + DIAG ROM 0.01 A


Common combinations:

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

The combination with the PlusDlite may produce too much heat when using an original ZX Power Supply, so using a modern 9V switching power supply is very much advised with such a combination.