USB powering of 08M alarm pcb

[Mal Weightman]

Can anyone advise if in secondary school use students will be able to make circuits using the 08M alarm circuit PCB which can be powered via a PC USB socket which I believe can deliver 5V at 100mA, instead of using batteries?

If they can be used will they be able to run any small motors (eg for a desk fan type project) or would they be restriced to running a few LED's?

 

[BeanieBots]

To which "08M alarm circuit PCB " do you refer?

How will you get USB power to it?
(the download cable cannot supply power).

100mA is not very much.
Very few motors (especially cheap ones) will even run at such low currents.
A solar motor would run OK, but that's about it.

The USB 5v line should be protected against excess current draw but do you really want to risk exposing an expensive mother-board electrically to students? If they connect it to something they shouldn't, it could be the end of your PC!

 

[Coyoteboy]

Unless it could be idiot-proofed I wouldn't bother, and that would require making up a new download cable or using a second USB slot for power only, and then fitting some sort of over-current protection. Pager motors might work with 100mA?

 

[hippy]

Welcome to the PICAXE forum.

The USB protocol doesn't support extracting DC power this way although it seems to work in practice.

All it takes is a faulty or mis-wired circuit or a board resting on an off-cut bit of wire or screwdriver to potentially destroy the USB port or damage the PC. You may get lucky, maybe not, I personally would not take the risk.

 

[demonicpicaxeguy]

even though you'll be suprised how bullet proof some of these usb ports can be *they are rare, personally i generally would never use the power from a usb port to power anything but a microcontroller and even then these days i tend to give them a seperate powersupply, usb ports while some of them can be bullet proof, a simple short circuit can mean either the magic smoke come out and one very expensive short circuit, or the usb host controller simply resets and windows has a mild heart attack,
it's cheaper in the long run to just use a whole heap of wall wart power supplies with a 7805 regulator and a capacitor

 

[Andrew Cowan]

I've used it on projects, but I wouldn't trust students to use it on PCs that don't belong to them.

Unregulated wall adaptors can be bought cheaply.

A

 

[SilentScreamer]

A reboot will fix a shorted USB ports and I believe the USB maximum current is 100mA for some laptops and 500mA for desktops and most laptops.

 

[MBrej]

PC USB ports should be current limited or short circuit protected, as this is probably in the USB spec, and should be included by all motherboard manufacturers. If you do short the power you get a notice given by the computer of a power surge on the USB port, and so the should be able to handle whatever you connect to them without damage.

As for the current you can take before it complains as above, you can take 100mA straight away, but they can supply upto 500mA, but the device should ask the host before it takes the extra power. However, most hosts would not complain, and it is something which is probably not conformed to by the various "gadgets" which you can get. Also if you can buy a USB powered fan, there should be no reason why you cannot connect a small motor to one via a PICAXE?

 

[Grant Fleming]

PC USB ports should be current limited or short circuit protected, as this is probably in the USB spec, and should be included by all motherboard manufacturers. If you do short the power you get a notice given by the computer of a power surge on the USB port, and so the should be able to handle whatever you connect to them without damage.

As for the current you can take before it complains as above, you can take 100mA straight away, but they can supply upto 500mA, but the device should ask the host before it takes the extra power. However, most hosts would not complain, and it is something which is probably not conformed to by the various "gadgets" which you can get. Also if you can buy a USB powered fan, there should be no reason why you cannot connect a small motor to one via a PICAXE?

The guy that offers that product shown in that web-link is certainly keeping his hand warm!
Ridiculous.

 

[Dippy]

Mal, hasn't come back so maybe he's blown up his PC by using the mug warmer...?
Lets face it, the makers from that 'neck of the woods' won't exist in a few months time so they don't give a monkey's if your PC breaks.
They'll have had your cash and you'll have to go whistle for warranty.

Certainly the USB current overload protection on my PC is working - I've accidentally tested it several times

But I agree with the previous sentiments - why risk it? - especially with untrained/uncontrolled children.

Or maybe get a powered hub for experiments?

Or just use batteries so that it still works after they've blown up the PC.

 

[Mal Weightman]

Thanks for all that.

I had an idea to try to limit the use of batteries with students in a school environment. The idea wasnt to mess with the download cable, merely to replace the battery with a separate power supply.

Generally regarding the use of USB supplies in schools, a number of secondary schools in the UK are 'doing' a USB 'mini torch' project which charges up something like a 0.47 F memory back-up capacitor to power an LED. The info we have had is that a USB supply is quite robust!

I'll probably steer clear of the USB powered PICAXE alarm PCB project (the Rev-Ed AXE102 one for reference)

Mal

 

[boriz]

"charges up something like a 0.47 F memory back-up capacitor"

Through a suitable current limiting system I hope!

How about you build a USB charger with integrated NiMh battery. The USB supplies (up to) 100mA to charge the batteries. A power socket on the battery box disconnects the USB when a plug is inserted. Almost any amount of current can be drawn, the USB is isolated.

It could also teach energy conservation. The students with the inefficient system may make a big noise, but they run out of power first.

 

[hippy]

The issue with rechargeables is that when class starts the batteries have to be ready, fullty charged. That may lead to leaving the PC on 'unnecessarily' just to charge the batteries via USB.

Rechargeable's may also overheat ( worse ? ) if the circuit is faulty or supply is shorted.

Alkaline batteries are therefore safer but costly. Regulated 5V wall-wart supplies may be the best option.

Another alternative may be a modified PC backplate for 5V out, with a regulator powered off the PC's +12V rail. That's quite a drop plus heat at higher currents so maybe LDO off +5V rail. It would need to be robust enough to withstand shorts without affecting PC operation in any way.

 

[Andrew Cowan]

I like the idea of having a 5V PSU at the front of the room, and charge up supercaps for the students to charge up and run their projects.

A

 

[BeanieBots]

NiMh batteries are expensive and likely to be pinched.
Super caps will only run a moderate load (a few LEDs) for a few minutes.
Also, how long before students realise the fun of shorting out a super cap?

It's hard to beat a cheap wall-wart with series resistor + 7805 regulator.
(plus a few caps as required by the regulator)
The series R chosen to only allow "safe" current draw.
Making a set of regulators to run off maybe just one wall-wart could be a project in it's own right.

 

[Andrew Cowan]

True. On the side of shorting out supercaps, cheap supercaps have a relativly high internal resistance (30 ohms ish), so shorting them out isn't that dramatic.

A

 

[BeanieBots]

That type of 'supercap' are only meant for memory backup.
They 'collapse' if more than a few 100uA are drawn which could limit there useability for 'powering' a PICAXE circuit.
They also take +48Hrs to charge.
They are more like a low capacity battery with series resistor in many respects. Great for what they are intended but probably not this app.

Try it!

 

[Haku]

Hope nobody minds me digging up this old thread.

On the subject of super capacitors, I saw a Gadget Show episode from the last series I think it was where they demonstrated a cordless screwdriver that charges in 45 seconds, the Challenge Xtreme 45 Second Flashcell Cordless Screwdriver. But when the screwdriver costs £50 I dread to think how much the capacitors cost, unless they're just cashing in on the novelty of quick charging cells that don't degrade over time like regular batteries.

I've been programming & powering my test projects almost soley from a USB port on my Asus Eee 900, it even gives enough power to run two servos (I took the analogue stick from a dead PS2 pad to control them with), but have to agree with an earlier poster; you don't want to give circuitry newbies the chance to short out USB ports, external PSUs are cheaper and easier to replace than PCs.