A friend of mine brought a little device he made its a bridge rectifier fed straight off the mains then fed into a 900 uF capacitor the purpose of this little device is to try to make a CFL stay on for 10 seconds I don't know why it's his project. We tried this device out and after a few switch ons it popped a 5 amp fuse in the feed to my test bench so the question is what would be the charge current for a 900 uF capacitor? It made the ammeter in the feed to the test board kick up. So is there a way to work it out?
Is this a sensible idea? The compact fluorescent lamp is designed to run on A.C. not D.C. A capacitor will discharge and supply a D.C. current. Try charging the capacitor through a resistor to limit the high charge current.
The initial inrush current will be very substantial. It is only limited by the impedance of the supply and the very low impedances introduced by the rectifier, the capacitor, and connections.
The inrush current may be limited by adding a resistance in series with the input. 10 ohms will limit the inrush to about 25 amps, which is within the very short term capacity of any likely fuse or MCB.
A better plan is to use an inrush current limiter or NTC device, this has a high resistance when cold, and thereby drops a significant proportion of the supply voltage and limits the current. As the device heats up, the resistance reduces and introduces only very limited losses.
I have done similar, to keep mains voltage LED lamps lit for a minute or more.
Thanks so the current is very high it's surprising just how much is involved. I know that some larger SMPS have a thermistor so when I se him again il suggest adding one. CFLs are happy with a DC or AC Iinput since the supply is rectified to DC at the input before its turned into high frequency As I say it's my mates project
The capacitor maker will quote an ESR, or a tan delta that you can work it out from. I'd expect fractions of an ohm, so inrush several hundreds of amps, but depending exactly where in the mains cycle you switch on - you may have been lucky a few times and switched near the zero crossing. But you do not need it to be that fast starting, and the violent surge is not good news for the capacitor or the light switch either.
As others have said some series R either before or after the rectifier chosen to dissipate less than a watt in normal operation is needed. Note that during switch on the full mains voltage will be across that resistor, and you may switch on at the crest of the mains (300 plus volts) when the cap is totally flat. beware of resistors not rated for 240V - 2 smaller ones in series may be preferred.
Most CFLs are indeed fine with a DC supply for the reasons already given. However use from a capacitor is not ideal as the reduced voltage from the partially discharged capacitor may have adverse effects.
A better choice would be an LED lamp, some types work down to 90 volts or so.
Domestically I perceive a use for capacitor backed LED lighting. A run time of a few minutes is enough to find a torch, light a candle, complete a trip up/down the stairs, get out of the shower, or otherwise avoid the perils of a power cut.
Hundreds of amps no wonder I had slight burning on the plug pins I will see if I can get hold of a LED light to try what would be a good brand or model to try? I'm guessing it would need to be one with either a SMPSU or resistive dropper rather than a capacitor dropper which I don't imagine would work on DC
also be aware that even after the lamp goes out, the capacitor will hold charge, possibly for hours after if the electronics switches off totally at low voltage.
A resistor across the C to bleed it down such that R * C is a sensible time - normally 5 secs in commercial products, but you actually want than that storage as run-on time, but perhaps a few mins.
900uF even if the lamp runs it downto 100V is 4.5 joules
energy is half C V squared - so 4.5 joules - which may not sound like a lot but it can still hurt.
To bleed down to 1/e of the voltage in say 50 secs , R*C=50 would be ~ 50 k ohms.
I would be happier with an L.E.D. emergency light. The energy stored in the battery will light the output for many hours. The automatic trickle charger will protect the battery from harm, and the whole emergency light will hopefully have been designed and made to safe standards. They can cost less than 20 quid each new. (Try C.P.C.). Mains on = battery charging. Mains off=light on. Even the little L.E.D. charging light conforming mains present can be quite bright in a dark room, even with the main L.E.D.s not working.
