So this would work, then? (In general. I might use a different 3V coin cell, and I probably have some polarities wrong. I usually just test the LEDs as I go to make sure I've got them the right way around.)

I really appreciate all the help!

Seems like you're missing some components. You've dropped that boost IC in the circuit in much the same way you could add a linear regulator to reduce voltages, but the boost IC doesn't work by itself. Look at the typical application circuit below.

It appears to require 4 external components. I'll admit I haven't put one of these circuits together before, so I don't claim any expertise, but just based on what I see in the datasheet, it looks like you need more parts.

 

Good catch!

I have no idea what the extra components are supposed to do, unfortunately. It's entirely possible that you're right and the circuit wouldn't work at all, but without knowing what they're intended to do, I have no idea what values to choose for my specific use case. They do provide some suggested values, which I guess I can go with if I can't figure it out, but I really wish I understood. (I would like to believe that driving it from a dinky coin cell means that I can do without some of the components that might be simply protective elements, but who knows?)

Honestly, I'd love some pointers in general for figuring out what extra parts in the datasheet are supposed to do. I did some googling, and it seems like the capacitor that usually shows up between vcc and ground is there to smooth out the input voltage (or something along those lines), but the random diode and inductor? No idea. The datasheets don't usually bother explaining it unless it's important for configuring the output of the IC itself, sadly.

 

Darn. The exact diode is apparently obsolete now too, and I don't know what specs are important if I can't find an exact match. At least capacitors and inductors should be easier to find equivalents of, I hope?

 

• Is this diode close enough? https://www.digikey.com/product-detail/en/B130LAW-7-F/B130LAW-FDICT-ND/725052/?itemSeq=272080530
• How important is capacitor type? Is it critical that Cl be tantalum and Cin be electrolytic? If so, why? I know tantalum capacitors are better... somehow...??? but I don't remember why.
• The specific inductor they recommend does exist (https://www.digikey.com/products/en?mpart=CR54NP-101KC&v=308), but it seems to be physically pretty chunky. Here's a bitty SMD inductor with the same inductance; would it do as well, or is there something important about being a wirewound inductor? https://www.digikey.com/product-detail/en/MLZ2012N101LT000/445-9496-1-ND/3909669/?itemSeq=272081330

If you all have any guidance on what kind of wiggle room I have on the values, that would be great too. Or, y'know, if I can leave any of these out without totally breaking my circuit.

 

• Is this diode close enough? https://www.digikey.com/product-detail/en/B130LAW-7-F/B130LAW-FDICT-ND/725052/?itemSeq=272080530
• How important is capacitor type? Is it critical that Cl be tantalum and Cin be electrolytic? If so, why? I know tantalum capacitors are better... somehow...??? but I don't remember why.
• The specific inductor they recommend does exist (https://www.digikey.com/products/en?mpart=CR54NP-101KC&v=308), but it seems to be physically pretty chunky. Here's a bitty SMD inductor with the same inductance; would it do as well, or is there something important about being a wirewound inductor? https://www.digikey.com/product-detail/en/MLZ2012N101LT000/445-9496-1-ND/3909669/?itemSeq=272081330

If you all have any guidance on what kind of wiggle room I have on the values, that would be great too. Or, y'know, if I can leave any of these out without totally breaking my circuit.

Unfortunately, I'm completely unqualified to answer your questions. I've seen and read enough about circuits similar to these to be pretty confident that you can't skip any of those components. None of them are protection - they're all part of how the higher voltage is developed. As for choosing appropriate substitutes, I'm no help.

Hopefully someone else who's already active in this thread can advise you better on the specifics. If not, perhaps @ebp will join us. I know he's intimately familiar with SMPS design, and I think circuits like this one are like the little kid brother of SMPS designs (a gross oversimplification, but still...)

 

Seems like this project is turning out more complicated than I expected... oh well. At least I'm gonna learn something!

 

Maybe I should just go with one of these instead: https://www.diodes.com/assets/Datasheets/ZXSC380.pdf

Just one inductor, and from the graphs, at a 3V input, the value of the inductor is pretty much irrelevant.

 

Whoops, was looking at the frequency graph. Regardless, it should be much easier to set up, although I might have to experiment with inductor values if I continue using 3V batteries. (Alternately, I could use a ~1.5V coin cell and be comfortably within their typical use case.)

 

Whoops, was looking at the frequency graph. Regardless, it should be much easier to set up, although I might have to experiment with inductor values if I continue using 3V batteries. (Alternately, I could use a ~1.5V coin cell and be comfortably within their typical use case.

Menandore, If this project you are trying to is a one time project, I will be happy to send you 3mm or 5mm blue and white LED's that match in forward voltage and will work on 3V in parallel and requires only 1 current limiting resistor. I also have 2ea 2032 battery holders for a printed circuit board. All I ask is you pay shipping for the parts.

 

Menandore, If this project you are trying to is a one time project, I will be happy to send you 3mm or 5mm blue and white LED's that match in forward voltage and will work on 3V in parallel and requires only 1 current limiting resistor. I also have 2ea 2032 battery holders for a printed circuit board. All I ask is you pay shipping for the parts.

Thanks for the offer - I appreciate it! If I can get this working, though, I'd like to make more of them - so I'm going to keep trying to figure it out.

 

Thanks for the offer - I appreciate it! If I can get this working, though, I'd like to make more of them - so I'm going to keep trying to figure it out.

ok

 

I recommend trying 8-10mA per LED, not 20mA. 20 is a maximum rating it can tolerate. Start at 8-10mA and see how it looks, adjust from there.

 

I recommend trying 8-10mA per LED, not 20mA. 20 is a maximum rating it can tolerate. Start at 8-10mA and see how it looks, adjust from there.

Ah, good to know. I was wondering how I could have been getting ANY kind of battery life at 20 mA from a 40 mAh battery - seems like I was probably running the LEDs at quite low current. They were still plenty bright, though.

Also, your username is fantastic and it made my day.

 

As usual answers on this thread are overly complex KISS

these key ring torches just connect an led across coin cell , no resistor , no nuffin!
For your project you can select leds that operate at the voltage and current that will give the life you require.

 

Problem solved! Here I'm driving 5 white LEDs in series from a 1.5v silver oxide coin cell with this little ZXSC380 chip - only one external component is needed, the inductor. I'm getting much less current than the datasheet indicates, but it might just be due to different battery chemistry (or else my multimeter is wonky). Still enough to light them up, though.


https://i.imgur.com/U9BxhHF.jpg


Mods Note:
Please upload the image file to the forum, otherwise when the links lose the connection then the image will be lose, the image already upload to the forum now.

 

I'm getting much less current than the datasheet indicates, but it might just be due to different battery chemistry (or else my multimeter is wonky). .

An ammeter does have a significant resistance .

Adding an ammeter to the circuit will add resistance so will reduce the current . The effect is not too noticeable at higher voltages , but will have a BIG effect on low voltage circuits ....

A clamp ammeter will affect the current less , but I think it still has an effect.

 

The tiny battery cell is not designed for the fairly high power it must supply to the voltage boost IC. Then the battery cell supplies as much current as it can which is only a few mA. But your LEDs are bright so the tiny battery life will be very short.

The battery life will be much longer if you add a circuit to blink the battery power to the voltage converter, so that the converter is not powered most of the time.

 

The tiny battery cell is not designed for the fairly high power it must supply to the voltage boost IC. Then the battery cell supplies as much current as it can which is only a few mA. But your LEDs are bright so the tiny battery life will be very short.

The battery life will be much longer if you add a circuit to blink the battery power to the voltage converter, so that the converter is not powered most of the time.

Not too surprising - I know I'm abusing the battery, and I'm fine with just a few hours of battery life for this version. But I do want to play with putting a bitty microcontroller on it for my next project.