D.I.Y. Illuminated Tailcap - gChart Editions

I just sketched out an overly simplified circuit diagram a minute ago, but it’s just pen on some rough paper. I can digitize it and upload if desired. At this point, I want to check out lower-resistance bleeders (say 470 instead of 680 Ohm) before assuming the tailcap needs modified. I had given thought to a diode, but after looking at my sketch, I don’t feel that that’s the answer.

Here’s the boost circuit diagram per the TPS61221 datasheet. Vin and Gnd are routed to the switch board, Vout is routed to the LEDs with a series resistor.

click for larger image

^

Worth a shot, but it seems the problem is power flowing into the driver via the normal route (i.e. Positive from terminal of cell, and Negative from battery tube (via the taillight boost circuit). The bleeder resistor will control current flowing from the positive terminal to the to the battery tube. It would be a kind of slow short circuit and not impact flow from the tailcap. It seems the problem is not the amount of positive power going to the tail pcb via the tube, but negative power feeding backwards through the tail circuit to the tube (and on to the driver).

Looking at the diagram, the booster chip bypasses the LED (the led being a diode itself will not allow reverse current). So a diode may be needed either after VIN or before the ground connection to the chip. Looking at the top side of your board, it appears there may be enough room to cut either the positive or negative trace and insert a diode. Questions is, what diode would be a good candidate. Also, will the voltage consumption via said diode be enough to make the circuit not work when a cell drains to a certain voltage. That might actually be good as a low voltage indicator.

I would be very happy to do some surgery and try when my boards arrive, but I would need some guidance on what diode to try.

Sounds logical enough! I should be able to scavenge a diode from a “for parts” driver and cut that into the Vin connection. It may not be a perfect match, but I would think good enough for testing. Hopefully I’ll have some time to try that tonight.

looks like you are using Eagle for design, if not Diptrace i cant help you giving a basic design with the diodes arranged

17/20mm board with LVP based on loneoceans design but with more stuff going on

0603 LED and 0603 resistor barely fits the central ring with custom pads for reflow only

for people who are afrid of dual layer reflows that red glue that gets hard above 85°C helps a lot, but its no problem to put the LED board on a post or clamp and apply 2. side solder paste with a tip and parts stick to the paste on both sides usualy fine if not too heavy

Thanks for the tips, Lexel. I have been using Eagle. I hadn’t heard of DipTrace - I see they have a 300 pin freeware version; is that what you do?

While you’re here, any thoughts on the strange behavior I was experiencing yesterday? I’m hopeful the diode will do the trick, but I’m open to ideas!

Update: I tried adding a diode (1N5819) at Vin, but I think we went backwards in functionality. Seems to be no change on Li-Ion, and now the tailcap doesn’t illuminate on NiMH.

maybe by using a small transistor instead a diode, so you got reverse blocking but a very low voltage drop

New theory… Is it possible that the capacitors are feeding the main driver instead of the tailcap (when the load from the driver is low)? Perhaps I need to put diodes between the capacitors and ground?

Edit: nope! I tried just removing both capacitors. I still get the same behavior on Li-Ion. On NiMH, the tailcap fails to light up at all, and the light strobes when the switch is off. So it seems like there’s voltage leakage (?) through the TPS61221 to ground. Would a diode at the ground pin address that?

Thank you for sharing your results. I wish I could be more helpful. I would be experimenting just like you are if I was in your shoes. When I looked at the datasheet for the TPS61221, I saw no mention of built in reverse polarity protection. So I'm assuming the chip is allowing current to flow through it backwards. Lexel's idea about a small transistor sounds great. Maybe google what they look like and see if you have any old stuff you can harvest one from.

Could you describe the strange behaviour of the main LED and the tail LEDs on clicks a bit more precise?

Schoki, if a picture is worth 1000 words, what’s a video worth?

This is where a feel a bit helpless. I understand a few core principles, but that’s about it. The extent of my electronics education is what I got from a breadboard kit when I was 10 or so. I appreciate any help you can provide!

Another theory I had today… in a “normal” illuminated tailcap, there’s voltage drop in the loop due to the tailcap LEDs. But in my boost circuit the VIN to GND path likely doesn’t have much drop - is that providing a suitable path for driving the main LED since there is negligible voltage drop?

On a lighter note, I put one of my Big Switch boards into a Convoy L2 tonight. Having the bottom of the board bare made the installation a breeze; no hacking away at things to make clearance for the resistors on the bottom of the board.

Red, White, & Blue - Happy Independence Day, America!

I’m wondering if you have tried changing the bleed and led resistors. I had a light that had the same behavior until I tried change the resistors to get a value that worked. The led’s in the tail look very bright to me, but I like mine very low anyway so they don’t light the bedroom up at night.
I usually set mine around 50 to 82k, probably a little low for some. You might try changing resistors.
I do understand your using a boost ic, don’t know if this will help or not just a thought.

I’ve tried 680 and 300 ohm bleeders so far, didn’t make any difference. I haven’t tried different tailcap resistors. I’m not against that, but hopefully this will be flexible enough where that wouldn’t matter much.

First, I want to say that the voltage regulator on the tail capis a really good idea. I hope we all can see more little projects like this. And you made a very good video! Explains all the little details (especially the dimming on the LiIon cell).

Now I will make a really technical theory that could explain the problem. But don’t think that if it’s me pointing out a problem, that it’s 100% correct. I could be wrong as well, i’m just guessing here:
It’s clear that there is current flowing from the boost circuit input to ground. And when you go higher in current, it starts working. There’s enough current flowing through the TPS61221 for the low modes, but not for the higher ones.
The only pin possible is the L pin.
When you have your light off, and screw the tail on, the tail starts lighting up. Then you click, and the main LED goes on, the tail goes off (all normal behaviour). Now if you then click, the main LED won’t go off. I think this has to do with the way the IC controls the boost operation. The TPS61221 has a so called “hysteretic current mode” control circuit (variable frequency control). It keeps the current ripple on the inductor at 200mA, and offsets the current ripple according to the load. This way the voltage stays constant. Now that’s a problem in your situation. It simply won’t reach the 200mA to start switching when you click in the low mode. This is why the lamp stays on. The IC tries to charge the inductor until the current reaches 200mA (by turing on the lower MOSFET from the inductor to ground —> current flow keeping the LED on), but it never reaches 200mA. The current keeps flowing and the main LED stays on.
On the higher modes it works, not sure why though. That’s why I’m not sure. It should be the same result all the time with my theory.
I don’t understand the exact coherence (is that the right word to use here?) of the mode switching and the 200mA hysteresis right now, but I think this is the reason.

I’ll try to completely understand though.

To be honest, I’m impressed the boost circuit works in the first place, it should never reach 200mA ripple because of the bleeder. On startup, the boost IC isn’t controlled with the hysteresis, but with a special startup controller with limited switching current (look at Figure 20 in the datasheet)

Nice work :+1:

Does the input capacitor help in it attaining startup? In my testing, I tried removing the input capacitor. No real change on Li-Ion. On NiMH it failed to startup in any mode… wouldn’t work at all.

What if a different IC was identified that had a lower ripple current? That’s the same as “switching current”, right? It looks like the Ricoh R1210N331D-TR-FE (3.3V version) has a 100mA switching current rating.

Ripple current is the difference between the highest current and the lowest current flowing through the coil (the delta current).
switching current is either the average current, or the absolute maximum current flowing through the inductor, depending on the subject you’re talking about (from x axis to the average, or the highest point)

The Ricoh could work, but I would keep looking for some alternatives, I’m sure there are better options. Does it have to be a package with leads, or is a no-lead package okay as well?

Thanks for the explanation. I’m good with a no-lead package as I’ll be hot-plate reflowing them. I’m definitely open to suggestions if you care to join the search!