EEVblog #291 – USB Power Supply Part 2 – The Housing

[darrenb]

I understand the concern with the housing. However I have yet to find another suitable case that meets the credit card size form factor with removable end and at a comparable price.
If you know of a specific one that does, please point it out.
I expect the PSU to be handheld a lot, and possibly operated while in the hand, rendering the curved bottom moot in those cases.
It's a compromise I thought was worthwhile given case availability and price.
As usual, people like to throw around all sorts of solutions, but they haven't really looked in to the finer details and costs of it, and that's when you often "come a gutsa".

Hi Dave,

Remember the logic analyser that you pulled out of the mailbag recently and you though it had a great case?  It was just an aluminium extrusion with some customisation done to the end panels.

I'm working on some at the moment and a picture of them is below.  On the left is a single prototype of a customised extrusion.  There are 250 more of them currently being manufactured and hopefully they will be here next week.  In the middle is an unmodified sample of another extrusion type which I will have customised.  On the right is a commercial battery charger which was the inspiration for my cases and which would probably be fantastic for yours as well.

The fully customised product on the left will cost $5.04 each (plus a bit more for shipping by the time they land in Castle Hill) which is a bit more than than the $2 that you're intending to spend.  But keep in mind that this is not just a bare case, it also includes solutions for all of the mounting problems you might have, as well as screen printing to finish off the appearance of the unit.

[EEVblog]

Using the LCD from the battery powered PSU project would save on inventory and improve the BOM scalability.

And cost about 15-20 times more.
I would have to change the bench display solution to suit the USB requirements.

Dave.

[EEVblog]

I'm working on some at the moment and a picture of them is below.  On the left is a single prototype of a customised extrusion.  There are 350 more of them currently being manufactured and hopefully they will be here next week.  In the middle is an unmodified sample of another extrusion type which I will have customised.  On the right is a commercial battery charger which was the inspiration for my cases and which would probably be fantastic for yours as well.

The fully customised product on the left will cost $5.04 each (plus a bit more for shipping by the time they land in Castle Hill) which is a bit more than than the $2 that you're intending to spend.  But keep in mind that this is not just a bare case, it also includes solutions for all of the mounting problems you might have, as well as screen printing to finish off the appearance of the unit.

I have the same Tunigy charger.
Where did you get them customised from and what brand/model?
That's very cheap for low run extruded alloy case.

Dave.

[Zad]

Crikey, I didn't realise I2C LCDs cost so much more than 7-segment LEDs!

[EEVblog]

One thing though - since this is a USB powered PSU, do not you think the 7-segment LED displays will chew up a lot of current from the USB bus ? Considering minimum and maximum current draw, assuming you are not doing any fancy multiplexing on the display segments.
If the display shows "1", it means only 2x LEDs are lit up, while worst case scenario could be all displays showing "8888". That would not leave much room for the DC-DC converter. Or am i missing something here ?

You aren't missing anything. That is why I said I am dropping the 7segment LED displays.

Dave.

[EEVblog]

I think Dave's video raises a couple of really important points, for anyone who is considering making a low-volume product (maybe a few hundred or less). Any custom case (especially any custom mouldings) could easily end up costing you more than the elctronics! You want something cheap, and yet it doesn't want it looking like something a seven-year-old threw together, and that is this.

Start with the case first, then figure out how to get your electronics in to it!

The second point, which he only briefly alludes to, is designing your front of back panel at the same time as your PCB. Choose a sensible colour (like a black solder mask with white screen printing and viola - a professional looking part, for pennies.

I used exactly these two tricks for a bunch of miniature nixie clocks that I have designed. The result (I think), is quite pleasing and professional.

That looks very nice, and yes, I should have actually been clearer on those points, that is exactly what I was implying.
Although for any volume machine assembled run you don't want to combine them like this, as it chews up your usable circuit panel space. So you won't save cost you'll likely increase it. It's the bomb for prototypes though.

Dave.

[Uncle Vernon]

Where did you get them customised from and what brand/model?
That's very cheap for low run extruded alloy case.

A lot of us would be more than interested at those kind of dollars for <1000 batches . Me included!

[darrenb]

I've PM'd Dave and Vernon with the the name of the factory.

I don't think this factory is unique however and there are plenty of manufacturers out there.  To find them go to alibaba.com and search for "Aluminium Extrusion Enclosure".

You'll then need to spend heaps of time learning how to deal with these factories.  Dealing with their MOQ's and learning what FOB or Ex-works means.  But it's worth it.

For example I need to have 4 tactile switches in each unit I'm making.  At Jaycar they are about 90 cents each and the cheapest I can find them at Digikey is about 30 cents each which doesn't sound like a lot, but when you multiply by four and then add four button caps as well it's adding dollars to the project.

From a factory in China I can buy them for 1.6 cents each and can therefore buy 1000 of them for $16.  At this price I can put them on my cereal.

[EEVblog]

How much freight would you pay to get like a 500 MOQ delivered to Oz? 

Dave.

[darrenb]

Hi Dave,

The MOQ is somewhat negotiable.  The minimum order quantity on these was 500 but they were able, after a fair bit of negotiation, to supply 250.

Shipping quoted by them for these 250 pcs is USD450 for approx 43kg.  This is for a 2-3 day express courier like DHL to Castle Hill NSW.  I'd like to find a cheaper shipping option but it's something I'm yet to solve.

[ModernRonin]

I like the idea of a clear top, but how about this... Make the clear top be the sides too!

Basically, make it too long left-right, then have the extra length bent downward so you end up with an upside down U-shaped thing. This will give your binding posts something to screw in to.

Keeps the BOM count low, looks more professional, theoretically a little stronger than a top and two end panels.

Only question is, how much does it cost to get perspex/polycarbonate bent like this?

[T4P]

It does have capability to do a proper USB handshake and thus negotiate power consumption, has USB remote control interface for the supply and provides virtual serial port that goes to the isolated side as isolated 5V TTL level RS-252.

Handshake? I wouldn't bother with that one. Every computer i have ( Old thinkpads and new llano laptop ) doesn't need negotiation

[Bored@Work]

Handshake? I wouldn't bother with that one. Every computer i have ( Old thinkpads and new llano laptop ) doesn't need negotiation

How should I explain that to you? The sample of computers you have is insignificant.

[T4P]

Handshake? I wouldn't bother with that one. Every computer i have ( Old thinkpads and new llano laptop ) doesn't need negotiation

How should I explain that to you? The sample of computers you have is insignificant.

Thinkpads from the IBM era
Thinkpads from the now-Lenovo era
HP tx2028AU
An acer 5560G
Acer cheapie from 2008
Fujitsu LH520

Insignificant ? I don't think so

[hlavac]

USB port on my Mac keyboard has limited power

[Christopher]

Hi Dave, I have access to a laser cutter and a pile of 3mm Acrylic/MDF if you want some prototypes cutting out. Im in the UK so shipping could take a while.

Brill project, make sure you include a DC jack, I'll probably use this as my main PS.

[proxime]

How mutch power will the usb power supply be?

Is it possible to make it usb 3.0 (current=900mA) or even more with a Y-cable? (http://www.cavalrystorage.com/cbusb20002.aspx) or even better more and more usb ports support battery charging (1,5A)

[TopGunPk]

One thing though - since this is a USB powered PSU, do not you think the 7-segment LED displays will chew up a lot of current from the USB bus ? Considering minimum and maximum current draw, assuming you are not doing any fancy multiplexing on the display segments.
If the display shows "1", it means only 2x LEDs are lit up, while worst case scenario could be all displays showing "8888". That would not leave much room for the DC-DC converter. Or am i missing something here ?

You aren't missing anything. That is why I said I am dropping the 7segment LED displays.

Dave.

Perhaps you can consider 7-segment display LCD screens, much like you have in DMMs. Ok, they may or may not have a back-light AND probably they would require a lot of I/Os or some driver chip(s), BUT they are very low on power consumption and quite DMM like.

I am designing around such a display in one of my projects - its a Petrol / Diesel (gasoline for the US people) dispenser system. And one of the Key requirements of the client was that the dispenser should continue to display the last result / current result of the counter when the power failed. The LCD keeps displaying the last amount and Litres dispensed even though the system is OFF, with just a AA cell and draws to the tune of uA (without back-light).

[hlavac]

There are nice low current LED displays like Avago HDSP-A101 that need only about 1mA per segment, no need to drop the display!

[Short Circuit]

36mA is quite significant if you only have 500mA to play with...
(2 x 3digit x 6segments on x 1mA

[Christopher]

36mA is quite significant if you only have 500mA to play with...
(2 x 3digit x 6segments on x 1mA

Do you mean 7 segments . Also what about a decimal point?

(Sorry for sounding like a tool but im a bit drunk right now)

Actually not all 7 segments will be lit.. What do you do regarging resisors when only say 2segments are lit compared to all 7, surely 2 will be brighter than 7?

[hlavac]

No you don't do that you multiplex one digit at a time and cycle thru the 6 of them ~60 times per second, that is only 8 segments on max at once or 8mA, thats not so bad - 0.04W
You can do as low as 0.016W with a 2V power rail if you can take the extra cost of one more DC/DC converter. Probably not worth it for the 1mA displays (would be with 20mA segments).

[hlavac]

Actually not all 7 segments will be lit.. What do you do regarging resisors when only say 2segments are lit compared to all 7, surely 2 will be brighter than 7?

No, each segment has its own current limiting resistor so the segment brightness is aways the same no matter how many other segments are on.

For common anode displays it goes: power rail -> digit switch transistor (PNP) -> display anode -> segment bus (8x) -> current limiting resistor(8x) -> I/O pin for the segment (8x)
Digits are switcghed on with another I/O pin per digit. You output LOW on pins for segments you want to light up, then turn on the digit with its pin (low on I/O pin ->resistor->base of the switching PNP transistor). Let it shine for a while, then turn off, activate next segments and turn next digit and so on.
For low power LEDs like the 1mA ones where one I/O pin can take current from all the 8 segments you can skip the transistor and just connect the display anode to the digit I/O pin directly and turn digits on with HIGH.

[ttp]

Hi Dave
Could you please fix the download link and QuickTime button, both point to VCR teardown episode. Thanks!

[EEVblog]

Hi Dave
Could you please fix the download link and QuickTime button, both point to VCR teardown episode. Thanks!

Fixed.
Weird, I think Wordpress is screwing up somehow...

Dave.