Full Wave Converter Question

[appv128]

Looking for some advice on adding Full wave Converters to big box store LED strings. Once the led strings are converted would it draw a higher amount of current than while operating as half wave? Trying to confirm that the wires on these strings will hold up. I have about 60 strings and want to know if any cons on doing the conversion. Thanks!!

[jimboha]

Yes, they will draw twice the current.

How do you plan to use these lights? Will they be always-on, or part of an animated/sequenced song where they will be at constantly changing dim levels? If the latter, you will likely not have any issue.

Even if always on, I think the wires are heavy enough (smaller AWG or gauge, means thicker wires) to handle the increased current. Remember, for LED lights, the string normally carries 20ms to 30ma. Even doubled this is less than 50ma and the wires should be able to handle a couple hundred milliamps. To be absolutely sure, you must determine the gauge of the wires, then look up the wire capacity.

I cut one of my strings (actually, it was already cut). The (stranded) copper wire has a 0.026" diameter. That means it is about 22 AWG. At 60 °C that should carry 3 AMPS. That is well above the doubled current of your fullwave conversion. I would not think that wires to be a concern.

You might want to check the resistor "size". Not necessarily the resistance, but the heat dissipation. Remember that dissipated heat is W (watts) = I^2*R (Current (Amps) squared times resistance (ohms)) Doubling the current makes the heat go up by a factor of 4. If it's a concern, you could replace the resistor by same resistance, higher wattage rating. Again, constant on could be an issue, but varying dim levels should be a concern.

Jimboha

[appv128]

Thanks Jimboha for the detailed response. Most of the time the lights are on they will be playing a sequence or doing random flashing. I had a few strands (200-C9s) that were connected to each other to line the driveway that was the ones that worried me.
Still learning- so did not follow which resistor value did you recommend checking and replacing above? Thanks again.

[jimboha]

The resistance needed to maintain the proper current should be roughly the same before and after full-wave conversion. But since the resistor will be conducting current twice as often (both halves of the now full-wave circuit) it will have to dissipate more heat. Not really twice the current, but twicer as often so we can assume that to make it easy. If it were twice the current, it would generate 4 times the heat. So you might have to get a larger (ie. higher wattage resistor). Eg on one of my strings, the resistors are 2600 ohms 1/2 Watt. To go fullwave, I would consider changing to at least a 1 W 2600 ohm resistor. Just to be safe (eg. in case the string was indeed left fully ON for a longer time - maybe a stuck relay?), maybe use a 2W 2600 ohm resistor. I think doing this is borderline paranoid - likely you don't have to worry, but it would be good to know what size resistor your strings have.

The 200 C9's shouldn't matter - each string is actually a couple of smaller sub-strings (usually 25-30 LEDs) each with their own resistor. All the same calculations apply as long as you find and swap out ALL of the resistors in each sub-string. Note that sometimes the circuits are more involved, so you'll need to find and inspect the 'blobs' in each string. If they contain more than a single resistor, then the conversion will be a little more involved.

I am most familiar with WalMart LEDs. Which big box store brand are you using?

For you various strings, can you find the specifications? eg. how much wattage does each string draw? How long (in LEDs) is each string, and can you find how many sub-strings it consists of? You can find the sub-strings but looking at the wire count. At both ends of strings (assuming they have plugs on both ends) you will only see 2 wires. Near the end, that will change to 3 wires usually in one LED socket that is a little larger than the others. Then, after perhaps 25 to 35 LEDs, that will convert back to 2 wires, a short distance, then convert back to 3 wires. These are the 'boundaries' between the sub-strings. Within in a sub-sting, you will also find a 'blob' (as I call it) that is again different (usually bigger) than the LED sockets. These contain the resistor (or other current-controlling circuitry). Sometimes they are like larger LED sockets with caps or plugs that you can remove to see the resistor size and value. Other times they are sealed units - you would have to partly disassemble the sub-string to measure them with a ohmmeter.

Tell me what you seem to have and I can tell you how to proceed.

Also, how to you plan to convert from half-wave to full-wave? Most strings don't have any rectifying diodes because, well, the LED itself is a diode (all 25 in a substring). Somehow you have to add a fullwave bridge (4 diodes in a special diamond configuration), and then address the fact that each substring usually changes the polarity. The way you plan to do that will help determine how you complete this conversion.

Jimboha

[P. Short]

The power dissipated by the resistor in the string is doubled with a half-wave to full-wave conversion. The reason for not using the 4x formula is that the 4x formula assumes that the current through the resistor is obtained by doubling the voltage across it, which is not the case here. He may still need to go to a higher-power resistor, but let's not get carried away.

[1pet2_9]

Are there actual part numbers you guys use for half-wave rectifiers, or is this something you just found out the store-bought LED's use? I always noticed that if I run 50% PWM through a DC LED, the LED seems to glow 90% perceptibly bright. I can handle that. It would be nice to get an automatic, free hardware PWM on my DC LED's; save half the power.

I noticed some of my China-direct solar LED's run double-throw half-wave rectifiers, opposite polarity. i.e. for positive AC voltage, one strand turns on, while for negative AC, the other turns on. Brilliant idea.

On the Walmart LED's, have you experimented with snipping the current drivers altogether and just running DC (obviously with a current limiter)? I've got some broken Walmarts in my garage.

[P. Short]

People have designed and provided boards with full-wave bridge rectifiers that were connected to the AC-input of big-box AC-powered LED strings. The LEDs in those strings were then driven on both halfs of the AC power sine-wave, reducing perceived flicker (the main reason for doing this) and some increased brightness (incidental).

It's unclear what you mean by 'just running DC'. Do you mean using pulsating DC (generated from AC with full-wave rectifiers), filtered DC (using capacitors to filter pulsating DC), full-regulated DC or something else?

[1pet2_9]

Filtered. But for experimentation, you would probably clip the store-boughts and put them on an adjustible power supply, and that would be full-regulated.

Good point about the flicker. PWM is probably best run around 120Hz; not 60.

[MartinMueller2003]

I agree with Phil on the 2x statement.

Here is a long explanation for unregulated, unfiltered conversion from an AC signal to a varying DC signal:

The formula Power (w) = I * V is a "Constant Voltage DC" Formula. It assumes you are using an RMS voltage in the calculation. The RMS voltage for a half wave rectified input voltage is about 60% (very rough number) that of the RMS voltage of a full wave rectified voltage when the input AC peak voltage remains the same.

The Peak forward voltage going to the LEDs is the same for both full and half wave rectifiers.

In this case, when calculating the peak voltage drop on the series resistor, you need to look at the peak voltage (which does not change) and the peak current (which also does not change). That means the "Value" of the resistor does not change.

To calculate the power dissipation on the resistor you need to use the RMS voltage value. Because the RMS voltage has gone up when you switched to a full wave rectifier, the difference in the power calculation for the Halfwave and fullwave RMS Voltage values will show a doubling of the power dissipated by the resistor.

[rayhjr]

I am in the process of converting some Wal Mart 225ct multi color LED strings now. The original resistors were 1k 1/4 watt. I have a bunch of 1/2 watt 4.7k resistors and have decided to use them. They are slightly dimer but that's not always a bad thing. They have 45 LED's per circuit and I'm changing from 5 circuits to 3 circuits per strand. Changing my Mardi Gras Fan from 8 channels to 15. I had 2 spare strings never used......... until now.