PSU Voltages.

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flaviosr
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PSU Voltages.

Post by flaviosr » Fri Aug 10, 2018 9:31 pm

Dear All,

I am putting order in my small Sinclair Collection (1 × ZX80 in kit, 1 × ZX81, 2 × Spectrum, 1 × QL), testing everything.
I was using one of two Spectrum when it simply turned off... I was a bit scared when I decided to measure the voltage from the PSU and it was 0.9V...I breathed a sigh of relief.
After this I decided to measure the voltage of all my 3 PSUs and I found what follows:
° ZX81: 19V
° Spectrum 1: 12.5V
° Spectrum 2: 14.5V (after a while the "dead" PSU decided to get back to life).
Anyway the above voltages are much above the 9V declared...
I started looking over internet and I have found in one site that Spectrum PSUs are normally 15V without load but nothing about the ZX81.

Can you help me understand what is happening?

Thank you

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RetroTechie
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Re: PSU Voltages.

Post by RetroTechie » Sat Aug 11, 2018 3:01 am

Yes it's common actual supplied voltage is much higher than the nominal of about 9V (both for Spectrum and '80/81). But no-load voltages aren't very useful. Measure under load.

High input voltage does not directly influence operation (since internal voltage regulator), but it does affect how much waste heat is produced inside the machine. To keep that in bounds, it's better to ditch original psu and use a regulated DC supply instead (modern smallish / lightweight DC adapters usually are :) ). About 7 to 9V DC is good as input.

:!: Most important: IIRC the ZX Spectrum psu uses a "center pin negative" connector. Don't take my word for that! Check elsewhere or with original psu. So don't replace with a run-of-the-mill DC adapter pulled from your junk bin. "Center pin positive" is de facto standard these days & will -at best- not destroy anything if plugged into a ZX Spectrum. So make sure to find DC adapter with correct polarity plug (or switchable, or do some re-wiring on the plug), and check both polarity & voltage before plugging in.

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1024MAK
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Re: PSU Voltages.

Post by 1024MAK » Sat Aug 11, 2018 4:21 am

Have a read through this thread ;)

ZX Spectrum 9V power supply units are similar to the ZX81 power supply units, but are able to output a greater current and have a 2.1mm/5.5mm barrel mini power plug.

If trying a non-Sinclair PSU, always double check the polarity. Get it wrong and you will blow up a number of components inside the computer.

Common faults in these PSU include damaged rectifier diodes, life expired electrolytic capacitors and, most common, dry/cracked solder joints and output cables with broken copper conductor(s).

Mark

flaviosr
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Re: PSU Voltages.

Post by flaviosr » Sat Aug 11, 2018 1:44 pm

1024MAK wrote:
Sat Aug 11, 2018 4:21 am
Have a read through this thread ;)
Thank you for the link! Very interesting... Why Mr. Sinclair decided for such (poor?) solutions? :(
I always check polarity before going on, comparing the original PSU with the modern one!
And I have decided not to use anymore "old" Sinclair PSU for two main reasons:
1. to save them...
2. to have the less heat dissipation!

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1024MAK
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Re: PSU Voltages.

Post by 1024MAK » Sat Aug 11, 2018 6:37 pm

Power supplies using a transformer, rectifiers and smoothing capacitor(s) were very common before the cost of switch mode power supply technology fell in price. Similarly, series pass regulators like the 7805 were very common for devices that need a regulated 5V rail. The 7805 was one of a whole range of series pass regulator chips.

Because these were all existing well known technologies, they were the lowest cost solution for home computer manufacturers.

Mark

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RetroTechie
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Re: PSU Voltages.

Post by RetroTechie » Wed Aug 15, 2018 6:53 am

1024MAK wrote:Power supplies using a transformer, rectifiers and smoothing capacitor(s) were very common before the cost of switch mode power supply technology fell in price.
DC voltage regulation is very often used inside equipment. Practically required for digital circuitry. Before switch mode psu's became the norm, any equipment using low voltage DC internally would use a similar "transformer -> rectifier diodes -> smoothing capacitor(s) -> voltage regulation" setup in one form or another. Or batteries. There's only so many ways to Rome. :P

The only choice for designers is where to put those components - inside a device or as part of an external power brick. Personal preference is for the latter as it makes the device itself simpler / smaller / lighter, and makes replacement of a psu easier.

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