Electronics > Projects, Designs, and Technical Stuff
HOW DOES THIS WORK?? Bench light circuit has me stumped...
engrguy42:
Okay I took apart a simple bench light (the kind with tons of super bright LED's) and tried to reverse engineer the tiny little circuit board.
First, all the tiny SMD components (diodes and transistors) have no markings so I used LTSpice defaults. No big whoop. I think.
It's got two, paralleled 4V sealed lead acid pieces of crap, and they're charged directly via the 120v wall outlet thru a full bridge (8-10 hours to charge, and only 5 hours usage on HI :palm: ; which means maybe 2-4 AHr batteries? No markings on anything). It has a switch to select HI, LOW, and OFF, and I think all those do is switch in some different biasing resistors for a couple of transistors to vary the current thru the LED array. BTW, I only show the HI configuration in the LTSpice circuit .
I measured the actual and it's about 360mA thru the LED array when ON HI.
If you look at the attached LTSpice I made, the simulation results do pretty much match what I measured on the real unit. But I'm trying to figure a couple things:
- WTF is the D5, and why is the bridge output circuit hooked up like that? Maybe I goofed? Makes no sense, but seems to work and I triple checked the connections on the board. Makes my brain explode.
- The RC frontend (200k) I suppose blocks DC and limits the battery charging current? I don't understand how the output of the 120V rectifier matches the required 4+volts to charge the battery. DOH!!!
- And one other PITA about this piece of garbage is that you can't have the light ON when it's plugged into the wall (it doesn't charge the battery). But I don't see how that is reflected in the circuit.
Anyway, thanks for any help.
thm_w:
D5 or D7 is not a zener?
Watch some of BigClives videos, he does a good job of explaining capacitive dropper circuits. Although I don't know a specific one to match that circuit.
Here is a solar/ac lead acid video: https://www.youtube.com/watch?v=IMYaWd_wuf8
C1 is the key in the front that allows a limited amount of charge to pass into the rectifier, R1 is to discharge C1 for safety.
engrguy42:
--- Quote from: thm_w on March 26, 2020, 10:52:26 pm ---D5 or D7 is not a zener?
--- End quote ---
Thanks. Yeah, D5 or D7 could be freakin' anything. No markings, and so tiny that all I can see is the "D" designation for both on the board.
Biduleohm:
I don't know about D5. At first I thought it was used to create a 0.6 V drop to do something with Q2, but nop, makes no sense.
That's a capacitive dropper. You use a cap instead of a resistor to limit the current thanks to its impedance. The 200 k is just here to discharge the cap so you don't get zapped if you touch the plug just after unpluging it.
My guess is it's Q2 who prevent the light to be on while charging but we don't know how the LEDs are connected so it's hard to tell. Can you add the LEDs on the schematic?
engrguy42:
--- Quote from: Biduleohm on March 26, 2020, 11:00:04 pm ---I don't know about D5. At first I thought it was used to create a 0.6 V drop to do something with Q2, but nop, makes no sense.
That's a capacitive dropper. You use a cap instead of a resistor to limit the current thanks to its impedance. The 200 k is just here to discharge the cap so you don't get zapped if you touch the plug just after unpluging it.
My guess is it's Q2 who prevent the light to be on while charging but we don't know how the LEDs are connected so it's hard to tell. Can you add the LEDs on the schematic?
--- End quote ---
Thanks. Yeah I'll see if I can add the LED's.
BTW, there's also an "SOS" position of the switch, and that connects a biasing resistor to the Q2, but I left that out because my brain already exploded. It's supposed to set up an SOS code flashing but of course doesn't work. Not that I care... :D
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