Here's the gig... I've got a component which uses an incandescent bulb to light up a sight hole for speed adjustments. The light has failed and I'd like to replace with either 1 or 2 hi intensity blue led's.... somewhere in the area of 3.5 - 5v forward voltage. My hic-cup is the bulb is run on AC and I'm unsure if a LED can be powered (generally directly) from this source without rectification. Can I wire in a small string and use a cap or something to smooth out the ripple? Or am I stuck with some bridge rectification? Like a nice simple means (or circuit) on the cheap to implement this lighting change. Any ideas?
Errr.. check the max reverse voltage of the LED first. I'd not be happy running a LED off AC tbh.. I'd rectify it first.
Or at least block reverse current with a single diode in series. Also leds need smoothing when working at high frequencies or they disintegrate.
Code: // (<-- those little arrows that indicate it's a LED) ---|>|-----|>|--------/\/\/\--- | | --||-- the left diode is a plain diode, 1n4xxx should work Just use any small electrolytic, and select the resistor based on the AC voltage You could ditch the cap if it's running on 50 or 60 hz.
Gigahertz? The bridge rectifier system is maybe best, 50Hz half-wave-rectified flicker is fairly obvious, 100Hz full-wave-rectified much less so.
Hey! Thanks for the fast responces! But now I'm even more uncertain. So can I, say, use a small rectifier hooked to the + / - of the circuit where the existing bulb drop is.... Then make a string up with a resistor and my LED's Perhaps a CAP to smooth the ripple and i'd be good? The draw would be minimal but would the rectifier get really hot? Pardon the in-experience and..... Thanks for the reply's!
The rectifier wouldn't get hot at all, you can use the smallest you can find. The rectifier will drop about 1.5V, so at 25mA that's only 33 milliwatts, much less heat than from the LEDs. There is no +/- with AC, hook the existing wires to the squiggle signs on the rectifier and take new wires from the + & - to your LEDs and resistor. What voltage is the bulb?
how do you mean is its job just to light up a control or is it from a record player type thing where the light lights up a spinning object with markers on it. you will know what i mean if you look at the edge of a old record player
Have a look here. I use this circuit since years as AC power indicators for the pumps, heater, thermostat, PH-meter and timer of my fishtank. The first version didn't have the serial resistor and I blew up a few LEDs due to the inrush current. CD
Several things I've taken apart had that same circuit for powering the led off the ac line. A clever way to power led's from ac mains.
Way better manner for AC power indication than the boring old orange (or sometimes green) neon indicators (that flicker or stop working after a few years). The only drawback of this schematic is the direct connection to mains (no issue if well insulated and properly integrated into a plastic housing). CD
Good point, if it is acting as a strobe then an unsmoothed LED with a single diode either in series with the LED or in inverse parallel will be ideal. Or fit a second LED in inverse parallel with the first, sizing the series resistor for a single LED.
Sorry the component is nothing too exotic The light is just to see inside to allow adjustment of some trim pots in a motor circuit. The bulb is a small unmarked light so I thought I could use what I have about. I grabbed a rat shack rectifier and tied it into the existing bulb drop and ran into a series of 2 led's. a 1k 1w resistor and called it quits. Thanks for the schema Still would like a diode in the line so good to see a 1N4148 will fly. It needs a little smoothing as well so I'll try this circuit directly as opposed to the rectifier. The bridge worked however! Thanks for the help on this one folks! Much Appreciated
you just a diode in reverse BIAS iethe other way compared with the LED. as long as your dealing with max rms voltage does not exced as the LEd forward bias voltage youll befine
