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## Solid States Devices => Joule Thief => Topic started by: carlprad on September 23, 2013, 04:35:23 AM

Title: How do I measure the efficiency of a Joule Thief?
Post by: carlprad on September 23, 2013, 04:35:23 AM
Hi everyone

I really could use everyones help to figure out the best way to measure the efficiency of a Joule Thief.

Lets say I have a 12v battery on the input. I plan to attach an "inline watt meter" to monitor the input power.

However, what should I place on the output end to see how efficient the Jt is?

Should I place a 12v "rechargeable" battery on the output end and see how much of the "input" battery gets drained in order to charge the "output" battery?

In other words, what is the best setup to get a clear representation of how much I'm getting out of the JT?

Thanks everyone.

Title: Re: How do I measure the efficiency of a Joule Thief?
Post by: Dark Alchemist on September 23, 2013, 05:04:43 AM
http://rustybolt.info/wordpress/?p=452

I can't say it any other way that isn't covered there and more on that guy's blog but you will need to scan his pages for the info as he learns as he goes.

Good luck.

Title: Re: How do I measure the efficiency of a Joule Thief?
Post by: carlprad on September 23, 2013, 03:38:23 PM
Dark

However, and maybe I'm having trouble explaining my goal, I want to know how much power I take out of one battery, in order to charge the other battery.

Meaning, if I have one battery as my input, how can I monitor its power depletion, as I charge the battery at the output.

Is their a way to monitor the charge of both batteries, as one is being depleted, and another charged, in order to see exactly what the ratio is between them and how efficient the Jt really is?

Or, is there a better way to see exactly what the gain, in performance, I'm getting from the JT?

Thanks

Carlos

Title: Re: How do I measure the efficiency of a Joule Thief?
Post by: Dark Alchemist on September 23, 2013, 03:50:58 PM
The only way I have seen is using the 1 ohm method or simply having two meters with one at the giving battery and the other at the receiving battery since both have resistances it works.  If there is another method I have not seen it.
Title: Re: How do I measure the efficiency of a Joule Thief?
Post by: carlprad on September 23, 2013, 03:54:33 PM
I'm new to all this.

So, if you would kindly explain the "1 ohm" method and give me an idea of what the setup would look like for the two meters, I would be very grateful.

Thank you for your patience with a neewbie.

Carlos
Title: Re: How do I measure the efficiency of a Joule Thief?
Post by: Dark Alchemist on September 23, 2013, 04:35:57 PM
I'm new to all this.

So, if you would kindly explain the "1 ohm" method and give me an idea of what the setup would look like for the two meters, I would be very grateful.

Thank you for your patience with a neewbie.

Carlos
The one ohm method is in that video I linked and he shows you on his scope but you could use a DMM as well but that is for LEDs which I can't even attach a probe to in my Multisim simulator when running DC Sweep Analysis.
Title: Re: How do I measure the efficiency of a Joule Thief?
Post by: carlprad on September 23, 2013, 05:20:52 PM
is it as simple as attaching an ammeter at the input and an ammeter at the output and measuring the difference?

Title: Re: How do I measure the efficiency of a Joule Thief?
Post by: Dark Alchemist on September 24, 2013, 03:04:04 AM
is it as simple as attaching an ammeter at the input and an ammeter at the output and measuring the difference?
For an LED no but for a battery that has a resistance I am not sure.  What I am thinking is that if you simply hooked up a meter at the outgoing battery it would be skewed due to that battery having a voltage.

Maybe adding a fast switching diode before the battery and measuring the opposite side that is away from the battery?  At least that would prevent the battery from injecting its voltage/current into the meter since the diode would stop that.
Title: Re: How do I measure the efficiency of a Joule Thief?
Post by: MileHigh on September 24, 2013, 03:21:22 AM
A real Joule Thief operates off a single 1.5-volt battery and has one or more LEDs as the load.  Because of it's somewhat unusual operating characteristics and the fact that LEDs complicate the output power measurement because they are what is called non-linear electrical devices it's actually much more complicated and difficult than you think.

The real way to do it would be with a digital storage oscilloscope.  Preferably you would have one with a built-in math function.  You have to measure the instantaneous voltage and current output by the battery, as well as measuring the instantaneous voltage and current across the LED.

It just happens to be a circuit where multimeters and a basic scope will not cut it and you can't really make the power measurement unless you have the right equipment and you really know what you are doing.

MileHigh
Title: Re: How do I measure the efficiency of a Joule Thief?
Post by: Dark Alchemist on September 24, 2013, 03:53:24 AM
A real Joule Thief operates off a single 1.5-volt battery and has one or more LEDs as the load.  Because of it's somewhat unusual operating characteristics and the fact that LEDs complicate the output power measurement because they are what is called non-linear electrical devices it's actually much more complicated and difficult than you think.

The real way to do it would be with a digital storage oscilloscope.  Preferably you would have one with a built-in math function.  You have to measure the instantaneous voltage and current output by the battery, as well as measuring the instantaneous voltage and current across the LED.

It just happens to be a circuit where multimeters and a basic scope will not cut it and you can't really make the power measurement unless you have the right equipment and you really know what you are doing.

MileHigh
Does that go for his battery to JT to battery setup as well?
Title: Re: How do I measure the efficiency of a Joule Thief?
Post by: MileHigh on September 24, 2013, 04:11:12 AM
It might be somewhat easier and you could make some approximations even with an analog scope.  TinselKoala often does manual power or energy calculations from digital pictures of scope displays.  It would remain a significant challenge.

One trick would be to substitute the charging battery with a largish cap in parallel with a variable resistor.  The higher the near-DC voltage across the cap, on first inspection you would expect to get a slightly higher power output reading.  That's because the higher the cap voltage the shorter the output current pulse and therefore the less energy lost in the diode.  Note if you switch to a battery or virtual battery done with a cap and a variable resistor, then the diode does not count as part of the output.  The load is on the other side of the diode.

If you use a fresh source battery where the voltage does not droop too much then you could probably get a decent input power reading.

None the less, these measurements would all have to be very carefully made.  You really and truly need to know what you are doing.

MileHigh
Title: Re: How do I measure the efficiency of a Joule Thief?
Post by: Dark Alchemist on September 24, 2013, 06:16:49 AM
It might be somewhat easier and you could make some approximations even with an analog scope.  TinselKoala often does manual power or energy calculations from digital pictures of scope displays.  It would remain a significant challenge.

One trick would be to substitute the charging battery with a largish cap in parallel with a variable resistor.  The higher the near-DC voltage across the cap, on first inspection you would expect to get a slightly higher power output reading.  That's because the higher the cap voltage the shorter the output current pulse and therefore the less energy lost in the diode.  Note if you switch to a battery or virtual battery done with a cap and a variable resistor, then the diode does not count as part of the output.  The load is on the other side of the diode.

If you use a fresh source battery where the voltage does not droop too much then you could probably get a decent input power reading.

None the less, these measurements would all have to be very carefully made.  You really and truly need to know what you are doing.

MileHigh
From everything I have read people are putting .1uf to 22uf in parallel with the battery to help stabilize the circuit from a battery that would be constantly draining.  For me I would kill the battery and use a benchtop power supply as the source voltage and current then go from there.

You are correct you need the tools and the know how, or a lot of reading and patience, to get this done but the tools are mandatory.
Title: Re: How do I measure the efficiency of a Joule Thief?
Post by: TinselKoala on September 24, 2013, 10:09:23 AM
You measure the input power like this:
You charge a good quality capacitor, like 2 Farads, to the top running voltage of your JT, like 1.500 V. Then you monitor the voltage on the cap with an accurate voltmeter, and you monitor the time with a stopwatch. You turn on the JT and let the thing run until the cap is down to, say, 1.200 volts, and you record the time this took, say it was 20 seconds (or whatever).
The energy you put in is the difference between the energy of the cap charged to 1.500 v and 1.200 v. The average power is this energy divided by the time.

You measure the output power like this:
Say your JT is powering an LED. You measure the LED's light output in any of several ways: photoresistor, phototransistor, intensity-frequency converter sensor chip, etc. You have previously calibrated your sensor using DC power to the LED. So now you can get instantaneous power output readings during your input power measurement tests for comparison.
Or, you can use common methods to charge an output capacitor through a diode. Again, the time it takes to charge an output cap from one voltage up to another voltage will give you a total energy out and the average power during the time interval.

The efficiency is then the ratio of the output power to the input power. Or, the ratios of the energies in and out during a specific time period.

You don't even need a scope for this, just a couple of accurate voltmeters, and some accurately known capacitances, and a good timing method.
Title: Re: How do I measure the efficiency of a Joule Thief?
Post by: Pirate88179 on September 25, 2013, 07:07:58 AM
You measure the input power like this:
You charge a good quality capacitor, like 2 Farads, to the top running voltage of your JT, like 1.500 V. Then you monitor the voltage on the cap with an accurate voltmeter, and you monitor the time with a stopwatch. You turn on the JT and let the thing run until the cap is down to, say, 1.200 volts, and you record the time this took, say it was 20 seconds (or whatever).
The energy you put in is the difference between the energy of the cap charged to 1.500 v and 1.200 v. The average power is this energy divided by the time.

You measure the output power like this:
Say your JT is powering an LED. You measure the LED's light output in any of several ways: photoresistor, phototransistor, intensity-frequency converter sensor chip, etc. You have previously calibrated your sensor using DC power to the LED. So now you can get instantaneous power output readings during your input power measurement tests for comparison.
Or, you can use common methods to charge an output capacitor through a diode. Again, the time it takes to charge an output cap from one voltage up to another voltage will give you a total energy out and the average power during the time interval.

The efficiency is then the ratio of the output power to the input power. Or, the ratios of the energies in and out during a specific time period.

You don't even need a scope for this, just a couple of accurate voltmeters, and some accurately known capacitances, and a good timing method.

I think this post should be preserved in a thread of it's own so folks can reference it.  This method is simple but yet accurate and all of us playing with JT's should use this.  (In my opinion)  I always thought that using caps was a good idea but, I never really knew how to do it...until now.

Thanks TK.

Bill
Title: Re: How do I measure the efficiency of a Joule Thief?
Post by: MileHigh on September 25, 2013, 07:53:03 AM
Using a cap for the load is fine for a JT-type circuit because the output comes from a discharging inductor.  However, be careful and don't take it for granted that a capacitor will work as a load in a generic sense for other circuits and measurements.  Note that how much power goes into a load is determined by the impedance of the source compared to the impedance of the load.  Capacitors are not a constant impedance so you could be in a situation that's fraught with problems.

How you make your measurements is in a way related to how far you want to "stray" from the original JT circuit.

Anyway, the fundamental thing at play in a Joule Thief is that you charge an inductor and then discharge it into a LED and repeat.  There are losses when you charge the inductor due to wire resistance and similarly you have the same wire resistance losses when you discharge it.  There are also losses due to the hysteresis loop of the core.  The other thing is the quasi "golden rule" where you don't want to energize the inductor for too long to minimize the resistive losses.   A very similar technology is used in computer switching power supplies and small DC-to-DC converters for printed circuit board use.  They are like a high frequency JT charging an output capacitor, and there is an output voltage sensing circuit that is used in a feedback loop to control the switching and keep the voltage output constant.  They can be greater than 95% efficient.

Just look at a modern motherboard where the processor socket is like this special throne with various voltages ready and waiting for the chip.  It's surrounded by capacitors to give the CPU chip a happy "nest" where the voltage is stabilized for "the beast."  Most PC hobbyists are familiar with tweaking the various motherboard voltages to squeeze out the maximum performance.  So it's probably fair to say that a motherboard has a whole bunch of "smart" JT-like circuits to keep the processor beast happy.  The JT chips are probably on the "I-squared C" bus and wait for their commands from the BIOS.  A modern motherboard is quite an amazing piece of technology and one we (happily) take for granted.

MileHigh