Request: Power Supplies UK700 vs UK1200 Schematic diagrams (and why does my UK1200 sup run much hotter under same load?)

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escher
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Request: Power Supplies UK700 vs UK1200 Schematic diagrams (and why does my UK1200 sup run much hotter under same load?)

Post by escher »

Hi

I have both original UK700 and UK1200 Sinclair power supplies for the ZX81 and have tested them running the 25th Anniversary demo with a modified (WRX Hi-Res, 2 diodes + resistor) 16k Ram Pack for several hours.

The UK1200 power supply runs MUCH HOTTER, is that normal? Or do you think I have a failing component in that supply?

I have opened the UK700 one and it is really very basic, just a few capacitors (edit: just one capacitor, the white component under the coil is a fuse I think) and resistors not sure there is even a bridge rectifier diode circuit there (edit: Of course there is! Not sure why i thought the four diodes were resistors). Due to damage to the hard to remove screws I don't want to unscrew the UK1200 one.

Does anybody have a schematic for both supplies?
Last edited by escher on Sat Jun 25, 2022 8:34 pm, edited 1 time in total.
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Paul
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Re: Request: Power Supplies UK700 vs UK1200 Schematic diagrams (and why does my UK1200 sup run much hotter under same lo

Post by Paul »

What does your multimeter show for both supplies?
Please check with DC20V and with AC20V
I had a supply with a broken diode that ran significantly hotter.
Or did you mean the zeddy is running hotter when using uk1200?
In theory, there is no difference between theory and practice. But, in practice, there is.
escher
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Re: Request: Power Supplies UK700 vs UK1200 Schematic diagrams (and why does my UK1200 sup run much hotter under same lo

Post by escher »

I'm not sure how you mean to check between DC20v and AC20v.

Do you mean just test the power supplies with no load attached, and switch my multi-meter between Ac and Dc?

What reading would indicate a fault?


edit: the 700ma supply shows 16.4 V on the multimeter, and the 1200ma supply shows 15.5 V, I think this is ok with no load attached (and the ZX81 regulator doesn't get too hot with either supply)
escher
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Re: Request: Power Supplies UK700 vs UK1200 Schematic diagrams (and why does my UK1200 sup run much hotter under same lo

Post by escher »

Ok I opened up both supplies, the only difference seems to be a 4700 uF capacitor in the 1200ma supply vs 2200uF in the 700ma supply and the UK1200 one has 4 bigger diodes for the rectifier circuit (not sure why I thought those were resistors previously) (there are no markings on the diodes in UK1200, the Uk700 has 1n400x diodes, (can't see the value of x))

Both power supplies perform reliably and the capacitors loook in good shape (no bulging, leakage etc), so I just assume the Uk1200 one tends to run hotter, maybe due to less efficiency.
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1024MAK
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Re: Request: Power Supplies UK700 vs UK1200 Schematic diagrams (and why does my UK1200 sup run much hotter under same lo

Post by 1024MAK »

Keep in mind that all the Sinclair PSUs for ZX81 were made by third party manufacturers, hence there are a number of different “versions” depending on which manufacturer made each one. Hence there are a number of different UK700 PSUs, all of which look exactly the same from the outside.

UK1200 are less common, so I don’t have many to compare, but it’s possibly the same with these.

The circuit is just a standard transformer, a full wave bridge rectifier, and one or two electrolytic smoothing capacitor(s).

The transformer either has a thermal fuse under the tape covering up the windings, or on the PCB under the transformer.

1N4001 or any of the higher voltage versions can be used in the UK700 (700 mA) versions, as a 1N400X is rated at 1A.
Higher current diodes are used in the UK1200 (1200mA or 1.2A) PSU.

Normally how hot a unregulated PSU like these gets, depends on the quality of the construction (including materials) of the transformer. Small, cheap transformers tend to get warmer, as there are more losses in the transformer. As different manufacturers were involved, some will run hotter than others.

Mark
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Re: Request: Power Supplies UK700 vs UK1200 Schematic diagrams (and why does my UK1200 sup run much hotter under same lo

Post by jojo »

one of them ;

adaptors & eliminators ltd

14 thames st louth lincs

also long gone
1 transformer , 4 diodes , 1 capacitor .
m_WhatsApp Image 2022-06-26 at 6.jpeg
Zx81-16k exp.- Spectrum 48 - Spectrum 128- Interface 1 - DIVide - Zx flashcard- Zx printer - Ram turbo-Velleman interface system-Currah microspeech -Ql - 512k exp.- Ql printer - Cst floppy interf.- Double 3.5 floppy- Minerva - Qimi-and much more .
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Re: Request: Power Supplies UK700 vs UK1200 Schematic diagrams (and why does my UK1200 sup run much hotter under same lo

Post by bola_dor »

Just a thought
The bigger rated (non regulated) PSU at the same load should be delivering a higher voltage. This necessarily will get the linear regulator on ZX81 and the heatsinc hotter. Remember that the 7805 regulator converts the extra voltage over 5v to heat..
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1024MAK
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Re: Request: Power Supplies UK700 vs UK1200 Schematic diagrams (and why does my UK1200 sup run much hotter under same lo

Post by 1024MAK »

Not necessarily…

A ZX81 typically consumes between 300mA to 450mA. The current consumption does not depend on the output voltage (unless the voltage drops too low). But the output voltage of the unregulated PSU does depend on the output current.

The output voltage of an unregulated PSU depends on the following: the mains input voltage, the number of turns on the primary and secondary windings on the transformer (the ratio), the copper losses (power lost through heat caused by the slight resistance of the wire), magnetic (iron) losses, the losses due to the forward voltage of the rectifier diodes, the amount of ripple voltage across the smoothing capacitor (and hence it’s capacitance), and most of these are affected by how much current is being drawn from the PSU.

For the transformer, there is a term used to describe the difference between the unloaded output voltage and the voltage at full load. It’s called transformer regulation. Small cheap transformers can have a regulation figure as bad as 20%. Bigger transformers, especially higher quality types have much better (lower) regulation figures.

So if we assume (because I don’t know the actual figure) that the transformer in a UK700 PSU does have a regulation figure of 20%, this means that at full load (700mA) it’s output voltage will be as per its stated output voltage.

But at no load, it’s output voltage could be 20% higher than its stated output voltage. With its output voltage smoothly varying between these figures.

Then there is the effect of rectifying the AC from the transformer. At no load or light loads, the DC voltage on the smoothing capacitor will be approximately equal to the peak sine wave AC from the transformer, that’s appropriately 1.41 times the RMS voltage (which is the normal way of measuring AC voltage). But as the current to the load increases, the voltage on the smoothing capacitor will fall whenever the AC sine wave is at or below the voltage on the capacitor, and the capacitor is therefore supplying current to the load. The difference between the no load voltage and the average output voltage will increase. Also the ripple voltage will increase (the AC component measuring the actual difference between the minimum and maximum voltage over one AC waveform cycle).

The upshot of this is that a larger, higher current unregulated PSU will have a lower variation between its no load output voltage and the current for the load compared to a smaller, lower current unregulated PSU. The larger, higher current unregulated PSU starting off with a lower maximum output voltage with no load.

As a practical example, the off load voltage of a UK1400 1.4A 9V Sinclair ZX Spectrum PSU does not normally go much above 13V. On one I have to hand, I measure 12.5V off load.

Compare that with a typical UK700 ZX81 PSU, which have a typical off load voltage of 16V (and indeed one I have to hand does output 16V off load).

ZX81 / TS1000 current and power consumption (all using a UK700 PSU)

Issue 3 board (TS1000) with NEC D780C-1 CPU, D2364C ROM, NEC D40116D SRAM and 2C210E ULA.
PSU output voltage = 12.4V. Current = 335mA. Power = 12.4V x 335mA = 4.154W.

Issue 1 board (ZX81) with SGS Z80A Z8400AB1, Motorola ZCM38818P ROM, 2 x MCM21L14P SRAM and 2C184E ULA.
PSU output voltage = 11.6V. Current = 362mA. Power = 11.6V x 362mA = 4.199W.

ZX81 Issue 1 board with NEC D780C-1 CPU, D2364C ROM, 2 x NEC uPD2114LC-1 SRAM and 2C184E ULA.
PSU output voltage = 11.8V. Current = 402mA. Power = 11.8V x 402mA = 4.744W.

Incidentally, I tested the current consumption of one ULA. The one I tested was a 2C184E ULA. It has a current consumption of 130mA to 133mA at +5V. So ULA power = 665mW.

Now, there is some debate on whether the Sinclair UK1200 1.2A PSU are in fact using the same transformer as the UK700 PSU. And that the difference is just the higher current rectifier diodes and the larger value smoothing capacitor(s). I don’t have an answer to this, as it’s not easy to work out the specifications of a transformer just by looking at at it. Some very detailed investigations would be needed.

I can say that I suspect that there may have been some mixups with the wrong PCB and transformer being put in the wrong PSU case…

Comparing a number of UK700 PSU with the one and only UK1200 PSU that I had in 2016, I found this:

UK1200
Transformer
52 x 43 x 18 laminations
52 x 43 x 40 overall (excluding PCB pins)

UK700
Transformer
52 x 43 x 18 laminations
52 x 43 x 40 overall (excluding PCB pins)

Mark
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1024MAK
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Re: Request: Power Supplies UK700 vs UK1200 Schematic diagrams (and why does my UK1200 sup run much hotter under same lo

Post by 1024MAK »

So I have a different ZX81 board today (all the results in my earlier post were from 2020 and were using their original 7805 voltage regulators).

This is a ZX81 Issue 1 board with 7805 voltage regulator, NEC D780C-1 CPU, Mostek MK36809N-5 ROM, Alliance AS6C62256-55PCN (modern 32k byte) SRAM and 2C210E ULA (a replacement, not the original ULA). It also has a Kayde keyboard attached which uses a little bit of power as well. Note that the modern SRAM chip means the current draw is significantly lower compared to the older SRAM chips that Sinclair used.

Using a ZX81 UK700 PSU:
PSU output voltage = 13.27V. Current = 257mA. Total power used by the ZX81 = 13.27V x 257mA = 3.41W.

Using a ZX Spectrum UK1400 PSU:
PSU output voltage = 11.35V. Current = 257mA. Total power used by the ZX81 = 11.35V x 257mA = 2.92W.

I don’t have any UK1200 PSU to hand to test :(

Mark

Edited to correct an error, I noticed that I had written UK1200 instead of UK1400. And to clarify the text for the figures.
Last edited by 1024MAK on Tue Jul 19, 2022 9:04 am, edited 2 times in total.
Reason: Edited to correct an error, I noticed that I had written UK1200 instead of UK1400. And to clarify the text for the figures.
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1024MAK
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Re: Request: Power Supplies UK700 vs UK1200 Schematic diagrams (and why does my UK1200 sup run much hotter under same lo

Post by 1024MAK »

Here is another example of a UK700 PSU:
4ADD9FF5-8B91-4571-B8C1-11B01FB5B5A5.jpeg
CC8CC41F-740D-4C93-9AB0-139D4C5AA099.jpeg
The transformer is at a funny angle because at some time in it’s past life, this PSU has been dropped. The plastic former that holds the PCB pins where the transformer pins are soldered to the PCB have broken, one PCB pin is bent and one has developed a loose/dry solder joint on the PCB.
EF43347C-87D7-41F1-BDC0-BA4F9320DF70.jpeg
(I’ve never actually used this particular PSU).

Mark
ZX81 Variations
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