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Fixing the AEG CX8-50EB, battery powered vacuum cleaner
#21
Hi - I was thinking this might do for the POWER-BRICK?
https://www.ebay.co.uk/itm/PC-mini-ITX-H...1391959658

And this for the DC-DC converter:
https://www.ebay.co.uk/itm/DC-Converter-...2050865537?

I'm settling for 10A devices to contain costs.
Regards.
Marc

(02-27-2019, 12:55 PM)DarkRaven Wrote: A bench power supply would work, if it's powerful enough, but don't forget that you can't (or shouldn't) run a several meters long DC wire to power the vacuum. That means you basically need to attach the PSU directly to the vacuum to keep the distances as short as possible. An ATX PSU would work as well, with a boost converter to boost the voltage from 12V to 29V.

But I wouldn't suggest either. Instead I'd get a powerful yet compact as possible PSU, like a laptop power brick or similar, and build a suitable DC-DC converter into the vacuum. The power brick can simply be strapped onto the outside of the vaccum and will supply power to the DC-DC converter. It's not pretty but would work well and you can use the AC power cord to connect it to the mains. It's certainly more practical than a bench PSU or ATX PSU.

Or you make a new battery. This sounds like you have Samsung INR18650-30Q cells, these are suitable for the job. Don't use the battery holders though, the current will be too high. You can either spotweld the cells together with nickel strip (around 8x0.2mm should be fine for 10A to 15A) or solder the cells together. I initially wanted to spotweld the cells together but I could only get 6x0.15mm nickel strip which is a bit too small for my taste. If I pick it up again at some point I'd probably solder the cells together using a solid copper wire.

Hi - I was thinking this might do for the POWER-BRICK?
https://www.ebay.co.uk/itm/PC-mini-ITX-H...1391959658

And this for the DC-DC converter:
https://www.ebay.co.uk/itm/DC-Converter-...2050865537?

I'm settling for 10A devices to contain costs.
Regards.
Marc

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#22
No, it doesn't work like that. You need to understand that voltage and amps multiplied is power and 10A alone is useless and doesn't mean anything. Also, you can't just say that you're settling for 10A devices to save costs. What if the vacuum draws more than that? You'll end up with it not working properly (or at all).

The power brick would need to be at least 250W. Keep in mind that this is based on guessing the total power of the vacuum. Mine is around 230W, but yours could be a bit higher. Also, DC-DC conversion doesn't happen at 100% efficiency and you have to take this into account when sizing the PSU. I guess you have no way to actually figure out how much power your vaccum uses I'd say go for 300W or even 350W. Your vacuum will need at least 230W and the DC-DC conversion will make you loose another ~10%.

The brick you've found will do 10A but only at 12V, that is just 120W maximum. This is barely half of what you need. You could get away with a 1A PSU, if it puts out 350V. Something like this doesn't exist, it's just an example. But it shows you that it doesn't matter how much amps you get on the input because you're doing DC-DC conversion anyway. The power brick can be any voltage as long as it's within the input spec for the DC-DC converter and the converter needs to do ~30V at ~10A at least.

You could also try to find an industrial PSU, maybe Mean Well (just to name one manufacturer) offers suitable PSUs that directly convert AC mains voltage to ~30V DC.

If you're looking for a cheap solution you're probably better off buying a corded vacuum cleaner.
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#23
Hi - yes I know how to work out power (my calculation was 10A x 29V = 290W as a ballpark which I thought might be enough). 


 I didn't consider losses due to inefficiency etc.

However, I still think I may not fully understand things - my logic was:

Use a power-brick that can put out 12V @ 10A and step-up the output voltage using the DC-DC Boost Converter to 29V @ 10A to be fed to the motor.

If the devices were 100% efficient (which I know they are not and hence with these particular items this is not a practical solution) is the above correct in principle? in other  words I just need higher-power devices?

Sorry if I'm being thick!

Marc
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#24
(02-28-2019, 08:46 PM)Oramen Wrote: Use a power-brick that can put out 12V @ 10A and step-up the output voltage using the DC-DC Boost Converter to 29V @ 10A to be fed to the motor.

It doesn't work like that. If that was possible then you would be generating power out of thin air and we wouldn't need specs on devices anymore Smile

A 12V PSU with a maximum output of 10A will never put out more than 120W because that is 10A at 12V. You're feeding a maximum of 120W to the DC-DC converter and it will step up the voltage to 29V. If it was 100% efficient then it would output the same 120W that you have put in. At 29V that would be ~4.13A. 29V at 10A would be 290W and these 290W have to come from somewhere, but the 120W input is less than half of what you need. And in reality there are conversion losses so you're probably getting only 90% of what you have put in.

For this to work with a 12V PSU it would need to be a 350W unit capable of almost 30A (12Vx30A = 360W). Then the DC-DC converter could take it (if it can take 30A input that is) and step it up to 29V while its output current will be limited to ~10.862A (315W, 90% of 350W) because it won't have enough power to provide a higher current, even if it was a 1000W capable unit with a maximum output current of 100A.

Within a closed system the maximum power is never going up, only going down affected by coversion losses. With DC-DC conversion you can convert the voltages but it will affect the current as well. Output voltage going up, maximum output current going down. And vice versa, output voltage going down, maximum output current going up.
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#25
I was breaching the 'law of conservation of energy' or something like that? wasn't I?

I think I get it now  Wink

Thanks for your input and patience!
Marc
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