1.2 volt to 5 volt boost converter - Page 1
Forum section: electronics / beginners

author: msuffidy
author_rank: Frequent Contributor
timestamp: May 01, 2025, 12:45:54 am
content: I got a solar light string from a store at a very low cost. I cut the line to the lights and measured the voltage, which was 1.2V. I am guessing it is more or less straight out of a nicd or some other chargeable AA battery inside the solar box. I was looking for a easy way to boost the 1.2V line to 5V to charge old power banks which then can charge more expensive devices. I found some videos describing crude converters that do not have regulator ICs. It seems like a lot of the stuff maybe can be found in a dead CFL bulb. I am not sure if these parts I found are adequate or not. This video wanted a BC547 transistor, but there are 2 4120PL transistors on this CFL as well as I have 2N222A here, which I think are all interchangeable? I was going to put that 200V 10uf capacitor across the output to try to reduce spikes.

author: msuffidy
author_rank: Frequent Contributor
timestamp: May 01, 2025, 05:37:51 am
content: I got these parts from the CFL bulb and I was kind of tempted to just use a 2 lead inductor controlled by a resistor capacitor oscillator. The inductors say 2.2? and 1mh. I guess I could install spice and simulate this and see what happens.

author: MrAl
author_rank: Super Contributor
timestamp: May 01, 2025, 06:08:13 am
content: Quote from: msuffidy on May 01, 2025, 12:45:54 amYou might check out converters like the Joule Thief. It works down to low voltage and provides higher output. You may need some other parts too though.That's if you don't want to use a chip made for this, of which there are many.

author: Zero999
author_rank: Super Contributor
timestamp: May 01, 2025, 08:04:27 am
content: It will take ages, days, possibly weeks, to fully charge a USB power bank from a solar light string. Heck, it might not even be able to provide enough power to make up for that lost in the charging circuitry.

author: jwet
author_rank: Frequent Contributor
timestamp: May 01, 2025, 01:44:49 pm
content: One of my favorite youtubers- "Bigclive" did a teardown and mini analysis for these circuits. Entertaining and knowledgeable- all you could want to know.

author: tunk
author_rank: Super Contributor
timestamp: May 01, 2025, 01:59:36 pm
content: Recently Diodegonewild had a video describing how they works:https://youtu.be/8JeSQ_8Ly5U

author: xvr
author_rank: Frequent Contributor
timestamp: May 01, 2025, 02:53:04 pm
content: Quote from: msuffidy on May 01, 2025, 12:45:54 amThen connect LED USB lamp to power bank and light solar light string. Voilà, you have a perpetuum mobile.

author: msuffidy
author_rank: Frequent Contributor
timestamp: May 01, 2025, 03:03:31 pm
content: Quote from: Zero999 on May 01, 2025, 08:04:27 amWell I think the way it works is the nicd battery can supply a good amount of current granted there are no in line resistors. Mine does light up like 60 LEDs. So you may be able to generate an adequate 5V supply, just the battery will discharge pretty quickly and you would have to keep charging up the solar box so in that way you are correct. You can see from what this guy did that seems to be the case.

author: IanB
author_rank: Super Contributor
timestamp: May 01, 2025, 03:10:44 pm
content: Quote from: msuffidy on May 01, 2025, 12:45:54 amI have, from the days before lithium power banks, an "emergency phone booster". It takes two AA cells, and it outputs 5 V to charge a phone. It is, however, borderline functional. It only works in any kind of satisfactory way if you install two fresh Energizer Lithium cells into it, and even then, it struggles to produce much useful charging current, and this is with a professionally designed and engineered boost converter.In short, voltage is not the problem, but power and energy is. Small 1.2 V cells do not contain sufficient energy, and cannot produce sufficient power, to charge bigger devices.Your quest is hopeless and cannot succeed.

author: Andy Chee
author_rank: Super Contributor
timestamp: May 01, 2025, 08:03:22 pm
content: Co-incidentally, I was toying with a similar themed project made from junk parts I have lying around.In my case, I am building a 5V converter with 4-6V input. Haven’t decided on SEPIC topology or 4-switch boost (see p.18 Figure 17)https://www.onsemi.com/pub/Collateral/AN920-D.PDF

author: Analog Kid
author_rank: Super Contributor
timestamp: May 01, 2025, 08:20:03 pm
content: Quote from: MrAl on May 01, 2025, 06:08:13 amYes, a Joule thief.I've built many of these to drive high-brightness LEDs, using this circuit:The transformer is a little hand-wound jobbie with the # of turns as indicated, wound on a little piece of ferrite (1/8"/3mm) rod.Very easy to build and might just work for your application. It delivers pulses with ~7 volt peaks.I've run up t 4 LEDs in series/parallel with this, plenty bright: somewhere around 80 mA.I don't know what the limit is. Give it a try?

author: msuffidy
author_rank: Frequent Contributor
timestamp: May 01, 2025, 08:26:05 pm
content: OH I more or less agreed it was going to be a problem. I was just out and shopping and I got another $4 solar thing. So I have a bunch of panels I got for around $10 Canadian now. I think I can just join them in series and parallel now and get a decent usb charger. I think I can mount the bigger panel on the back by drilling some holes and then it is some soldering and glue gunning. Ironically I suspect the bottom PCB has some of the guts of the Joule Thief and has an inductor to drive 2 bright LEDs. I measured like 1.2 volts across the LEDs so I don't really know why you need to boost it, but.

author: msuffidy
author_rank: Frequent Contributor
timestamp: May 01, 2025, 10:50:45 pm
content: OK Well I hope it is ok. I measured the solar cells under a fluorescent light and got 2, 1.5, 1.5. So I concluded 3.5 was not going to cut it and changed the wiring to full series. Problem is that it is cloudy here and I get 6V right now, so it could maybe go to 7.5 and this is to charge power banks. I used a breadboard end so I could maybe combine with another USB panel I have or use it for something else.

author: msuffidy
author_rank: Frequent Contributor
timestamp: May 02, 2025, 12:46:43 am
content: I fixed up some better USB breakout cables.

author: Gyro
author_rank: Super Contributor
timestamp: May 02, 2025, 08:03:15 am
content: Those solar cell leds almost certainly use the Chinese QX5252 IC. A single chip + inductor solution that works from 0.9 - 1.5V input...https://www.opengreenenergy.com/wp-content/uploads/2020/07/QX5252-Datasheet.pdfhttps://asset.conrad.com/media10/add/160267/c1/-/en/002268915DS01/datenblatt-2268915-tru-components-qx5252f-to-94-linear-ic-bulk.pdf

author: tunk
author_rank: Super Contributor
timestamp: May 02, 2025, 12:16:21 pm
content: At least some cheap solar power banks use this setup:5-6V solar panel -> (Scottky) diode -> BMS -> battery/cell

author: msuffidy
author_rank: Frequent Contributor
timestamp: May 03, 2025, 03:11:40 am
content: Quote from: Gyro on May 02, 2025, 08:03:15 amThey likely did before I just cut that part out. The weather has really sucked he in the last 2 days but I just want to see a basic bank charging application, which has not been realized yet.

author: Sacodepatatas
author_rank: Regular Contributor
timestamp: May 06, 2025, 12:50:16 am
content: Quote from: Analog Kid on May 01, 2025, 08:20:03 pmThe topology is so simple that can be used to convert anything linear to switched mode... I succesfully used it to upgrade a linear DC motor speed controller by inserting a small toroid and a schottky diode into the BJT + potentiometer set. Torque was much better. Here i attach a picture of a similar circuit that converts a typical linear regulator in a mixed linear/switched one...

author: Analog Kid
author_rank: Super Contributor
timestamp: May 06, 2025, 07:40:40 pm
content: That actually looks like the (sorta) classical boost converter topology, not a Joule Thief.A Joule Thief is a blocking oscillator. Similar but different.

author: Sacodepatatas
author_rank: Regular Contributor
timestamp: May 06, 2025, 11:52:17 pm
content: Quote from: Analog Kid on May 06, 2025, 07:40:40 pmThe one in the picture is also a blocking oscillator. It's just a redrawn JT put in series with the load. A JT is a boost Converter but if you put It in series with a load (and you redraw it conveniently) then you substract the boost voltage, hence you create a buck converter. If you look carefully, this circuit is just the BJT hack for increasing the current of a linear regulator, but, the original BJT (a PNP here) acts as a current source of a very high output impedance when working in linear mode (and the voltage is commanded by the low output impedance of the linear regulator). Because you insert the transformer between the base and the collector of the transistor, then a positive feedback path is created (because the transformer shifts 180 degrees) and then the transistor begins to oscillate creating a pulsed waveform. The diode is there to give a path to the current stored in the collector's coil. Because once the transistor no longer works in linear mode, it doesn't act as a current source, then, for achieving the same effect, there is an extra inductance in series with the collector coil with a value >> 10 times the former, so it acts like a choke, that also, with the output capacitor in the linear regulator, then a LPF is formed.