Electronics > Beginners
What the heck does this circuit do?
poodyp:
So I pulled apart the flashlight that came with my set of tools and found this board inside. I can't tell what it does other than possibly a super complicated 12v regulator. It's an 18v Makita li-ion battery with a 12v incandescent bulb. I'd like to replace it with a high power LED and I don't want to accidentally pull out some protection circuitry.
Attached is my attempt at reverse engineering it, and here are pictures of the back and front. It's super late so it's highly possible I failed at it, but I can't wrap my head around what it's supposed to do. I couldn't get the right models for all the parts so I tried it with other parts. Q1 is a C9015, Q2 is a B773, and U1 is a TL431A. All it seemed to do is drop 6 volts, and when I lowered the input voltage the output dropped proportionally. Also somehow V1 was sinking 5KA...
Can anyone please help?
Psi:
You've got some errors in there, but i think it's intended as a regulator and also a protection system to stop the lightbulb from discharging the lithium battery below it's min volts and causing battery damage.
BravoV:
Although I have not try to analyze that circuit, I recognized that you're using LTSpice for drawing the circuit.
Actually if you could substitute some of the transistors with similar one provided in the LTSpice library, and try run the simulation to have an idea how it works, provided that you've captured the schematic correctly.
Btw, that TL431 can not be replaced by LT1009.
I included as attachment in this post the TL431 model for that LTSpice, just save the TL431.ASY inside the \SYM folder or your own custom made folder and the TL431.SUB in the \SUB folder.
Hope this helps.
amspire:
Lots of errors in the circuit.
Q1 will be NPN I think - a 2SC9015 transistor.
Q2 will be PNP - a 2SB778 6A power transistor that will control power to the light bulb.
I suspect that the emitter of Q2 probably connects to the battery +ve and the collector connects to the light with the other end of the light going to the battery -ve if it is a switching regulator.
That probably means R5 is an emitter to base resistor.
U1 probably has the reference and cathode leads swapped.
I think Psi will be right, the torch has to turn hard off when the lithium cells drops below 3V each. It is also possible that a switching circuit is used so that instead of regulated 12 V, the battery voltage is pulse width regulated to make the average volts 12 V. If this is right, then the capacitor must form the time constant of the switching regulator to set the switching frequency.
A few too many errors to take a stab at exactly what the circuit is meant to look like.
You better check the transistor and TL431 lead pinout again and have another stab at it.
Richard.
poodyp:
--- Quote from: BravoV on September 23, 2011, 12:29:56 pm ---I included as attachment in this post the TL431 model for that LTSpice.
--- End quote ---
Thank you for the model, I've added it to my circuit.
--- Quote from: amspire on September 23, 2011, 04:33:45 pm ---Lots of errors in the circuit.
Q1 will be NPN I think - a 2SC9015 transistor.
Q2 will be PNP - a 2SB778 6A power transistor that will control power to the light bulb.
I suspect that the emitter of Q2 probably connects to the battery +ve and the collector connects to the light with the other end of the light going to the battery -ve if it is a switching regulator.
That probably means R5 is an emitter to base resistor.
U1 probably has the reference and cathode leads swapped.
I think Psi will be right, the torch has to turn hard off when the lithium cells drops below 3V each. It is also possible that a switching circuit is used so that instead of regulated 12 V, the battery voltage is pulse width regulated to make the average volts 12 V. If this is right, then the capacitor must form the time constant of the switching regulator to set the switching frequency.
A few too many errors to take a stab at exactly what the circuit is meant to look like.
You better check the transistor and TL431 lead pinout again and have another stab at it.
Richard.
--- End quote ---
You're absolutely correct about Q2 and U1, I had those mixed up. But are you certain about Q1? Searching for C9015 comes up with this, a pnp transistor, but I can't find anything for 2SC9015.
I took the pictures of the board and mirrored the bottom pic and used it to draw the traces on the top one you can take a look here if anyone wants to try their hand at simulating it. I made another attempt at simulating the circuit and it still seemed to not do much. Tried with Q1 as both a pnp and npn. This time the output pretty much matched the input, and never dropped out. I found a multimeter (mine's in a box somewhere) and measured the output and currently the battery is at 26v (Isn't that really high for 5 li-ions? Maybe this multimeter is a little janky), and the output is about .5V less. Maybe I can set up a battery logger and see exactly what happens.
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