The brightness is mainly due to a greater on-percentage. It probably depends on your sensitivity and ambient light. One easy test to see if the rectifier is working properly is to wave a bulb sideways in front of you. You should be able to see it turning on and off.
I was actually going to try that, but don't have a 120V rectifier now (I do have a 50V one).
I'd expect it to be brighter, with nearly twice the power.
Does adding the capacitor make any difference (that is, if you have a large capacitor that'll work on 180V or so)?
You should see the drop across 2 diodes, 1.4V. Also, are you sure you switched the meter from AC to DC?
Neither an analog VOM nor a DMM (other than a very expensive one) will give accurate RMS readings except on a good sine wave. They respond to average voltage (or current, if that's what you're measuring) and attempt to display this as RMS (correct for sine waves).
I made some measurements of current for Xmas lights, including:
string of 25 C9 lights: 1.5A
string of 25 C7 lights: 1.3A
string of 50 miniature (incandescent) lights: 200mA
string of 70 LED lights: 50mA
Supply voltage is almost exactly 120V here.
BTW, with the LED lights, the blue ones are noticeably brighter than the other colors (I have some red, some blue, some green, and some yellow).
Yes, with the full wave rectifier you will see very short off period (when waving the lamp). With half wave, you see the 50% on and 50% off. I did notice a strange thing in the strings which I bought (LED Lights). The one color 70 LED string (2 series strings of
35) has the limit resistors in sockets 2,3,33,34 for the 1st half and 37,38,68,69 for the second string of 35. You can actually feel the heat on these sockets. I have them on a full wave rectifier. The multicolor sting of 35 apparently has distributed resistors in every socket as there seems to be no socket getting warm. Also, on their web site they mention that some color LEDs (white and blue) are run at higher currents than red. So, my guess is that there are resistors in each socket.
Yeah I waved two bulbs back and forth, one on a string running on AC and the other on the rectifier and it was easy to see that there were twice as many flashes on the rectifier.
Haven't tried that yet.
Soitenly. Putting it on DC gives an even more ridiculous number.
If enough LED's would be connected in series could they dispense with the resistors entirely?
I tried using a diode and capactor. You need a series diode to rectify the AC to half wave DC. The capacitor filters the bumps and provides smooth DC. I used a very small electolytic capacitor; 2.5uf (that's what I had in my junk bin) provided smooth DC. I didn't pull out the scope, however, waving the LED showed NO blinking. This was for a double string of 70 single color LEDs. The difference in LED brightness between full wave rectified DC and pure DC (with a capacitor) was very small.
Not having the total text from the above statement, I'll try to reply, so I hope this makes sense. First, every meter reads differently; some read peaks, some take an average and some are true RMS. When taking these type of measurements ( AC and pulsing DC), you should use a true RMS meter, however, any meter can be used to compare one reading to another. The readings just may not make mathematical sense. Also, the voltage drop on LEDs may not be, and is usually not, the typical .7 volts. Differernt color LEDs may also have different voltage drops.
Would a capacitance of 220uF be OK for this?
Oscilloscope?
Supposedly you can't, since a LED in breakdown (necessary for it to light, voltage needs to be at least 1.5V or higher for blue LEDs) has a very low resistance, and would draw too much current without a resistor. I have seen LEDs used without resistors on small batteries, with high internal resistance.
Resistors do dissipate some power (reducing energy efficiency of the lighting system). Maybe someone could figure out a way around this someday. Maybe a duty-cycle controller like that used in incandescent light dimmers.
BTW, I did once (accidentally) connect a LED to 12V with no resistor. There was a POP and half the LED package disappeared (moved too fast to be seen, like a flea jumping). The remaining half (attached to the leads) did look burned around the chip.
Another thing, I learned about LEDs in college. The semiconductor material used determines the color. There were no blue LEDs at that time (blue is at the high end of the visible spectrum). There are blue ones now. However, I still don't understand WHITE LEDs (there is no single frequency for white, like for the other colors). White could be made from red, green, and blue but I see no evidence of them in a white LED.
A capacitor works with a full-wave rectifier too. you could use a smaller capacitor (It won't have to maintain 120V for as long).
scopes will show peak. A new one may also show the RMS equivalent (may work right for sine waves only).
Aren't those expensive?
.7V (700mV) is the common forward breakdown voltage for silicon diodes (300mV for germanium). That's the voltage required for the diode to conduct when forward biased.
LEDs emit light when the forward voltage exceeds the forward breakdown voltage (some people might think "breakdown" is something destructive, but it isn't when current is limited). LEDs are made with different semiconductor materials, and so can have different breakdown voltages. I have measured about 1.5V on a red LED. I have not measured any blue ones yet, but have seen advertisements claiming around 5V.
Also, I measured 50mA current to my string of 70 LED lights (I measures this on an all-red one). 50mA is a common maximum forward current for LEDs and exceeding it would damage the LED. It could be the string has 2 35-LED series, running on opposite polarity and drawing the maximum current (blue ones may be different). Once I use a scope, I might find out more about this.
I haven't measured a blue LED yet. I've seen catalogs that claim around 5V.
Supposedly it's a blue chip exciting a phosphor.
So then, white LEDs should have the same forward voltage as blue LEDs? I do intend to test that sometime.
Then they could actually make it any color they want.
But why would twice as many flashes or continously lit look only a little bit brighter than the original half wave arrangement? I know in film projection a projector shutter that is open twice as long will seem more or less twice as bright even though peak instantaneous brightness has not changed.
Twice as bright is not really as much brighter as most people would expect. I would also guess that peak is important for the ability of the human eye to detect and that might reduce the effect even more. An interesting thought that. I wonder if anyone has done any studies on peak vs continuous light sensitivity of the human eye?
It is that way because the eye, and the ear, for sound, sees (or hears, for the ear), logarithmically. So, twice the sound or light will only appear as a small increase.
True for hearing certainly but I have never heard of vision described that way. Decibels is a logarithmic scale but footLamberts is not.
Well, I guess I was thinking F stops on lenses. I seem to remember that one F stop doubles (or halves) the light. Am I remembering this correctly?
I don't know if anyone has done the testing, but I believe it is true for sight as well as hearing.
The loss of 2 volts from the bridge rectifier may be part of the explanation. But I think a bigger part is that human vision is nonlinear, and doubling light output of a lamp does not make it look doubled.
Look at the lumens on packages of various wattages of lightbulbs. A "standard" or "soft white" 100 watt one produces close to twice the output of a 60 watt one, and that is almost twice as bright as a 40 watt one. A
100 watt lightbulb produces about 3.5-3.75 times as much light as a 40 watt one. Does it look that much brighter?- Don Klipstein ( snipped-for-privacy@misty.com)
In my experience, halfwave in an LED string has duty cycle a lot less than 50%, since much of the time the instantaneous line voltage is too low to make any current flow through the LED string at all. With fullwave, expect the duty cycle to be not that high despite being doubled (or a bit less than doubled due to voltage drop of the bridge rectifier).
- Don Klipstein ( snipped-for-privacy@misty.com)
Current inrush when you plug this thing in could cause nasty sparking where contact is made and may be hard on the rectifier unless it's rated a good 3-4 amps or more. I would add a power resistor of a few ohms in series with the AC line. I would also add a fuse in case a voltage transient breaks down the rectifier or the capacitor fails.
The usual white LEDs have a blue LED chip coated with a phosphor that absorbs some of the blue light and fluoresces yellow light. The phosphor output is a fairly broad band from mid-green to mid-red in the spectrum, so white LEDs are not really horrible with color rendering.
- Don Klipstein ( snipped-for-privacy@misty.com)
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