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5 volt supply straight from 240v AC mains

Started by techie_alison July 13, 2006
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? > > Thanks, > > Aly
The relay will draw a substantial amount of current and it is challenging to make a circuit that can supply this much current from the mains without wasting (and having to get rid of, as heat) more than a watt (and that's if you use a 48V relay too, low voltage relays will be worse.) A small transformer would be a nicer solution. By the way, a "solid-state relay" will draw less current than the coil of a mechanical relay though it may not like charging the capacitors of a switched-mode power supply. If you must have something cheap, would it suffice to get rid of the relay (which I guess was to apply power to the computer) and then open up the computer and insert a circuit with a PIC or whatever to hold the computer in reset for 30 seconds after the computer's power comes on? That way there is no new mains wiring, just a bit of logic. If you can't open the computer for whatever reason, is there a connector on the computer which provides +5V from the computer power supply, and also a reset signal which can be pulled low to reset the computer? I am guessing that this is a *really* old computer, correct me if I'm wrong. Chris
Tom Lucas wrote:
...>
> 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. >
Actually, the neon lamp will explode :-). Unless you're careful to get a neon _indicator_, which will have the necessary current limiting resistor built into the package. John Perry
> If you must have something cheap, would it suffice to get rid of the relay > (which I guess was to apply power to the computer) and then open up the > computer and insert a circuit with a PIC or whatever to hold the computer > in reset for 30 seconds after the computer's power comes on? That way > there is no new mains wiring, just a bit of logic.
Or add a capacitor/resistor combination across the reset jumpers that will pull the reset signal low for however long you need to keep the PC from starting.
Tom Lucas wrote:
> "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. > >
Actually Some power strips use and LED. And a large resistor.
On 2006-07-13, techie_alison <retro@dial.pipex.com> wrote:
> Hi, >
> 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?
there's 5V on the disk drive power connector plug (inside the external case) there's 12V there too which is probably better suited to driving a relay with mains contacts. 555s will operate off 12V, and can drive many relays directly. Bye. Jasen
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. > >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 >
Piece of cake if you know the current consumption of your circuit. I run LED's and small circuits directly from the 120 VAC 60 HZ mains all the time. Use a capacitor to drop the voltage. - no heat and no energy wasted. You take your circuit - measure the current and voltage it needs. Subtract the voltage from the mains supply voltage and calculate a "dropping capacitor" for the voltage you want to drop with the current you need.. Use the formula for capacitive reactance. Put the cap and a 100 ohm 1/2 W resistor in series with one side of the mains - the resistor is to limit inrush current as the cap charges and works as a fuse if the cap shorts. That goes to your four diode full wave bridge rectifier. Here I use ~.47 ufd/250 Volt caps for lighting a LED or two or three in series from the 120/60 mains If you are just interested in using an LED on the mains supply - you can just bypass the LED with a small diode to keep the reverse voltage from getting over .6 volts instead of using a rectifier or use back to back LED's and light two with no rectifier. The cap should be a non-polar type with an AC rating for the mains voltage or 3X the DC rating. Don't use the circuit with no load - the output voltage will be high. When powering circuits you have to take into consideration your load - if you're driving a relay, for instance, the current will be much higher when the relay is energized. So in addition to solving the capacitive reactance for the relay current + circuit, you also have to give it some means to prevent voltage overshoot when the relay is not energized - a simple shunt regulator with a single 1 watt zener may be all you need - I posted the data from the Siemens 1990 Optoelectronics data book "Operating LEDs on AC Power, Appnote 6 " on alt.binaries.schematics.electronic a couple of months ago Another way to steal a little bit of power that may come in handy for your disc spin up timing - a current transformer. Just add a few turns of wire to a transformer with an open core (like a toroid or side by side bobbin transformer) and put the load on the low voltage windings - A trick we frequently use to turn on solid state relays when an X-ray machine or other dangerous piece of equipment is running - to sound horns or work warning lights. A little trial and error involved - but that ties the output to a load - whenever the load is drawing current the output is there. ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =----
In article <n9CdnRSafcwThCvZnZ2dnUVZ8tCdnZ2d@bt.com>,
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.
First off it's a dangerous one. A non isolated, non stepped down, non regulated circuit connected to 240V is a terrible accident waiting to happen.
>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? Any old wall wart you have laying around can safely bring the voltage down to a safe level. There's no apparent reason as to why you wouldn't want to take the much more safer route.
>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.
Unless you have a cost concern here, there's no justification for throwing safety out the window. None whatsoever.
> >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 you are determined, Microchip has a Technical Brief for a transformerless power supply. You can find it here: http://ww1.microchip.com/downloads/en/AppNotes/91008b.pdf This is dangerous. The brief says that it's dangerous. I'm telling you that this is dangerous and that you shouldn't do it. It's a safety hazard and a fire hazard and there's no good justification. My suggestion: DON'T DO IT! 'nuff said. BAJ
In article <jqCtg.332610$5Z.119785@dukeread02>, John Perry  <jp@no.spam> wrote:
>Tom Lucas wrote: >...> >> 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.
>Actually, the neon lamp will explode :-). Unless you're careful to get >a neon _indicator_, which will have the necessary current limiting >resistor built into the package.
Absolutely. BTW you can connect an LED directly to the mains under a couple of conditions: 1) You use sufficient current limiting. 2) You make sure to wire your LED with another diode (which can be another LED) in antiparallel configuration. This limits the reverse voltage for each of the diodes. I rigged up a cheap 120VAC motion sensor by tying the LED of an optoisolator directly to the 120V light fixture of motion sensing lights. I used a 10K 2W resistor and a reverse LED as a local indicator. The optoisolator converted the dangerous 120VAC into a safe optoisolated 12VDC. BTW the current limiting applies to neon bulbs also. BAJ
"techie_alison" <retro@dial.pipex.com> writes:

> 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?
Check out this: Step-down rectifier makes a simple dc power supply http://www.edn.com/archives/1998/040998/08df_06.htm Link was picked from http://www.epanorama.net/ -- Tomi Engdahl (http://www.iki.fi/then/) Take a look at my electronics web links and documents at http://www.epanorama.net/
"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 > >
You could use a 12K (12000 ohm) limiting resistor in series the the LED to limit the average current of the LED to around 20ma. The 12K resistor will need to be at least a 5 watt unit and will get pretty warm during operation. You will also need a bypass diode hooked across the LED with the anode of this diode to cathode of the LED and vise-versa to limit the reverse voltage of the LED. The avoid the loss of power and subsequent heat generated by the resistor you can use a 0.22 uf capacitor in place of the resistor. Make sure the voltage rating of the capacitor is at least around twice the peak value of the AC or around 600 volts. It must be a non-polar capacitor (most electrolytics are polar). C is calculate from the following formula: C = 1/2*Pi*f*Xc this formula is an algebraic variation of the capacitive reactance formula We use to use this technique to limit current for indicator lights in 440 volt power distribution switchboards. NOTE: You can still get shocked by this arrangement if either or both of the diodes open. 20ma of current across your heart is enough to kill you. Don't even try this if your not familiar with the proper electrical safety measures. The safest technique, by far, is to use a small, step-down transformer because of the increased level of isolation it offers. Dorian