Proper voltage for ZX81 AC adapter

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msellan
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Proper voltage for ZX81 AC adapter

Post by msellan »

I was using one of my ZX81 adapters last night and when I went to unplug it it seemed much warmer than I remembered them getting. It's an original Sinclair one - 9.75V 600MA. When I metered it no load, it measured just over 14.5v. I got out two other adapters (one Sinclair and one DVE) and they both measured just over 12V. Is that kind of variation normal or is one of my supplies going bad?

Thanks much,

-mark
3 ZX80s (1 UK issue 2, 1 US issue 2 & 1 US issue 1), 5 ZX81s, 1 TS1000, 2 Memotech 16k RAM,
2 ZX81 16k RAM, 2 TS 16k RAM, 1 ZX81 unbuilt kit, 1 TS2040 printer, 2+ ZX8-CCB and counting

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1024MAK
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Re: Proper voltage for ZX81 AC adapter

Post by 1024MAK »

The so called 9V PSU supplied by Sinclair, is an unregulated type. If there is no load (ZX81 not connected), the transformer will produce a higher voltage than the rated value, plus the smoothing capacitor(s) will charge up to the peak of the rectified AC waveform. Typical small transformer off load voltage increase is 15 to 25%. To convert AC RMS to the peak of the sinewave, multiply by 1.414. So 9V AC plus 25% is 11.25V AC, then multiply by 1.414 gives the peak value of 15.9V. This is the voltage that the smoothing capacitor(s) will charge up to. Once the PSU is connected to a ZX81 (loaded), the output voltage will fall to anywhere between 9V DC and 12V DC. ZX81s that have been upgraded to a switching regulator/ DC-DC converter (in place of the 7805 voltage regulator) will draw less current, so the ZX81 mains PSU output voltage will be higher than when supplying a unmodified ZX81.

Mark

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povvercrazy
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Re: Proper voltage for ZX81 AC adapter

Post by povvercrazy »

I have 2 zx81 power units, one marked uk700 the other uk1200 which unloaded give 16v and 13v respectively, both seem ok in use thou.
Because older power supplies are inefficient and unreliable I'm basically replacing all my systems with modern 9v switching units and changing the internal 7805 to a switching alternative too, ( ie spectrum, zx81, nes, 2600 and so on I've even just picked up an amstrad gx4000....stop laughing! :lol: )

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1024MAK
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Re: Proper voltage for ZX81 AC adapter

Post by 1024MAK »

It's the series regulator (7805) that is the inefficient part of the system. The Sinclair mains PSU is made up of a transformer, four silicon diodes (forming a bridge rectifier), and one (or two) electrolytic capacitor(s) to act as DC smoothing.

Mains transformers are one of the most reliable components, and are reasonably efficient (the larger higher quality transformers are often better than 85% efficient at full load). They are very safe. The downside of mains frequency transformers are their size and weight.

You will find a silicon diodes forming a bridge rectifier in modern switch mode PSUs - so no difference there.

You will also find electrolytic capacitor(s) to act as DC smoothing in modern switch mode PSUs - so again no difference there.

The heat that you feel on the case of a Sinclair ZX PSU mainly comes from the mains transformer, and is the relatively small amount of heat caused by "iron" losses. This caused by eddy currents that flow in the iron plates that make up the magnetic circuit.

Due to there being no proper ventilation for air to flow through the case, instead the heat warms up the case.

The downside of "modern" cheap switch mode PSUs is that often imported units do not comply with safety standards (poor isolation between the mains voltage circuitry and the low voltage output circuitry), poor regulation (but this does not matter in this application) and an unknown reliability. The properly designed more expensive types are however fine.

The best thing you can do to a ZX81 is to replace the 7805 series regulator with a DC-DC switching type converter :(
You can then continue to use a Sinclair ZX PSU.

Mark

gozzo
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Re: Proper voltage for ZX81 AC adapter

Post by gozzo »

my zx81 and spectrum psu's always seemed to me as if you could fry eggs on the things...😉 but...using a modern regulated 9v psu on a zx81 could be a problems as some rampacks used the 'higher than 9v' line as a 'near to 12v' for the '12v' for a bank of 4116s, so 9v wont work, but if 11 ish, probably will...genuine sinclair packs do it properly so will usually be ok on 9v

gozzo
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Re: Proper voltage for ZX81 AC adapter

Post by gozzo »

as for replacing the 7805 with a switchmode thing, this doesnt help with the rampack voltage problem, but will certainly reduce the heat in the computer...or you could try fitting a full width heatsink? sprayed with a thin layer of matt black barbecue paint...??

gozzo
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Re: Proper voltage for ZX81 AC adapter

Post by gozzo »

and..beware,..i have a UK1200 psu with the same smaller transformer as the UK700..and it appears to have got VERY hot as the pcb is blackened, although still works, gets fairly warm even with no load,..so...there may be others out there....

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povvercrazy
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Re: Proper voltage for ZX81 AC adapter

Post by povvercrazy »

The heat generated by the transformer is wasted energy is it not? :p
Even if there were vents to dissipated the heat.
It would be very interesting to see a comparison of performance, might be an interesting experiment.
There's many posts saying running on 9v allows the computer to run cooler and is better for it.
Heat is the enemy of a computer so surely anything that will reduce heat or the generation of it is better?

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1024MAK
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Re: Proper voltage for ZX81 AC adapter

Post by 1024MAK »

Having looked at the UK1200, I get the impression that the 1.2A rating is for intermittent loads, not for continuous loads. Sinclair supplied it so that a ZX Printer could be used.

An overloaded transformer will produce more heat, as then as well as the iron losses, the small amount of resistance in the copper wires will produce heat. The greater the load, the more heat.

The other thing, they are supposed to be used on a hard surface with good ventilation, not on a deep pile carpet.

Mark

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1024MAK
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Re: Proper voltage for ZX81 AC adapter

Post by 1024MAK »

povvercrazy wrote:
Thu May 11, 2017 9:16 pm
The heat generated by the transformer is wasted energy is it not? :p
Even if there were vents to dissipated the heat.
It would be very interesting to see a comparison of performance, might be an interesting experiment.
There's many posts saying running on 9v allows the computer to run cooler and is better for it.
Heat is the enemy of a computer so surely anything that will reduce heat or the generation of it is better?
Every electronic component gives off heat.

If using the original 7805 regulator, and a not using a RAM pack that needs the 9V line, the best input voltage is a regulated 8V input. Then the 7805 only has to drop 3V.

If using a DC-DC switching regulator in place of the 7805, it does not matter too much what the input voltage is (within it's rating), as they are very efficient and run cool.

Mark

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