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LED String Circuit

cmaxwell

New Member
I have a string of blue LED's in my garden, which look very nice. I'm powering them from a 3.7V DC supply.

I wanted to add more lights. All that I can find are LED stings with multiple flashing modes, I just want steady blue light. The strings on sale consist of LED's in opposing direction, so that changing the supply polarity causes different LED's to illuminate, and give a flashing appearance.

So, I bought some, thinking all I need is a power pack and a bit of circuitry to swap the polarity of the output fast enough to make it appear that the LED's are all on. Buying the power pack was easy, but the circuitry has proved too much for my old brain.

I'm currently running them from an 18v AC (250 mA) supply, and it works fine, but AC power packs seem really hard to find, and switching the DC polarity electronically should be possible, shouldn't it? Ha.

Any ideas? I'd be every so grateful if you did.

Thanks

Colin Maxwell
 

danadak

Active Member
An H Bridge part easily to switch polarity. You alternate turning on diagonal fets.
Eg, Q1 Q2 on Q3 Q4 off, then Q3 Q4 on, Q1 Q2 off. Alternating.....

1662728618056.png


Popular part is L298. Yiou can get it in modules (ebay, banggood, amazon)-

1662729048830.png


You need a clock to switch it, I am sure you can find a module with L298 on it and a PWM
control of it, eq a pot or V control to control brightness as well.

Most of the applications use an Arduino board to create the PWM signal to the board, and a dir signal
for polarity which can be a clock signal to reverse the polarity you are seeking. Or use a dual 555 timer
board, one for the PWM signal, the other to switch the direction (polarity) control of the L298 module.

Using Arduino using mBlock to program it. This converts your block configuration to code and
programs Arduino.

1662730967863.png


Note you could possibly tie the PWM signal high, full brightness, and just clock the DIR pin (polarity)
to do the design.

The L298 is a dual H bridge so do 2 strings.....a possibility. Its rated for 35V and 2A in a single bridge,
1 A each bridge is using both internal bridges. 25W total.


Regards, Dana.
 
Last edited:

DrDoggy2

Member
Yes, thoes ac packs are always switching polarity at a frequency > 60hz , and just supress the volts out when the leds are off. Also you cant just make them all blue with a wart as there are 2 colors per polarity, 4 colors total. The modes only change flashing rates. Always on is an option, but not for just 1 color.

You could buy 4 strings and switch out all the leds to make each string one color each, if the leds are even changeable.

(maybe just mine)
maybe post yours, mine have only 2 wires feeding leds.
 
Last edited:

Tony Stewart

Well-Known Member
Most Helpful Member
You did not say which colours and thus what voltage the LEDS were.
If you are using 18v AC (250 mA) supply, on +/-3.5V GREEN or BLUE LEDS Something will be overheating like the transformer. which may have a rating of 18/4=4.5VA and will be dissipating about 4Watts.

The L298 is problematic as it has about a +2 (hi) / 1.6V (low) drop at 2A or a Vsat of 1.0V at 0.4A.

So your DC supply needs to match the LED's. You will also have voltage drop in the wires so extending them from your existing string will dim the 1st string. It is better to use an old PC supply with 3.3V on the blue and 5.0V dropped with a 3A power diode to 4.3V so that the L298 will drop some more. Better is to use a current limiter pair of NPNs and a current sense resistor with pullup. Measure your LED voltage at each end and touch your power supply. It should not burn your finger after 40 minutes. (>=55'C)
 

cmaxwell

New Member
Thanks Tony.

The LED's are blue. There are 300 LED's on the string, but it is a combination of series and parallel wiring. 18v brings the LED's on to a mid level brightness.
 

Tony Stewart

Well-Known Member
Most Helpful Member
OK then the LM298 will drop it to 17V at slightly less current and brightness but you need DC not AC for the LM298 with a 5V regulator on board for logic level clocks
 

danadak

Active Member
L6201 mosfet single H-Bridge driver -


.3V drop at 1 A. Also has onchip Vref for logic level establishment.

If you wind up at higher current L6203.


Regards, Dana.
 

Tony Stewart

Well-Known Member
Most Helpful Member
LED voltage usually has a dynamic range of about 10% and the L298 may drop 1V @ 0.4A or 1/18 or <5% so it may be significant starting from the mid-range. CMOS half bridges are much better. Also dual opto-isolated SCR's for dimming each AC polarity.
 

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