ZXpand+ compatibility with TS-1500

[1024MAK]

It’s likely the DC/DC converter circuitry rather than a 7805 that’s causing problems…

Mark

[FrancoisLanciault]

Ok I can’t keep the TS-1500 working from the bench power supply for ever, so I went to my nearby electronic store to buy a 10V wall regulated PSU, but they only have 9 Volts (that don’t work) and 12 volts. I got the 12 Volts, and it works… But I am afraid that it could make the computer overheat if plugged for too long ?

Does 7805 heats more when supplied with 12 volts than with 9 volts ? In general, can you power a zx-81 with 12 volts?

François

[1024MAK]

7805 voltage regulators are normally rated at 35V maximum input voltage. The limiting factors are the voltage rating of the capacitors (typically either 16V or 25V) and the heat dissipated by the 7805 via the metal heatsink.

This applies to the ZX80, ZX81, TS1000, TS1500 and some ZX Spectrum boards.

A 7805 voltage regulator works (in simplistic terms) by clamping the output voltage at the specified voltage. So, for a 7805, thats 5V. To do this, it has to drop the difference between the input voltage and the output voltage. You can't just get rid of something when it comes to energy, that energy has to go somewhere. The somewhere is to convert it to heat, then use the heatsink to transfer the heat energy to the air...

The calculation to work out the energy being converted to heat is not too difficult:

Power (heat energy) in Watts = (Input Voltage - Output Voltage) x current

Taking a typical ZX81/TS1000, the current is typically 350mA or 0.35A (some use more, some use less, it depends on which exact parts have been used).

So if the input is 9V, the electrical power that the 7805 voltage regulator will convert to heat is 1.4W

With an input of 12V, that's 2.45W.

However, the original PSU supplied is an unregulated type. Unless the output current is at or near the maximum rating, the output voltage will be higher than the rating. Hence a ZX81 when used with a UK700 9V Sinclair PSU (and no expansions) will actually see between 10.5V and 12V approximately. Not 9V.

So, yes, with a higher input voltage, the 7805 will generate more heat. The computer will get hotter. If used for say only one hour at a time, this should not be generally be a problem. Even if left on It continuously, will not 'overheat' as such, as long as there is adequate ventilation around the computer, and the air temperature is in the normal range (at or below 25°C).

However, if you are intending to use it for many hours at a time, then one method for reducing the temperature that can be done, is to wire a couple of suitable rectifier diodes in series with the positive lead of the PSU. Each diode will drop between 0.7V and 1V, hence it will reduce the input voltage to the 7805

Alternately, the 7805 can be replaced with a modern DC/DC converter/switching regulator (such as RECOM, Traco Power, or MuRata type) which are much more efficient, and hence don't get hot.

It's generally considered that a cooler computer will last longer and be more reliable.

Mark

[FrancoisLanciault]

Very clear answer. Thank you!