So here in this post we are going to see how we can make one small 1.5V to 12V converter circuit using just two common transistors and one cheap coil. This whole idea was actually requested by one of our visitor friend Mr. Kliff, and we tried to solve it step by step.
Now What Kliff Asked Us
So Kliff said like this:
"Hello, I was searching through your blog and checking many circuits but could not find exact answer for my requirement. So I thought better I ask you directly.
I want to do one thing like this that I have 4 pieces of 3V CR2032 coin cells and I will connect all of them in parallel. That I am doing so that they can last longer and not drain fast.
Then I want to use one Joule Thief kind of circuit to boost that low voltage and make it around 12.5V to 14V, so that I can light up 3 pieces of color changing LEDs or maybe even one 12V LED bulb which I found on eBay. I have even added the link of that bulb here.
But now my actual problem is, I don't want to use any transformer or any hand-wound coil! I want to make this circuit with only transistors and other simple parts which we can easily find from shops. That is because finally I am going to put this whole thing inside transparent resin and make it permanent.
If you can help, it will be very great. Also if you can give me full wiring diagram with part names written clearly that will be more helpful for me.
Thanks a lot,
Kliff"
Then What We Replied to Keith
So we told like this:
Hello Kliff,
Thanks a lot for contacting me!
But I am really sorry, without using a coil this idea is not going to work because in a Joule Thief circuit that coil is the main part which does the full energy extraction job from the low voltage battery. Without coil, we cannot create that flyback effect.
Regards.
Now What Kliff Replied Again
Then Kliff understood the point and he replied back like this:
"Yes I was thinking maybe you will say something like that. So then I want to ask you – can we use any ready-made coil with ferrite core, like the ones we get from Radio Shack or DigiKey, so that I don’t have to wind anything by hand?
And can you please help me by designing one such circuit which uses those readymade coils?
Also I have one more question, since I am going to seal this whole circuit in clear resin, will that cause any overheating problem for the coil? Like, it won’t get air to cool down so will it burn or get damaged?
I will be using mercury switch for on and off.
Also I saw one 330uH coil value used in one project here:
https://www.instructables(dot)com/id/Joule-Thief-no-IC-and-no-Transformer/
Will that work for me?
But if this idea is not good and not going to work, then please just give me a full circuit diagram with proper wire coil and all part names, and I will make it that way only.
Thank you once again,
Kliff"
Now Our Final Answer and Circuit Analysis
So after checking everything we said like this:
The circuit which you have given from that Instructables link looks like it will work nicely for your need. That circuit is able to boost from 1.5V to 12V easily using just basic components.
Also the second link from DigiKey, that coil which you are talking about, is perfect match for this kind of Joule Thief circuit. So you can surely try using that one.
About the resin sealing – no worries, you can safely put the coil and full circuit inside the resin. That coil will not get too hot and the resin will not cause any issue. Joule Thief circuits are normally very low current and low heat type.
So you can proceed without fear.
Regards.
Circuit Diagram
Inductor 300uH or similar (Picture)
