PSU alternative?

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Cristian
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Re: PSU alternative?

Postby Cristian » Tue Nov 07, 2017 7:33 pm

Thank you Martin for your explanation. Learning something is always important for me.


Martin_Head
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Re: PSU alternative?

Postby Martin_Head » Thu Nov 09, 2017 10:56 am

I was looking at the circuit diagrams of the +/- 12 volt supply on the QL and the PSU trying to figure out how they work.

The PSU circuit states that it's a 16v AC winding on the transformer.

On the QL circuit the +/- 12 volt regulators use the 9 volt DC line as a ground/reference?

You can get plus 16 volt, or minus 16 volt from an 16 volt AC winding, but how do you get both? Unless the two diodes by the +/- 12 volt regulators on the QL are some kind of a voltage doubler. (Which I've never really figured out how they work)

But then I would expect the +/- 12 volt rails to be referenced to the 9 volt rail. i.e. +21 and -3 volts.


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tofro
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Re: PSU alternative?

Postby tofro » Thu Nov 09, 2017 12:17 pm

Setting the ground reference of the +/- 12 V lines to the +9V line looks weird, at first, but you need to look into the PSU in order to understand it.

Sinclair has simply saved on one strand of wire, the 14VAC ground. Most probably, the complete transformer is one continuous winding, the input to the secondary coil being GND, the next pickup point the AC for 9V and the last the ~12V. The difference in potential for 14VAC is thus between the +9V and the ~12V output. The AC between GND and VAC is probably much higher that 14V, as it is the AC produced by both secondary transformer coils.

The diodes are there to let the positive sine wave pass into the 7812, the negative sine wave to the 7912.

I guess the whole circuit would make more sense when the PSU circuitry is included in the drawing.

Tobias


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Re: PSU alternative?

Postby Silvester » Thu Nov 09, 2017 12:29 pm

Martin_Head wrote:On the QL circuit the +/- 12 volt regulators use the 9 volt DC line as a ground/reference?

It's an error in the original service manual circuit, it is 0V. I checked PCB when doing PDF version.


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Re: PSU alternative?

Postby Martin_Head » Fri Nov 10, 2017 11:14 am

The PSU http://www.dilwyn.me.uk/docs/hardware/QLperip.zip shows the 16 AC as a separate winding with one end connected to the 0v (GND) line.

If as Silverter says, there is an error in the service manual circuit, So that the + and - 12 volt circuit would make sense. Then the AC signal arriving at the QL would have to swing between about + and - 16 volts. ie. about 32 volts peak to peak around ground.

Thinking about it a bit more, it does makes sense. The 16 volt AC, if measured with a multi meter, would be 16 volts. The meter would half, or full wave rectify the AC signal, so you would only 'see' the upper, or lower half of the AC signal. Whereas if you looked at it with an Oscilloscope, you would see that the signal actually went from +16v to -16v.

So both are correct, The PSU supplies 16v AC, and the QL receives the quoted 44v peak to peak.


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Pr0f
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Re: PSU alternative?

Postby Pr0f » Fri Nov 10, 2017 11:45 am

This is just half wave rectification, Sinclair being cheap again ;-) The current requirements for the +12 and -12v are so light, they can get away with this, and the ripple is easily dealt with by the regulator..


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1024MAK
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Re: PSU alternative?

Postby 1024MAK » Sun Dec 24, 2017 8:10 pm

Apart from the "common" 0V / GND / "ground" wire, the 9V DC and the 15.6V AC parts of the power supply work independently.

The 15.6V AC from the transformer is referenced to the 0V connection. So on the positive half cycle of the AC waveform, the instantaneous voltage is positive with respect to 0V GND and a diode connducts and allows the "smoothing" capacitor for the 78L12 +12V voltage regulator to charge up. This capacitor then allows the regulator to continue to function during the negative half cycle.

During the negative half cycle, the instantaneous voltage is negative with respect to 0V GND and another diode connducts and allows the "smoothing" capacitor for the 79L12 -12V voltage regulator to charge up. This time, to a negative voltage with respect to 0V / GND. This capacitor then allows the regulator to continue to function during the next positive half cycle.

And yes, what you have overall, is two back to back half-wave rectifier circuits.

My earlier statement that two transformers would be needed, is true only if you are using off the shelf "standard" transformers. Of course, if you are happy to pay extra, there are companies that will custom wind transformers. Then you can have both windings on a single transformer.

Mark


QL, Falcon, Atari 520STFM, Atari 1040STE, more PC's than I care to count and an assortment of 8 bit micros (Sinclair and Acorn)(nearly forgot the Psion's)

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