Author Topic: Using varistor instead of pot. to autom. adjust res. in joule-thief for bat.life  (Read 4175 times)

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Offline DistelzombieTopic starter

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I'll happily await your response, JS :D

mmagin: I don't want to fully replace the circuit. As far as I understand my circuits performance in comparison to others, it's quite great, isn't it?  ??? I don't understand the circuit very much. I would need to have a scope to finally understand it.  |O
But... I think I get one of these in order to have something to compare mine to. LTspice says my efficiency isn't particularly great, (~63%) but it isn't very accurately simulated and it also does not have indicator for light-output. (And Light over Time is really good, I think)
I also found a thread on stack exchange where someone used a 2N3819 JFET. So now at least I know that many would work fine.
Is that ZXSC300 you recommended a good one? Are there better ones? Do you know which on is "the best"?  ;D

Offline JS

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63% ain't that bad, you have a serial diode with the LED generating losses right at the load, an oversized shottky would improve there. Then there is the transistor saturation voltage, if you could use a mosfet with low rds you improve there. The coupled inductor might have some considerable losses, etc. Adding up to 60+% is completly reasonable.

JS

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Offline DistelzombieTopic starter

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I tried using shottkys. The Light output was considerably lower and it had a higher current at the LED. I'm currently using normal N4007 diodes. I think 1N4007. They worked best for some reason.

A MOSFET? Hm, are they fast enough?
If I can I buy from digikey. (They also have that IC mmagin was talking about) I just hope I can buy there without a credit-card and that they send to germany. If you say "low rds" do you mean the "Rds On (Max) @ Id, Vgs" section/column in digikey or the "Drive Voltage (Max Rds On, Min Rds On)" column?
Right now the page doesn't work anymore, unfortunately.

Offline mmagin

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Is that ZXSC300 you recommended a good one? Are there better ones? Do you know which on is "the best"?  ;D

I've generally been happy with my limited experience with the formerly-Zetex led regulator parts that are now sold by Diodes Inc, though I suspect there are many others, since it's the standard kind of thing for single-cell flashlights these days.  I'm sure there are even cheap ones made by Chinese manufacturers I've never heard of.

I certainly get that it is more fun to design something from scratch using discrete parts :)
 

Offline DistelzombieTopic starter

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What is low Rds? Is 4 Ohm considered low?

EDIT: OK there are very expensive ones with less than a milli Ohm.
This is the lowest priced MOSFET with Rds under 0.8mOhm that is also available right now. Still costs 2.25€! xD
http://www.ti.com/lit/ds/symlink/csd16570q5b.pdf
Would it work? I need to know if I'd kill it when I connect it to the JT. (Huge Voltage spikes, I assume) It's only rated for 25V max and on my breadboard the JT charged capacitors up to more than 30V. Unfortunately this is another case where I need a Scope to see what is happening.
Next one with 40V costs 3.1€ already.
http://www.onsemi.com/pub/Collateral/NVMFS5C404NL-D.PDF
« Last Edit: July 10, 2018, 01:18:30 pm by Distelzombie »
 

Offline JS

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How low is low Rds depends on the rest of your circuit, how low you need to go depends on the efficiency you want. If your coil has a resistance of 2Ω having an Rds of 4Ω is too high but there's not much reason to go for single mΩ figures, ~200mΩ seems more reasonable.

JS
If I don't know how it works, I prefer not to turn it on.
 

Offline DistelzombieTopic starter

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Ok thank you.

but, again: Is Rds the only thing I have to look for? What about Drive Voltage? Are they even working at this low voltages? Do I have to look for a Vdss that is higher than 30V?
Please answer all questions.

Offline JS

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I don't know if it would work in that circuit as it is, probably not. Of course there are other specs to watch for, you don't need any mosfet with low rds, but the mosfet you pick should have low rds.

JS

If I don't know how it works, I prefer not to turn it on.
 

Offline DistelzombieTopic starter

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You're not helping. But on the other I don't know anything and i can't demand you tell me everything.

Offline JS

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  I'd just pointed out some inefficiency spots on the design, you should get it to work and see if efficency is a problem, maybe you are at 60% efficiency and your battery life is enough, so you don't need to mock around any longer, you won't get to 98% with this, efficiency in low voltage is quite low, as a small drop represents a big portion of the total supply.

  To be able to optimize a design you need a lot of things, lot of measurements and knowledge of basic devices are two of them. I can't make measurements and I don't have the time to search for your part, I can suggest where to look for.

JS
If I don't know how it works, I prefer not to turn it on.
 

Offline DistelzombieTopic starter

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  I'd just pointed out some inefficiency spots on the design, you should get it to work and see if efficency is a problem, maybe you are at 60% efficiency and your battery life is enough, so you don't need to mock around any longer, you won't get to 98% with this, efficiency in low voltage is quite low, as a small drop represents a big portion of the total supply.

  To be able to optimize a design you need a lot of things, lot of measurements and knowledge of basic devices are two of them. I can't make measurements and I don't have the time to search for your part, I can suggest where to look for.

JS
Yea, but you're contradicting yourself and not answering questions. Here:
I don't know if it would work in that circuit as it is, probably not. Of course there are other specs to watch for, you don't need any mosfet with low rds, but the mosfet you pick should have low rds.
And I explicitely said: "Please answer all questions.", because I knew you wouldn't do that. This is really frustrating. How am I supposed to even start? It actually sounds like you're not knowing much more than I do.  ;D

But thanks for the help you've done so far :)
« Last Edit: July 11, 2018, 08:34:01 pm by Distelzombie »
 

Offline BrianHG

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I'm curious as to why no one has recommended trying a slow CDS photocell held up to the LED, working on top of your existing design.  Since this type of photocell acts as a true pure resistor, as the battery drains and the led gets slightly dimmer, the CDS photocell will be wired to boost the transistors turn on current.  With this you get the same circuit, and an almost regulated brightness, though, photo cell placement and initial calibration may be touchy, it may be interesting to experiment with.
 

Offline JS

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I could do the design for you if you give me the specs, that would take me some time for which I charge a fee for, I'm pointing you in what to look for, I haven't seen measurements results neither run the sim to make better suggestions. I'm not contradicting myself, drop in replacement of a BJT for a mosfet doesn't usually work, but a circuit using a mosfet tends to be more efficent than one using a bjt. I've also said that bjt efficency is probably good enough, and I think you only need to address the brightness stability, that would make a viable product, all the rest is for later optimization, I don't think you need to learn all about mosfets to get this running.

JS

If I don't know how it works, I prefer not to turn it on.
 


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