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[SOLVED] joule thief output voltage

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marianodanilo

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hello another question,,I have a working JT,,but when I'm measuring the CE voltage (using DMM) with or without LED, I can only measure above 0.8V..I know my JT is working because it can light up my LED with source voltage of above 1.1V.. how can I measure the output voltage correctly..
 

betwixt

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You have to use an oscilloscope. The voltage is in the form of very short spikes with almost nothing between them, the meter is reading the average over time which is much lower than the peaks that actually light the LED.

There is a way that will not be accurate but give a better idea of the voltage you can try: connect a signal diode (1N4148 or similar) and a 1K Ohm resistor in series with the DMM and also connect a capacitor of about 1uF across the DMM terminals. It will make a crude peak voltage meter. If you reverse the signal diode you can measure either the positive peaks or negative peaks of the signal.


Brian.
 

rohitkhanna

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...If you reverse the signal diode you can measure either the positive peaks or negative peaks of the signal...
Brian.
Imho even the usual simplistic JT circuit would only give positive pulses. I would also suggest ceramic caps of smaller value (to avoid leakage pull-downs) and a DMM with high input impedance ~ 10's of megohm.
 

BradtheRad

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The C-E voltage rises to whatever is the operating voltage of your LED. It does not go higher than that. The LED gobbles any overvoltage immediately.

However if you remove the LED and operate the joule thief with no load, then voltage will spike to a very high level. It may go high enough to ruin your transistor.

- - - Updated - - -

I can only measure above 0.8V
I believe you are asking how to know what is happening as the battery drops below 0.8V?

The coil generates smaller amounts of current. It turns on the led for shorter amounts of time. Its light is dimmer.

There is a point where the transistor no longer turns on, and/or the coil no longer carries much current so as to generate sufficient kick when it is switched off.
 

marianodanilo

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i found that the output current is too small..is there any way i can amplify the current
 

BradtheRad

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Reduce the resistor going to the bias terminal. This will have the effect of increasing current through everything else.

Screenshot:



I'm not saying 100 mA is a safe amount to send through the led. It's up to you to measure average current and keep it under a safe limit.
 
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