Forums

5 volt supply straight from 240v AC mains

Started by techie_alison July 13, 2006
Hi,

Please may I ask what the arrangement is when you see a single LED powered
straight off of a mains power supply without any transformers or switch mode
circuitry?  In other words, totally uninsulated or regulated.

I have an old computer with an external hard disk which needs about 30
seconds to spin-up before the computer.  With a small timing circuit, 555,
or using a PIC even (have dozens) after a set time a relay would be set,
thus powering on the computer.  A 7805 could be introduced to take into
account the voltage swing.  Half wave rectification could result in 120v
too.

This doesn't need to be insulated from the outside world, safety is not a
concern, just that roughly 5v should be available for the small circuit and
the 3amp relay.

Any ideas??  Just interested to hear of how this is done.  Or would it be
easier to just buy a small tordial TX and make the box a bit bigger?

Thanks,

Aly


"techie_alison" <retro@dial.pipex.com> wrote in message 
news:n9CdnRSafcwThCvZnZ2dnUVZ8tCdnZ2d@bt.com...
> Hi, > > Please may I ask what the arrangement is when you see a single LED > powered > straight off of a mains power supply without any transformers or > switch mode > circuitry? In other words, totally uninsulated or regulated.
I think you'll find that that is a neon lamp and not an LED. LED's connected directly across the mains will give light in the form of fire.
> I have an old computer with an external hard disk which needs about 30 > seconds to spin-up before the computer. With a small timing circuit, > 555, > or using a PIC even (have dozens) after a set time a relay would be > set, > thus powering on the computer. A 7805 could be introduced to take > into > account the voltage swing. Half wave rectification could result in > 120v > too.
Why not buy a 5V supply to run the electonics off rather than mess with mains yourself?
> This doesn't need to be insulated from the outside world, safety is > not a > concern, just that roughly 5v should be available for the small > circuit and > the 3amp relay.
SAFETY IS THE PRIMARY CONCERN! Mains is not something to be fooled with or it WILL kill you. Insulate everything or when you (or somebody else) is not at full concentration you will touch something and die. There should be fuses and other protection in the circuit as well. This is why you should seriously think about buying a supply and letting someone else handle the high voltage design - and the legal concerns that go with it.
> Any ideas?? Just interested to hear of how this is done. Or would it > be > easier to just buy a small tordial TX and make the box a bit bigger?
If it were me then I would use 5V from the Hard drives supply to trigger a PIC (but a 555 or an RC circuit would be just as good) to control the PC's power switch. If the PC has an ATX supply, then great because you can stay low voltage and use a simple relay to replace the PC power switch (remembering that only a pulse is required to simulate a buton press). If the PC supply is AT then the power switch is mains and you could use a mains relay switched by 5V but you need to be sure that the terminals are properly insulated on the mains side. Seriously, your tone doesn't sound like you are giving mains electricity the respect it needs. I know enough people who have died from electric shocks and each time it was because they thought safety wasn't a concern. Be careful.
"techie_alison" <retro@dial.pipex.com> wrote in message 
news:n9CdnRSafcwThCvZnZ2dnUVZ8tCdnZ2d@bt.com...
> Hi, > > Please may I ask what the arrangement is when you see a single LED powered > straight off of a mains power supply without any transformers or switch > mode > circuitry? In other words, totally uninsulated or regulated. > > I have an old computer with an external hard disk which needs about 30 > seconds to spin-up before the computer. With a small timing circuit, 555, > or using a PIC even (have dozens) after a set time a relay would be set, > thus powering on the computer. A 7805 could be introduced to take into > account the voltage swing. Half wave rectification could result in 120v > too. > > This doesn't need to be insulated from the outside world, safety is not a > concern, just that roughly 5v should be available for the small circuit > and > the 3amp relay. > > Any ideas?? Just interested to hear of how this is done. Or would it be > easier to just buy a small tordial TX and make the box a bit bigger? > > Thanks, > > Aly
Half wave rectification would NOT result in 120V and a 7805 can't drop anywhere near that much voltage. If you don't know this then it would be a very bad idea for you to make any live circuit. I recommend that you use a wall wart for your 5V. Having said that, a mains rated capacitor in series can be used as a sort of voltage dropper followed by a rectifier to get low voltage. I actually found a circuit like this in a Russian battery charger from the 60s. Yikes! You get small size at the expense of safety and I don't think that its worthwhile. Peter
> >
On Thu, 13 Jul 2006 11:07:16 +0100, "techie_alison"
<retro@dial.pipex.com> wrote:

>Hi, > >Please may I ask what the arrangement is when you see a single LED powered >straight off of a mains power supply without any transformers or switch mode >circuitry? In other words, totally uninsulated or regulated.
Typically a series capacitor is used with a capacitive reactance sufficiently large at the mains frequency to limit the current to a desired value. Due to harmonics and other high frequency noise usually present on the mains, the capacitor reactance at these high frequencies are quite low, thus quite large peak currents could flow, thus, it is a good idea to also include a series resistor. These kind of circuits only makes sense with very low current demand, since at higher currents, the capacitors become quite large.
>This doesn't need to be insulated from the outside world, safety is not a >concern, just that roughly 5v should be available for the small circuit and >the 3amp relay.
In order to minimise the load current and hence the required capacitor size, it would be a good idea to use a relay with a large coil voltage (and hence low coil current) driven by a high voltage open collector output or a separate high voltage transistor. Paul
"Paul Keinanen" <keinanen@sci.fi> wrote in message
news:pb9cb2tgno2qqqpq8dmepmq9b2cvehhjnd@4ax.com...
> On Thu, 13 Jul 2006 11:07:16 +0100, "techie_alison" > <retro@dial.pipex.com> wrote: > > Paul >
Mmmm, hi Paul Thanks :-) I think I'll stick with a small toroidal. Much more in line with what I've done before, just had seen the LED concept out there before and wondered how applicable it was. Aly
techie_alison wrote:
> Hi, > > Please may I ask what the arrangement is when you see a single LED powered > straight off of a mains power supply without any transformers or switch mode > circuitry? In other words, totally uninsulated or regulated. > > I have an old computer with an external hard disk which needs about 30 > seconds to spin-up before the computer. With a small timing circuit, 555, > or using a PIC even (have dozens) after a set time a relay would be set, > thus powering on the computer. A 7805 could be introduced to take into > account the voltage swing. Half wave rectification could result in 120v > too. > > This doesn't need to be insulated from the outside world, safety is not a > concern, just that roughly 5v should be available for the small circuit and > the 3amp relay. > > Any ideas?? Just interested to hear of how this is done. Or would it be > easier to just buy a small tordial TX and make the box a bit bigger?
<standard disclaimers re: HVAC apply> Put a bridge across the mains (rated at the appropriate peak to peak voltage plus margin ~300V+). From the + output of the bridge, feed the anode of an LED. Connect the cathode of the LED to the collector of a high voltage NPN. Put ~22K from collector to base. Tie the base of this first Q to the collector of another NPN. Tie the base of that second Q to the emitter of the first Q. Connect base of second Q to second Q's emitter through ~330ohm. Connect emitter of second Q to bridge's - output. Make sure all your components can handle the rectified line voltage. And, you'll have that full rectified line across the first Q with Iled flowing through it so make sure it can dissipate the power. Adjust the 330ohm for Iled/brightness. If you don't know what you are doing, then don't do it! :> HTH, --don
Peter wrote:
> Having said that, a mains rated capacitor in series can be used as a sort of > voltage dropper followed by a rectifier to get low voltage. I actually found > a circuit like this in a Russian battery charger from the 60s. Yikes! You > get small size at the expense of safety and I don't think that its > worthwhile.
I still have a US made battery charger from the 60s that is composed of: * 2 safety interlock switches to disconnect both sides of the line when the lid is opened. * a rectifier. * a 4 watt night light bulb. Marc
techie_alison wrote:
> Hi, > > Please may I ask what the arrangement is when you see a single LED powered > straight off of a mains power supply without any transformers or switch mode > circuitry? In other words, totally uninsulated or regulated. >
If memory serves, I have seen this done once with an LED using a pair of resistors as a voltage divider - it might have even just been the one resistor. Personally I think it was cheap workmanship though. This will work with an LED, although it will flicker as it's only illuminated for less than half the time, I think it can work with only one resistor but only when you know the resistance of the device it is powering. For hard disks this is a non option - as thier resistance varies wildly as it spins up, settles, and does all its internals stuff, and the resistors would have to be big mothers as hard disks draw a hefty current. No no no no no
> I have an old computer with an external hard disk which needs about 30 > seconds to spin-up before the computer.
forgive me if I seem a little dense here... but if it's an external hard disk, does it not have its own regulated power supply? could this not be utilised in some fashion? but what sort of interface is it? ide, scsi, parallel? that might give us some clues as to what would be a good solution.
> With a small timing circuit, 555, > or using a PIC even (have dozens) after a set time a relay would be set, > thus powering on the computer. A 7805 could be introduced to take into > account the voltage swing. Half wave rectification could result in 120v > too. > > This doesn't need to be insulated from the outside world, safety is not a > concern,
safety is always a concern.
>just that roughly 5v should be available for the small circuit and > the 3amp relay. >
I need a bit more information on your set up really, but I can envisage two possible scenarios: scenario 1) you have a true external hard disk, in an external hard disk caddy with all the wiring and gubbins and some sort of parallel interface whotnot. The drive is powered independantly of the computer. If this is the case I would utilise the regulated 5v line powering the hard disk however, I think scenario 2 is more likely: scenario 2) You have a hard disk that is external of the computer - connected via an IDE lead and the computers power supply. You need the hard disk to power up before the computer - although I dont know why at this stage, but i'll pretend for now there is good reason for it. If this is the case, you NEED a good regulated power supply for the drive, and not just the 5v line if its a 3.5" drive - 12v will be required as well or it wont spin up. 2.5" disks generally only require the 5v line. Your best bet is a regulated 5v/12v power supply - which should be fairly simple to construct. then simply run the timer circuit off that supply.
Marc Guardiani wrote:
> Peter wrote: > >>Having said that, a mains rated capacitor in series can be used as a sort of >>voltage dropper followed by a rectifier to get low voltage. I actually found >>a circuit like this in a Russian battery charger from the 60s. Yikes! You >>get small size at the expense of safety and I don't think that its >>worthwhile. > > > I still have a US made battery charger from the 60s that is composed > of: > * 2 safety interlock switches to disconnect both sides of the line when > the lid is opened. > * a rectifier. > * a 4 watt night light bulb.
Yeah. Sounds like a good idea. In my youth I had an early model airplane R/C set. The battery pack was charged with an on- line charger. I can painfully remember touching the wrong part and doing the chicken walk.
Go to Microchip's website and look at the AP note TB008 "Transformerless 
Power Supply".  The example is for 115 VAC, but the concept is the same.

Don