cheap led driver board questions (mc34063) - Page 1
Forum section: electronics / projects

author: gobo
author_rank: Newbie
timestamp: April 02, 2014, 04:37:49 am
content: i've been trying to "re-purpose" a cheap mc34063 based led driver board to use a lowercurrent led. one was dissected and the schematic is attached. the .63 resistor (Rsc) wasreplaced with 2.2. at 12vdc input, the led current is about 34mA.i must confess this thing does not really work the way i expected. with the comparatorinverting input grounded, no voltage regulation, the current limiting is reversed. loweringthe input voltage causes the led current to increase.then there is that 22 ohm in series with pin 7 (current sense.) i have really no ideawhat they were trying to do here. hysteresis maybe?any thoughts?

author: pa2ees
author_rank: Contributor
timestamp: April 02, 2014, 04:11:19 pm
content: Hopefully some of my thoughts will lead you to your answer. I looked at the datasheet, and thishttp://www.intusoft.com/onsemipdfs/an920-d.pdfapplication note on using the MC34063, but I didn't see any reference circuits that looked like the one you drew.At a glance, the wiring to pin 1 and 8 doesn't seem right. In all of the reference circuits that I saw, they had VCC connected directly to pin 1 and 8, as those are the collectors of the internal transistors. The way your diagram is, it looks like both diodes are reverse biased, thus current will never flow into the pin 1 or 8, and subsequently, will not flow into the internal collectors. I'm assuming your 12V input is at VCC_CIRCLE. I think it might be prudent to double check your schematic, especially with reference to the LED and Schottky.If indeed the diodes in the circuit are drawn correctly, you have me stumped also.- Erik

author: SL4P
author_rank: Super Contributor
timestamp: April 02, 2014, 11:18:42 pm
content: If you read the ISWE and ISWC as emitter and collector of the power switching component - then the LED simply looks like it's back to front !I didn't look at the rest of the pins - but that seems like a good place to start.

author: gobo
author_rank: Newbie
timestamp: April 03, 2014, 12:59:15 am
content: yes, i had the led drawn backwards. it was correct on my pencil drawing...vcc_circle is the +12vdc. i hastily put that drawing together to have somethingto post here. the corrected drawing has been inserted.i found a similar schematic on the web. they used pin 5 to modulate the brightnesswith a pwm signal. but it did not have that 22 ohm in series with pin 7.

author: pa2ees
author_rank: Contributor
timestamp: April 03, 2014, 05:32:34 am
content: After looking at it a couple more times, here are some more thoughts that I had.The current sense input (pin 7) should not be sinking any current, ideally. So if no current is going in, then there is no current going through the 22 ohm resistor, and thus no voltage drop across the resistor. Essentially pin 7 is seeing the voltage on the anode of the LED. Essentially the 22 ohm resistor may as well be a short.As far as the issue with LED current going up when the supply voltage is decreased, here is a possible explanation:First, lets go through the scenario of 12V: when the output transistor is on, current flows through the 2.2 Ohm resistor, through the inductor (putting energy into the inductor) and through the transistor. There is a voltage drop across the 2.2 Ohm resistor, which is sensed on pin 7. If this voltage drop is triggering the current limiting, then the duty cycle of the output transistor is changed (the on time is shortened) by charging the timing cap faster. Here's the kicker, during the off cycle, the energy stored in the inductor wants to go somewhere, and flyback voltage causes a voltage polarity switch across the inductor, so now the schottky is forward biased. Current flows throught the schottky, but has to return somewhere, and does so through the LED.Now when the input voltage is lowered, the voltage drop at the current sense input is lower (the drop is smaller), and we'll assume the current limiting circuitry does not turn on, thus the duty cycle of the output transistor is not changed (on time is about 6 times off time). The current is about the same, but the inductor is being charged for a longer period of time, thus more energy is stored in the inductor, and so during off time, it wil conduct more current than in the 12V scenario.This is obviously just a hypothesis, as I don't have the circuit to play with. My suggestion is to measure the output transistor duty cycle wth an o-scope. Do this at different input voltages, and see if it's changing the on time. Also you could measure the voltage drop across your 2.2 Ohm resistor, to see if it would trigger the current limiting circuit (I think the threshold is around 290mV). You should also see a voltage drop across the 2.2 Ohm resistor during the off cycle. That would give you insight as to how much current is going through the LED when the inductor is discharging.Investigate with your scope, and let me know if I'm way off.- Erik

author: gobo
author_rank: Newbie
timestamp: April 06, 2014, 09:58:16 pm
content: at 12vdc, the drop across the sense resistor (2.2) is 0.08v, the current through the led is about 38 mils and the frequency is about 89khz.decrease the input, and the sense drop increases and the frequency increases.at 10vdc the sense drop is 0.097v, led current 45 mils and the freq is 104khz.i built a different drawing. see attached.normally, the comparator output would be high during the ramp up of the coil charge. once the voltage set point is reached, the comparator output switches low, the AND gate goes low, drive off. so the clock and comparator form a ramp.here, the comparator input is grounded, so the comparator output is always high leaving only the clock to drive. there is not really a ramp. the drop across the sense resistor goes up, the oscillator freq goes up. the higher the voltage on pin 7, the faster the clock. faster clock = higher current.well, maybe. i guess. it was meant to be cheap, not good.

author: valley_nomad
author_rank: Contributor
timestamp: May 28, 2017, 09:45:25 pm
content: I just recently got this controller and had the same question. After some thinking, I found that the way in which it "controls" the current is to use chip's current limiting feature. But due to the fact that it controls the peak current instead of average value its performance is very poor. So it is not the correct way of using this chip for constant current control.