Velleman LED driver kit - Page 1
Forum section: electronics / projects

author: steve30
author_rank: Frequent Contributor
timestamp: October 08, 2013, 04:34:30 am
content: I bought this LED driver kit (K8071) last year. I've been using it a lot to power some LEDs and its working well.But I was wondering if someone could help me to understand how the circuit works.I can see that we are somehow controlling the amount of current that goes through the FET, and comparing something to a reference voltage. But I'm not sure what all the parts do.I'd be grateful if someone could explain how it works. I've attached a copy of the schematic.Thankssteve30

author: homebrew
author_rank: Frequent Contributor
timestamp: October 08, 2013, 09:37:01 pm
content: Hi Steve,I'm just a hobbyist but I think I got a clear understanding of how that regulator actually works ...All in all, this is a switch mode regulator, regulating the output current...Let's see:top-left corner:That should be fairly clear, just basic 4-diode (D1-D4) rectifier and smoothing capacitors C6, followed by a linear regulator (7805). C1 and C5 are required for the 7805 to operate stable. Why C5 is not of the same 100nF value as C1 isn't clear to me. According to the datasheet of the 7805 they could have come away with only 100nF in the output... But the regulator is not used to supply other devices (especially not the LED) it is instead just used to provide a stable voltage as reference to control the output current. R1 and R2 are a simple voltage divider and according to Ohm's law the reference voltage therefore has to be U*R2/(R1+R2) = 0.342 Volts.bottom-left:This is just the power-supply of the comparator. The LM393 comes in packages of two but they needed only one comparator. To eliminate any unintended switching of the device they simply tie the inputs to the supply rails. For the theory of operation of that circuit that section is totally irrelevant.central right section:Here is where the magic really happens:It's a bit hard to see but T1 D5 and L2 from a switch mode regulator in buck (step-down) configuration. L1 is the power storing device here. When L1 is 'charged' (it's magnetic field has stored energy) and T1 is not conducting, current is flowing through D5, L1, the load, R7 and R8. Turing on T1 will magnetize L1 as current flows through it. The voltage drop over the resistors R7 and R8 is proportional to the load current. As R7 and R8 are both 1Ohm, the voltage-drop therefore is 0.5 mV per mA.IC1B compares exactly that voltage-drop against the reference voltage. If the voltage drop is lower than VREF, the transistor is turned on, otherwise it is turned off. I your case the output current should therefore 684mA. That most likely resembles their 700mA model ...R4, T2 and T3 are just a driver stage to switch the transistor rapidly. R4 is needed, as the LM393 has an open collector output (look into it's datasheet) and therefore can't source any current. T2 and T3 basically work as voltage followers (gain is approx. 1). As T1 is a large power mosfet, it's gate has a significant capacitance and therefore high currents could flow every time the transistor is switched. They limit that switching current to some 100mA (depends on the input voltage) by having R3 in series with the gate.Last components:R5 and C4 are limiting the bandwith of the control loop. According to Fc=1/(2*PI*R*C) that should be something around 1MHz. Otherwise that circuit would oscillate wildly ...Not the most modern circuit and most probably that thing will produce a lot of RF noise that might cause EMI problems. Especially when the wires to the LED are a bit longer ...I hope that I could give you some insights on that circuit.So let's see whether the experts will rip this description apart ...Best Peter

author: dannyf
author_rank: Super Contributor
timestamp: October 09, 2013, 12:38:13 am
content: It is a self-oscillating switching mode power supply: the comparator compares the desired voltage to the (smoothed) voltage. If Vsample > Vref, the comparator goes high, which turns off the mosfet, causing Vsample to deline, until Vsample is no longer greater than Vref.That process turns off the bjts but turns on the mosfet, charging up Vsample....

author: steve30
author_rank: Frequent Contributor
timestamp: October 09, 2013, 02:26:55 pm
content: Thanks for the replies. That makes quite a bit of sense.I think I'm going to try and build something similar on a breadboard to experiment with.