Sinclair ZX80 / ZX81 / Z88 Forums

Hi. Would it be advisable to use an external LM7807 or LM7808 to feed the internal LM7805 jut to avoid the extra heating inside the case?
I know that some rampacks need the 9v line but I am planning to use a 4164 modified rampack or a custom made SRAM one..
My min xon er is about a ZX80 .. I have two original linear PSU from CZ1000 (argentinian TS1000 @50Hz PSU made for AC 220v ) that renders almost 14v unloaded and more than 10v at 7805 input.. or a new switching 9v PSU...
My plan is to build an intermediate box with a heatsink to feed the ZX from.. without any internal modification..
I guess I'll use any of those rampack with it
Will this help with the noise added by the switching PSU?
Thanks

Yes you can use a 7808 voltage regulator to drop the voltage down, then feed it to the computer.

Only third party RAM packs that use the 4116 or equivalent DRAM chip need the +9V rail to be at around 11V. All others should be fine with a lower voltage on the +9V rail.

Rather than a hard to get 7808, it is also possible to use a 7805. Use a 2.4V or 2.7V 400mW or 500mW Zener diode wired between the 7805 common/GND/0V leg/terminal and the supply 0V/GND. The use of the Zener diode raises the output voltage.

Use of a series pass regulator like a 7808 or 7805 will reduce the noise from a switching PSU to the computer, yes.

It is also possible to use a good quality modern switching PSU (SMPSU switch mode power supply unit) that has an output of 7.5V to 9V.

Mark

1024MAK wrote: ↑Thu May 28, 2020 9:29 pm
Rather than a hard to get 7808, it is also possible to use a 7805. Use a 2.4V or 2.7V 400mW or 500mW Zener diode wired between the 7805 common/GND/0V leg/terminal and the supply 0V/GND. The use of the Zener diode raises the output voltage.
Mark

This looks really Interesting..
Just to be sure I did understand .. is this the right place to put the zenner? Anode pointing were to?
Capacitor bridges would be usefull?

Mark

1024MAK wrote: ↑Fri May 29, 2020 3:30 pm Increasing the output voltage of a 7805 using a Zener diode.png

Mark

Grrat
Thanks you!!

And here are photos of a similar version from 1993. This design uses a 4.3V Zener diode to produce a 9V DC output. This version has a 1N4001 diode on the input, and C1 is 1000uF 35V (so it can be used on unsmoothed DC supplies), C2 and C3 are both 100nF (did I say that it was built using only parts I had to hand), there is no C4, and a red LED and a 560Ω resistor are included to act both as a power on indicator and to discharge C1.

Note that because the Zener diode is being operated at a low current, it's operating voltage will be slightly lower than you would expect. Hence the actual output voltage of this unit is 9.09V.

Sorry that the photos are not very good, the device I used is not very good at taking close ups.

Mark

Looks strong enough!!

I ve made this but used 4 Diodes I had at hand I series instead of a zener..
I guess this also will protect from a mistaken polarity input.. Input pin is similar to that in spectrum but 2.1center positive because that's the PSU I had to test but I will change that... I had enough mistakes this weekend already
Board doesn't have stripes so I used only a short side I can put sideways in box with a disipator in a future