Which power supply? Original sinclair zx or modern? Which one do you prefer? Which one to choose?

[Mr.ToR]

Hi speccy friends.
The original power supply for the ZX Spectrum is a simple transformer+full rectifier+smoothing capacitor. I am wondering if a modern power supply is better in terms of noise? A modern one is definitely more efficient but is it at a cost for noise? Should I go with an original ZX Spectrum power supply or a modern one?
Also I can put a modern one inside an original sinclair power supply.
What do you prefer?
What are your thoughts?

 

[SiriusHardware]

Switch-mode power supplies (which 99% of modern PSUs are) are much worse sources of noise than old-school iron transformer + capacitor + rectifier types due to all the fast switching waveforms going on inside. They have a tendency to generate a lot more in the way of radio interference than old school 'simple' unregulated supplies. The low frequency ripple on an unregulated supply will be quite high but that is what the onboard regulator is for, to regulate the power supply voltage down to 5V and at the same time flatten out the bumps.

Arguments in favour of modern switch-mode supplies - lighter, more efficient, regulated. Yes, you could put the works from a modern SMPSU inside an original Spectrum supply, there are screw heads under the felt feet which you can take out to get into the PSU non-destructively. I would assume you would take great care to make sure that the PCB was secure and could not possibly ever come into contact with the mains wiring / mains pins on the inside of the enclosure.

What is the starting point for your question, is it that you have neither type currently and are trying to decide what to go for, or...?

 

[cj7hawk]

Running a SMPS at 9.0v and a Transformer based one probably putting out somewhere around 12v still even under Spectrum load - I'd go for the 9v one.

The linear 7805 inside the Speccy can only dissipate the extra voltage by turning it into heat... And heat is not good for older computers.

 

[Mr.ToR]

Thanks a lot for your comments. I have two ZX Spectrum+ computers without a power supply. I have a real janky cheap looking 9V SMPS that seems to work fine without any superficially observable adverse effects on both of them. I don't count it as a permanent power supply and I would like to get an appropriate power supply for them. I found an original EURO 1400 ZX power supply for ~27USD in my local currency. I'm thinking on buying it but not sure if I should use it or get a SMPS or maybe even put an SMPS in the original case etc. That's actually the starting point of my question.

About the 7805 and heat, I'm planning on replacing them with Traco TSR 1-2450 step-down switching regulators. Getting rid of the heat and efficiency is fine but I'm not sure if it has any adverse effects too.

I'm more familiar with commodore 8bit series original power supplies and they are usually considered as ticking time bombs since they go bad without warning and instantly give a high voltage at some point which results in chips getting fired in the computer. Probably due to the transpformer caps and board being encased in epoxy inside the power supply case.

 

[SiriusHardware]

Yes, even though I have never owned a C64 I have seen that problem with failure of the 5V regulator in a C64 'brick' supply, although in the particular case I saw it had failed with no output, whereas it is certainly a hazard of linear regulators that they can fail short circuit from input to output, passing on the full input supply voltage to what should be the 5V line with predictably catastrophic results.

In truth, this could just as easily happen with the Spectrum's onboard regulator but, not being encased in epoxy they are usually slightly better ventilated due to the fact that the air can circulate around the heatsink.

Even SMPSUs, if they are the sort which consist of a chip, a switching transistor and a 2-pin inductor with the switching device placed between input and output, can fail in the same way if the switching device (often a FET) goes short circuit.

All you can really do about that is to add a protection circuit, typically called a 'crowbar', which detects when the regulated voltage rises above a certain threshold and kills the power, hopefully before the excess voltage can do any serious damage.

 

[daver2]

And here is the 'rub'. An old fashioned 'analogue' voltage regulator has a lot of in-built protection. An equivalent 'switcher' may have very little protection in comparison.

An analogue regulator produces more heat, but less noise - although purchasing switching regulators from a major manufacturer should be a 'must'. Buying from a cheap Chinese source would be asking for trouble.

If you are operating an analogue +5V voltage regulator, you want to target an input voltage of +7.5V to +8V (or whatever the manufacturer recommends in their data sheet). Any higher voltage will just cause an increase in power dissipation and heat.

If you are planning on replacing an analogue voltage regulator with a switcher, check the data sheet for any recommended decoupling capacitors etc. Without the correct devices, the regulator can become unstable. The original circuit (with the analogue regulator) may not have the correct components fitted.

A transformer, full-wave bridge rectifier and smoothing capacitor is a pretty robust unit (again, if purchased from a reputable source). The only likely failure mode is a faulty electrolytic capacitor if it hasn't been used fir a while. In general, I have mainly seen these devices fail safely.

Dave

 

[twolazy]

Get one these old school switchable voltage archer/realistic power bricks with the switchable connectors.... Get best of both worlds. I have 3 round here, if you need one... At least it has a transformer. Less then 20 bux, and useful for other machines as well. The 300ma is conservative, its more like 600-700ma but voltage can drift a little on load.

 

[daver2]

The most likely failure mode with this unit (of course) is the switch...

Dave

 

[SiriusHardware]

There's always the question of how the switch is arranged as well, is it make-before-break so that the output voltage steps seamlessly from one voltage to the next, or is it break-before-make, allowing the output voltage to assume an uncontrolled value in the short interval between one switch position and the next? Not that you should ever be choosing the voltage AFTER you've connected the PSU to the equipment.

Those universal / reversible adaptors also carry with them the hazard of human error - selecting the wrong voltage and / or polarity by accident. For equipment which has already survived for 40 years or more I prefer to choose a dedicated power supply for it and keep that PSU with the equipment at all times.

If a power supply has a marked mA rating of 'x' mA I wouldn't usually push it further than that.

 

[twolazy]

There's always the question of how the switch is arranged as well, is it make-before-break so that the output voltage steps seamlessly from one voltage to the next, or is it break-before-make, allowing the output voltage to assume an uncontrolled value in the short interval between one switch position and the next? Not that you should ever be choosing the voltage AFTER you've connected the PSU to the equipment.

Those universal / reversible adaptors also carry with them the hazard of human error - selecting the wrong voltage and / or polarity by accident. For equipment which has already survived for 40 years or more I prefer to choose a dedicated power supply for it and keep that PSU with the equipment at all times.

If a power supply has a marked mA rating of 'x' mA I wouldn't usually push it further than that.

Being the voltage is 9v max, more isnt a worry...

 

[SiriusHardware]

In the specific case of using it with a Spectrum of course that's true. I was musing more generally.

However, did you realise that the original Spectrum PSU was rated at 1.4A? Granted, that rating will have been chosen with the (optional) Interface 1 / Microdrive in mind, but 300mA still feels very underrated even for the base machine.

 

[Mr.ToR]

Does anyone have any opinions about using the Traco TSR 1-2450 step-down switching regulators in place of the 7805? Apparently it is even possible to completely remove the large heat sink. I'm thinking removing the main heat source from inside the enclosure should be beneficial for the chips, especially for the ULA, right?

 

[daver2]

I posted some back in #6...

Less heat - but potentially more electrical noise and (depending upon the existing design of computer) potential instability issues.

Dave

 

[twolazy]

Alot of cordless phones used 9v transformers...

 

[Mr.ToR]

I decided to investigate how the original sinclair euro 1400 zx power supply is compared to a cheap smps.
Additionally I compared the 7805 regulator to a pin compatibe step down buck converter.
I checked voltages, pal signal and thermals.
Here are the results.

First I checked everything with 7805 and euro 1400 as a baseline.

ULA temperature: ~68C° 7805 temperature: ~58C° Heat sink temperature: ~45C° (I put a tape on the metal for the thermal camera to avoid reflections)

the sinclair euro 1400 zx power supply extrenal thermals when connected to a spectrum plus (after 30 min. use)	the spectrum plus extrenal thermals with 7805 (after 30 min. use)

This is the input voltage on the 7805 from euro 1400	12V pin of RAM
5V pin of RAM	-5V pin of RAM
Composite PAL Signal

In my next post I will check everything with a 0.6A 9V cheap smps and with a step down buck converter in place of the 7805.

 

[Mr.ToR]

here is my findins on when the following is used:
the smps i used is called t090060-2c1. you can find it branded under tplink or vasata.
the regulator i used is dd4012sa




this after about 30 minutes of use


input voltage at the step down buck converter regulator


buck regulator output


ram 5v pin


ram -5v


ram 12v


pal signal


an this is the consumption


The image on the monitor screen is vitually identical to before.

I can't see a reason to not use this.

Also ,the standalone consumption of the smps is 0 watts compared to 2.4W of euro 1400.

I hope this would be beneficial to someone.

 

[Mr.ToR]

If you are just thinking of just using the step down buck converter regulator in place of the 7805 with the original sinclair power supply (euro 1400 in my case);

this is the consumption of the original setup with 7805 and euro1400


and this is the consumption of euro 1400 with buck converter regulator


It seems that the 7805 alone wastes about 3.2 Watts of unnecessary power as heat into the system.
Considering that the total consumption becomes 5 Watts with SMPS and Buck regulator,
disregarding the smps inefficiencies. we can safely say thay 7805 alone wastes more than half of the total system concumption,
which is actually about 65%.

It is almost a crime not to get rid of the 7805.

 

[bobmoo79]

I agree, I changed out the 7805 almost straight away, mainly because I wanted to reduce the heat rather than due to economy but both are linked. Interesting results, thanks for sharing.
Which thermal camera did you use to take the photos?

 

[Mr.ToR]

I agree, I changed out the 7805 almost straight away, mainly because I wanted to reduce the heat rather than due to economy but both are linked. Interesting results, thanks for sharing.
Which thermal camera did you use to take the photos?

Sorry for the late reply.
Flir one for ios

 

[radventure]

As an option, I've also used L78S05CV where traco and similar weren't recommended due to noise etc(mega drive). They run a bit cooler, are rated for 2A and seem to be a halfway point.The lead size might need considerations though