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?
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.
Why not buy a 5V supply to run the electonics off rather than mess with mains yourself?
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.
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.
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.
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.
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.
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.
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! :>
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
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.
safety is always a concern.
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.
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.
that fuse from neutral to earth may look harmless but it'll mess with any ELCB/RCD etc that the circuit is connected to, much better to leave the connection to earth off, at and treat the setup as what it is: a "live chassis", fusing the Live side probably makes more sense too.
There's no question that a wall wart is safer than a non-isolated supply, for some applications. But this vociferous reaction about how dangerous it is - is just not justified.
Many of us learned on vacuum tubes where the plate supply was an order of magnitude more lethal than any mains voltage one might encounter. Ever see the inside of a 100 KW transmitter power supply? Build a Tesla or Induction coil? Rail Gun? Coil Gun? Vacuum tube amp or transmitter? Line regulator? Repair a TV set? Power Factor correction circuit? etc..
When all you need is a small indicator or circuit it makes sense to use a cap to drop voltage - more efficient than a wall wart, takes up less space, less cost, lighter, no waste heat to speak of. The enclosure provides the shock protection.
For tinkering with circuits on a breadboard - or just learning electronics, I'd agree it is too dangerous. But the op mentions it doesn't need to be isolated, so he probably already thought of using a wall wart.
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IMHO, 99% of the time this topic comes up, it falls into the category of "if you have to ask, don't use it". The 1% are mostly sensible reliability and regulatory compliance concerns from people who do know fairly well what they are doing.
Best regards, Spehro Pefhany
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At least in Europe, all tube televisions and many tube radios had a universal (AC/DC) power supply, with a half wave rectifier generating the B+ line about 200-250 V, thus there was only a rectifier between the other mains plug terminal to the B+, while the other mains plug terminal was directly connected to the metallic chassis. Depending on the way the mains plug is inserted into the wall socket, you either have the Neutral in the chassis or the full 220 Vac Live in the chassis. Also the tube heaters were in series and across the Live and Neutral, possibly with a VDR in series to limit the inrush current.
When working with such equipment I have used two main principles, before starting to work, I _always_ checked the mains plug orientation by measuring the metallic chassis voltage. When working with active equipment, I put my left hand in the pocket and only work with my right hand inside the equipment. This avoids the risk of having the current flow through your heart. If you get a muscular cramp in your right hand due to an electric shock, you still have the left hand operational to cut the power.
Regarding low power devices powered by a series capacitor, I would suggest using capacitors intended for mains filters.
Instead of a single capacitor on the live side, put two in series, each connecting one side to the respective mains plug terminal, while the other terminal of each capacitor goes to the load (rectifier etc.). In this configuration, the small signal circuit is floating around 110 Vac. If you accidentally touch the small signal circuit, there is still the other capacitor in series between the mains voltage and you, limiting the current through your body. If you want to limit the worst case current to 30 mA, the normal circuit current consumption must be below 10-15 mA, since in normal operation, there are two capacitors in series.
This is a good idea because it also provides double protection against a capacitor short failure. Also due to the possibility of such a failure a series, low current fuse is advisable.
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