Spectrum & ZX81 PSU Strange Voltages

dreamscape · August 2014

Hi all

I've just refurbished my ZX spectrum PSU after i accidently broke one of the wires i soldered the joints to the PCB nicely and all seems to be well.

That was until i tried it on my multimeter... The speccy PSU is putting out 13.4V and my ZX81 PSU is putting out 14.4V ???

Surely these are too high? It's supposed to be DC 9V, right?? :-? :confused:

Surely either both of mine are screwed or there is a regulator on the board???

balford · August 2014

The ZX81/Spectrum PSU's are of an older design and are unregulated, which means that the actual voltage supplied depends on the load being drawn from the PSU by the machine.

It's not uncommon to measure high voltages with no load with these PSU's, but anything over about 14V on load tends to kill the Speccy's voltage conversion circuitry.

Personally, I don't like using unregulated supplies if I can as the extra voltage has to be dissipated by the 7805 voltage regulator on the ZX81/Spectrum board as heat.

The closer to 9v that can be provided, the less energy is wasted, and the cooler the machine runs, aiding longevity. For this reason I use a modern 9v regulated supply.

B

dreamscape · August 2014

Ohhhh that makes sense then! thanks for clearing that up.

I presume then that even at these voltages these 2 are safe to use? Hopefully they will drop enough to not cause harm.

I don't want them killing a speccy or my ZX81.

1024MAK · August 2014

Around 13 to 14V off load for a ZX81 or Spectrum PSU is normal.

Mark

dreamscape · August 2014

That is reassuring, thank you for the information. :-D

Pierre · August 2014

When measured with no load, you will only measure the peak voltage produced by the supply. Measured on-load with a true RMS voltmeter will give you the correct output of the power supply. There are many posts somewhere on the forums that discussed the issue in-depth. I know it is difficult to search for a post, but give it a try.

Pierre

RobeeeJay · August 2014

Um, why is RMS important for DC? :o

1024MAK · August 2014

RobeeeJay wrote: »

Um, why is RMS important for DC? :o

Because the output of a non-regulated (unregulated) DC power supply unit is actually a mixed waveform. It is DC, but with an AC component part. The AC part is known as ripple.

RMS means root mean square. It measures the voltage, current or power that you get with any waveform. Its value is equivalent to the voltage, current or power that a regulated DC power supply or a good battery will produce across / through a resistive load.

Multimeters that measure using RMS are often called true RMS meters.

This is different to the peak to peak voltage (the maximum voltage that the waveform gets to).

Cheap multimeters do not measure true RMS, but instead use an average technique based on the assumption that the waveform is a sine wave. But ripple is not a sine wave.

However, a cheap non-RMS multimeter will still give you an approximate value, so don't worry about it too much.

Mark

Winston · August 2014

True RMS or not true RMS, it won't make much difference what multimeter (well, other than a decent true RMS meter is probably more accurate and probably calibrated properly) for a Speccy supply off-load. I just measured the ripple of a 128K supply off load and it was only on the order of 20mV.

Pierre · August 2014

As a matter of fact the specific meter that is used for measurement can lead to incorrect results. The ripple that is measured with off-load should be 0 or very close to 0. The ripple that you do measure is caused by the load of the multimeter (or volt meter) or some internal resistance in the PSU. A high impedance meter will show a very low ripple (or AC voltage) and a low impedance meter will show a high ripple. Read the specs of you meter to check the meter impedance and then you can calculate the load that you will connect to the PSU.

On the other hand, a low impedance meter will not necessary implicate that it is a cheapy as precision low impedance meters are very expensive and do have specific applications. For example when measuring a battery cell in a battery bank as you need to put a load on the cell to ensure that you do not measure the floating voltage of the battery charger.

The point that I would like to make is that one should ensure that you use the right test equipment with right specifications for each application and could very easily come to an incorrect conclusion by not measuring correctly.

RobeeeJay · August 2014

1024MAK wrote: »

Because the output of a non-regulated (unregulated) DC power supply unit is actually a mixed waveform. It is DC, but with an AC component part. The AC part is known as ripple.

I'm not sure measuring ripple with a multimeter would be very accurate, I'd assume you'd need an oscilloscope for that, and aren't calculations for AC depending on the waveform? So a sign wave is different from a square wave?

I'm not arguing that an RMS meter will be more accurate than a non-RMS meter, because it will most likely be made to a higher spec, merely arguing that the RMS calculation for AC isn't relevant for measuring DC and not even part of the equation when set to DC.

I also think a ?15 multimeter is good enough for measuring the DC output of a Spectrum PSU. :)

Spekkie · August 2014

dreamscape wrote: »

Hi all

I've just refurbished my ZX spectrum PSU after i accidently broke one of the wires i soldered the joints to the PCB nicely and all seems to be well.

That was until i tried it on my multimeter... The speccy PSU is putting out 13.4V and my ZX81 PSU is putting out 14.4V ???

Surely these are too high? It's supposed to be DC 9V, right?? :-? :confused:

Surely either both of mine are screwed or there is a regulator on the board???

That's funny, I had exactly the same question yesterday evening after measuring the output of my ZX81 and ZX Spectrum power supplies. I found this video, it answers the question(s) very well.

Paul

Spectrum & ZX81 PSU Strange Voltages

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