Will dimming incandescent bulbs with a rectifier diode cause noticeable blinking

[Jay112]

If I use a rectifier diode to dim incandescent light bulbs to around 50%, do you think it will cause noticeable blinking? A google search showed that some people suggested that the blinking might be noticeable, but they didn't seem certain about it; It either seemed conditional or they weren't speaking from experience.

What do you think? Is it conditional? Will the blinking be noticeable?

I have a halogen fixture that is dimmed with a 1N4007 diode (this is what originally gave me the idea), and I don't notice any blinking at all.

[Lightages]

I don't think you will see anything. Thermal devices like a light bulb can't respond fast enough I would think.

[helius]

You will not see noticeable blinking with incandescent lamps. They emit light as a function of the filament temperature, and the time constant is long enough that temperature will remain steady even if every other half-cycle if blocked. This should be intuitive since triac dimmers work in a similar way and don't cause visible (or really, any) blinking with incandescents. I have some disc-shaped devices labelled as "rectifiers" that were sold once upon a time as bulb life extenders. They do not cause any blinking.
You can satisfy yourself that it isn't a flicker-fusion effect, and the luminance is actually steady by blocking a lamp with an electric fan. If there was blinking faster than the eye could see, there would be nodes in the chart of brightness vs fan speed. If the brightness is flat independent of fan speed then it really is steady.

[Jay112]

Thanks for the responses!!

As an aside, I doubt a single rectifier diode will cause a noticible degree of dimming. Measureable yes, but noticible probably no. I suspect that the diode is there for another reason. One benefit would be longer lamp life as a result of the lower voltage.

The halogen fixture had a Low and High setting on a 3-way switch. One day the Low setting stopped dimming the bulbs (they were displaying full brightness, just like the High setting), so I opened it up to see what happened.

It ended up that the 1N4007 was visibly burned. The diode was connected to the Low side of the 3-way switch. When I replaced the diode, everything worked perfectly again, and the Low setting dimmed the lights just like before.

I'm just a beginner in this stuff, so I don't know if I drew some wrong conclusions.

[richard.cs]

On 240V lamps you certainly can see a flicker, I have a 100W one with a diode in series that I use for charging batteries. Lower voltage lamps have thicker filaments so maybe you can't see any flicker in the USA with 120 V.

[AF6LJ]

For me it depends.
low wattage bubs I can see the flickering...
Higher wattage bulbs and I don't notice it.

Your mileage may vary...

[Jay112]

Thanks everyone for your input! I ordered a 1000V 10A diode (http://www.ebay.com/itm/171505136204) and am planning on using it to dim eight 60-watt 120v incandescent bulbs. The item should arrive sometime within the next 7 days. I'll post my results here once I try it.

[Ian.M]

May be marginal for inrush current.  Your 60W lamps each need 1/2A current when running normally, but when cold their resistance will typically be ten times lower so they will each pull a 5A surge at switchon.   With eight in parallel that's a 40A surge.  Check the diode's datasheet carefully for its peak If rating and how long a surge it can withstand.

[fubar.gr]

Halogen lamps life expectancy drops significantly when dimmed. The lower filament temperature hinders the regeneration process.

[Ian.M]

Also, DC doesn't do them any favours (electromigration)

[Tomorokoshi]

For me it depends.
low wattage bubs I can see the flickering...
Higher wattage bulbs and I don't notice it.

Your mileage may vary...

Yes, I can see flicker on C7 Christmas lamps due to variation on triac triggering.

[Jay112]

May be marginal for inrush current.  Your 60W lamps each need 1/2A current when running normally, but when cold their resistance will typically be ten times lower so they will each pull a 5A surge at switchon.   With eight in parallel that's a 40A surge.  Check the diode's datasheet carefully for its peak If rating and how long a surge it can withstand.

Thanks! I will keep this in mind, and will check the datasheets once I'm able to see the part numbers.

Does anyone know of any other easy way to dim a line of AC incandescent bulbs to about half strength (I don't need it to be variable) by using very few components? I know they make all kinds of dimmer switches for this purpose, but I'm interested in learning how this can be done in other ways.

My main purpose is that I want to remove a certain regular light switch and put a wireless 4-way latching relay there instead, so that I can control/automate the lights through my Arduino. The 3 options will be: Off, Full Power, and Half Power. If a proper rectifying diode is able to dim the lights to about 50%, this would be easy and ideal for me.

[richard.cs]

A series capacitor of the right value would dim without introducing any flicker. Motor run capacitors are rated suitably and usually cover roughly the right range of values.

[Delta]

Connect each pair of lamps in series for half power.

[SeanB]

$3 gets you a cheap eBay triac dimmer, which will handle 2kW and which you can set and forget.

[HighVoltage]

This is a very old trick to make the incandescent bulbs last almost forever with a diode in series. But in Germany with 230V and 50 Hz, we always have a flicker, no matter if it a low wattage or high power bulb. In the USA with 60 Hz and 115V I never noticed the flicker as much but it is still there.

Both suggestions, to use a capacitor in series or a second light bulb work much better than a diode.

[Jay112]

Thanks everyone!

I'm interested in learning how to use a capacitor in series, and I was researching it for the last 10 minutes, but I feel like it's too complex for me for now. I'm still working on the beginners' electronics books.

Is the main idea that the capacitor lowers the voltage across the circuit? If so, how do we know what kind of voltage drop would be able to dim eight 60-watt incandescents to anywhere between 40-60% of their regular output? Is it straightforward, like where 60 volts (instead of 120) would result in a 50% reduction of light output?

Also, how do we calculate what size capacitor to choose?

[Jay112]

I found this information, and am curious what you guys think of it:

The capacitor type you need is an X2 class or Y2 class (both are standard capacitor types). This capacitor has the properties necessary to SAFELY implement a circuit (such as good transient withstand).

How it all works - Well, its really quite simple. The capacitor's capacitive reactance (Xc) acts as the limiting element, and unlike a resistor will not dissipate large amounts of energy due to losses that a resistor will. But remember, the capacitor + the circuit you are driving are acting as a voltage divider network, and variations in the load will cause variations in the voltage being supplied to your circuit. This means that it is only suitable for some applications. The advantages being a compact power supply can be designed that (can) be smaller and cheaper than its transformer-based equivalent.

What you need - A capacitor in series on its own is a bad idea. Xc is dependent on frequency, so high speed transients on the AC power supply (which do occur very regularly) will see the capacitor as a short circuit and will not be attenuated. This means that placing a suitably sized mains-rated resistor in series with the capacitor is a good idea, as the transients current will be limited by it (obviously the resistors resistance is alot smaller than the capacitors Xc). A zener diode on the rectified output side (if you are trying to drive a DC circuit) is a good idea to sink that nasty current-limited spike away from your electronics and prevent the voltage it sees from shooting up and damaging anything. Oh, and because it works using Xc = frequency dependent, rectify the voltage AFTER the capacitor and resistor combination.

Very Important !!

The capacitor voltage divider has a few serious drawbacks. The biggest one is that, unlike a transformer-driven circuit THE OUTPUT IS NOT ISOLATED FROM THE AC MAINS SUPPLY IN ANY WAY! You have to assume that your circuit and anything connected to it could at any time be at a dangerous potential, and as such it should be designed so that either you or the user can never touch it. The other is you are a little limited to the amount of current you can draw (and going to a very large capacitor will negate the advantages of using it in place of a transformer). Oh, and make sure there is a good voltage safety margin for both the capacitor and resistor. Say atleast 10% for the capacitor and say atleast 100% for the resistor in series with it (which will see those high voltage spikes across it)

I would not recommend this type of circuit (from a safety point of view) to anyone who is not confident and experienced with working with the AC supply. Oh and don't use the LED as a rectifying device (you wouldn't use your cell phone as a hammer would you).

http://cr4.globalspec.com/thread/13464/Capacitors-as-AC-Voltage-Drop-Components

[IanB]

Does anyone know of any other easy way to dim a line of AC incandescent bulbs to about half strength (I don't need it to be variable) by using very few components? I know they make all kinds of dimmer switches for this purpose, but I'm interested in learning how this can be done in other ways.

As Delta said, rewire the lamps as series pairs. It requires only a switch. It is cheap, simple, and reliable.

[Jay112]

Does anyone know of any other easy way to dim a line of AC incandescent bulbs to about half strength (I don't need it to be variable) by using very few components? I know they make all kinds of dimmer switches for this purpose, but I'm interested in learning how this can be done in other ways.

As Delta said, rewire the lamps as series pairs. It requires only a switch. It is cheap, simple, and reliable.

I would do this, but I want to be able to switch easily from full power to half power (and from an Arduino sending a wireless signal to a group of relays).

[Ian.M]

X10 Dimmer module + an X10 interface for the Arduino.   You get local manual control, a warranty and its code-compliant so it doesn't FUBAR your home insurance.

[IanB]

I would do this, but I want to be able to switch easily from full power to half power (and from an Arduino sending a wireless signal to a group of relays).

Switch easily using a relay:

[Delta]

On 240V lamps you certainly can see a flicker, I have a 100W one with a diode in series that I use for charging batteries. Lower voltage lamps have thicker filaments so maybe you can't see any flicker in the USA with 120 V.

I have heard of this, but thought of it somewhat of an old wives' tale (old sparkies' tale?).  But now I have a car battery that refuses to take a charge I am thinking of giving it a go... I think I'll go for a 60w lamp as it's my first time!

[Seekonk]

What about inductors.  I know of one famous sign that had a motorized core that moved in and out.  A primary of a transformer can be wired in series and low voltage secondary can be shorted for full brightness.  Not necessarily small.  Magnetic amps, two transformers in series controlled by a 9V battery and a pot.

 "But now I have a car battery that refuses to take a charge I am thinking of giving it a go... I think I'll go for a 60w lamp as it's my first time!"

I once started a car with a toaster and it made breakfast.  Wish there was Kickstarter back then. Could have been a product!

[Jay112]

Switch easily using a relay:

Thanks for the info!

The 8 lights I'm trying to control go around the room in a big "U" shape, with the long legs of the "U" about 30 feet long (with 4 bulbs spread out linearly on each long leg), and the short leg about 10 feet long. The wiring is run through exposed metal conduit.

Do you have any suggestions for the best way to wire something like this up into series pairs, while being as conservative as possible with the wires? Would the most efficient way be to wire each adjacent bulb as a pair?

Does having 70 feet of linear wiring make the series-pairs idea less practical?