5VCPC

[Bryce]

Update: The PCBs arrived last night. I'm still waiting for some other parts, but I have enough parts to make one. As soon as I have time to solder one up, I'll post some pictures of the final device.

Bryce.

[Bryce]

Managed to find time to solder one up tonight. Here's what the final device looks like. Over the weekend I'll be testing it to the extreme to find out exactly where the limits are.

Bryce.

[Gryzor]

Result! It looks so professional... (but of course )

[robcfg]

Nice!


Sent from my iPhone using Tapatalk

[1024MAK]

Managed to find time to solder one up tonight. Here's what the final device looks like. Over the weekend I'll be testing it to the extreme to find out exactly where the limits are.

Looking good 
No, wait! Something's missing. A vital part is missing 
Here, that's what's missing 

Mark

[||C|-|E||]

Very nice!    There are some big caps in there 

[Bryce]

Looking good 
No, wait! Something's missing. A vital part is missing 
Here, that's what's missing 

Mark

Still doing the QC at the moment, so the stickers can't go on yet (Unlike Chinese devices that get the sticker without ever having been tested).

Bryce.

[Bryce]

Parts arrived at the weekend and I was able to build a few more devices to do parallel testing. One is installed in my 6128 which my son is "testing" intensively with his favourite games - Sabre Wolf, Jet Set Willy, Harrier Attack and Scorcery.

I've also done some proper testing on the others and here's some interesting info:
- The 5V PSU needs to be able to supply at least 2.5A during peaks, anything below this will cause problems.
- The device never gets even warm no matter how much disk reading you do.
- The smaller device fits much better inside the CPC.
- If the PSU is not powerful enough the CPC will reset as soon as you try to access a disk, no other effects.
- Some picture darkening (as seen of the CPC6128+) will happen on the CPC if the PSU is close to it's limit, but only on CRTs, this doesn't happen on LCDs or other displays connected via an RGB to VGA/HDMI device.

One strange side effect I've found is due to something I completely overlooked: The very second that the motor starts in the drive, just happens to also be the exact moment that the CPC also needs as much power as possible. This is because almost every chip on the CPCs mainboard is being used at that exact time. Because of this, there is a sudden requirement for lots of current. Not an issue usually, but I discovered one CPC that would reset at that moment, although I was using the same PSU that didn't have this problem on other CPCs. It turned out that the electrolytic capacitor on the mainboard was bad so it couldn't handle the sudden current spike. The CPC would work fine otherwise. So if resets happen and you know your PSU is good, then there's a chance that the capacitor on the CPC is bad and you've just not known. The CPC will actually run without this capacitor in place if you have a decent sizes PSU attached, so I suspect this capacitor has been bad for a long time and IV'e never noticed.

Bryce.

[Bryce]

Had time to finish the orders today. After testing them I'll be contacting the interested parties for postal details...

Bryce.

[||C|-|E||]

Look at them! Very nice! 

[robcfg]

Hooray!


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[Bignumbas]

I like this idea but what about going the other way using 12v to provide 5v, surely a more efficient way.
Wouldn't one of those common external hard-drive power packs rated at 2A be enough?
My 6128 rating adds up to approx 15w (5V x 1.7A and 12v x 0.4A)
And you can buy small modules premade from China.
These could be used to supply 5v to external drives like goteks, 3.5 floppies etc.
I know this will upset people that like to make things themselves but for the rest of us.....


http://www.ebay.com.au/itm/1Pcs-Mp2305-Dc-Dc-Buck-Sent-Down-Converter-Module-7-20V-12V-19V-To-5V-2A-Car-I-W-/282283605224?_trksid=p2141725.m3641.l6368

• Input voltage: DC7-20V limit 23V
• Output Current: 2A
• Work efficiency: about 92% (input 12V, output 5V1A)
• Output ripple: less than 60mV P-P
• Overload protection: protective current of about 3A
• Short circuit protection:
• Output voltage: DC5.25V + -0.05V
• Reverse Polarity Protection: None

[Bryce]

I did consider this direction when I was designing the device, but there are a few disadvantages of going this way. The 5V rail can need a lot more than 2A quite often. The CPC is rated at 1.7A for the bare machine. Actually in reality it only pulls about 1.2A for a bare machine sitting idle, however, as you know, people now strap on extra RAM, ROM, RS232, USB Mice, WiFi, LAN, Spy Satellite Control Systems (maybe that's just me ) and other great stuff. 2A isn't enough for a modern CPC setup. A robust DC/DC converter that can supply 3A is a big chunky piece of hardware at least three or four times the size (and cost) of my device.
It's easier, smaller and cheaper to create a 12V 400mA (4.8W) supply from 5V than trying to create a 5V 3A (15W) supply from 12V.

Now let's take a look at the Chinese device, and a problem that people often get caught out with. The Chinese advert states the MAX figures for all values and these are the values for the IC, not for their circuit. You can not combine these: Can it supply 2A - Yes probably, Can it supply 5V - Most likely. Can it continuously supply 5V at 2A - Definitely not. You only have to look at the size of the components they've used to know that something's not right. 5V at 2A is 10W of power. Does that circuit look like it can handle 10W of power continuously? According to the datasheet, the chip can dissipate an absolute max of 1.39W. They claim that at 5V 1A it runs at 92% efficiency, so let's assume it runs at 88% efficiency at 2A (being very generous here as that's the value for 3.3V, no value for 5V in the datasheet and this assumes an ideal circuit). This means that at 5V 2A running at 88% efficiency, it would need to dissipate >1.2W, that's going to be one hot little chip with a relatively short lifespan.

External Components:
According to the datasheet, the minimum inductor current value can be calculated using the formula:
Ilp = Iload + (Vout / 2 x fs x L) x (1 - Vout / Vin) .... Let's grab the calculator.... = 2.428A
(I've chosen a 10µH Inductor, as it's the one used in the datasheet and most likely what they used)

Now check the internet for what a 2.5A 10µH Inductor looks like. It's 5 times the size of what they've used. They also chose a non-shielded inductor, so you have a very hot 340kHz transmitter inside your CPC. Now we've got and underdimensioned inductor to deal with too. This will completely invalidate their efficiency claim and the circuit will already have reached its limit somewhere slightly above 1A.
You'll also notice that they didn't even bother including the required electrolytic capacitor, just some cheap ceramic caps, so the ripple on the 12V side could and will be massive (and they conveniently don't state values for this). They also seem to have left out the bootstrap diode. So really they've just slapped together the absolute minimum required so that a voltage will come out at the other end.

There is a very good reason why this device costs $1.84... It's because that's all it's worth. I know the Chinese have drastically brought down the cost of electronics, but expecting to get a reliable 5V 2A supply from a tiny slapped together $2 PCB just isn't realistic.

Buy some for yourself, connect them up to a load and push them up to the claimed limits and do some measurements. Let me know how it goes and be careful not to burn your fingers.

Bryce.

[1024MAK]

And video it, so we can all watch the magic smoke come out and go "Oooo!" 

[||C|-|E||]

I guess that if I was using a 12V PSU I would make a module containing one of those good quality 5V voltage regulators, a decent heatsink and the required caps. Problem is that heat disipation would be noticeable in this case, at least pluging the 12V output directly to the regulator.

[Bignumbas]

Wow, Bryce, I am humbly whipped back in to place now!
I am now really scared of all those thousands of cheap chinese 12v/5v power pak devices I have driving all my hard-drives, modems etc.
I assume they would have the same standard of circuitry in them. All ready overheat and smoke.

While you are still full of info do you know how much current a 5v Gotek drive draws?
If I do get any of those 12v to 5v circuits I will definitely video what happens.
Reminds me of the other day when I shorted out 240v AC in a power supply, now that was a bang!
When I worked as an electrician it was good to cut a live cable once with your pliers cause then you had a wire stripping hole built into them.   

[Bryce]

Wow, Bryce, I am humbly whipped back in to place now!
I am now really scared of all those thousands of cheap chinese 12v/5v power pak devices I have driving all my hard-drives, modems etc.
I assume they would have the same standard of circuitry in them. All ready overheat and smoke.

While you are still full of info do you know how much current a 5v Gotek drive draws?
If I do get any of those 12v to 5v circuits I will definitely video what happens.
Reminds me of the other day when I shorted out 240v AC in a power supply, now that was a bang!
When I worked as an electrician it was good to cut a live cable once with your pliers cause then you had a wire stripping hole built into them.   

My intention wasn't to "whip you back into place", just inform.

I don't have a Gotek to test, but it's based on an ARM Cortex STM32F105 and a few external bits, so I can't image that it needs more than maybe 300mA at the very most depending on the USB stick used. To be compliant with the USB standard, the supply should be able to provide at least 500mA + whatever the circuit needs because that's what the USB standard requires.

You don't need to be scared of those cheap power packs as long as you aren't using anywhere close to the amperage they claim, but I still wouldn't leave them turned on when I'm not home.

Bryce.

[Skunkfish]

What 5V PSU's are people using for their CPC's?

Would something like the below be suitable?

https://www.lindy.co.uk/power-c8/power-accessories-c341/multi-country-switching-ac-adapter-5v-dc-2-6a-2-5mm-inner-5-5mm-outer-dc-jack-type-p7406

It seems all the ones I can find are 'switching' rather than 'linear'?

[||C|-|E||]

I guess that nowadays linear PSUs are used mostly for audio only    As Bryce said, you can also buy a nicely filtered switching PSU, that is more efficient and less bulky that the linear ones. I am kind of a dinosaur in that regard and I like to use linear PSUs and CRTs with my machines from the 80s, since it is they way they were originally conceived, but I am sure that Bryce can point you to a nice switching PSU    On the other hand, if you do not use a CRT, it is unlikely that the switching PSU will cause major problems, even if it is not perfectly filtered.

[Bryce]

What 5V PSU's are people using for their CPC's?

Would something like the below be suitable?

https://www.lindy.co.uk/power-c8/power-accessories-c341/multi-country-switching-ac-adapter-5v-dc-2-6a-2-5mm-inner-5-5mm-outer-dc-jack-type-p7406

It seems all the ones I can find are 'switching' rather than 'linear'?

For a bare CPC with no expansions that would be fine. Regarding the amps stated even on good PSUs: You should aim to be using slightly above 50% of its capacity, so if the device you are powering needs 2A, go for a 3.5A supply. The 2.6A stated on the PSU is the absolute maximum it can supply and you don't want your PSU to be running a max the entire time. It will get hot and die earlier than expected.

Bryce.

[Bryce]

All devices have been tested (just one failed). So I will be sending PM's out later today.

Bryce.

[Trixster]

Great stuff! Looking forward to getting mine.

[Bryce]

As I'll be shipping this week, a quick note to those who are waiting for their device to arrive: The connector to the drive is deliberately mounted at a slight angle. This isn't a mistake or something that happened in the post, so don't try to bend it back to 90° It's meant to be like that. The PCB sits at a slight angle so that it fits inside the case. It's a very tight fit. See attached picture.

Bryce.

[1024MAK]

Hi Bryce - thanks for this information 

Mark

[||C|-|E||]

That is actually a good advice!  knowing my OCD I would have probably tried to desolder the connector and put it straight