You can't measure Joule Thief output using a multi meter for the following reason: Multi meters are configured to measure AC voltages at 60 cycles per second, a Joule Thief, while still being AC, well I wouldn't say AC because in reality its Pulsed DC at a high frequency, that's why you can heat a whistling noise depending on your Joule Thief configuration. A multi meter is not going to be of any use for you to perform Joule Thief based measurements in DC or AC mode. If you would like to measure the irregular wave forms of a Joule Thief to determine their maximum up most voltage spike to get an accurate and correct pulsating voltage readings you will need an oscilloscope.

I forgot to add that there could be a work around that I haven't tested or confirmed for accuracy, its my theory that you can experiment with to allow you to measure the voltage using your multi meter. For that, you will need a high voltage capacitor with capacitance as less as possible, for example: 600 volts at 0.1 uF or less then connect the capacitor to the output of the joule thief. I theorize that the Joule Thief should charge the capacitor up to its maximum spike voltage and you will have a voltage in terms of Direct Current (DC) that you can, therefore, measure using your multi meter. This only would work if you expect the voltage coming out of your Joule Thief to be no more than 600 volts as most consumer grade multi meter is capped at measuring DC voltages at no more than 6...see more »I forgot to add that there could be a work around that I haven't tested or confirmed for accuracy, its my theory that you can experiment with to allow you to measure the voltage using your multi meter. For that, you will need a high voltage capacitor with capacitance as less as possible, for example: 600 volts at 0.1 uF or less then connect the capacitor to the output of the joule thief. I theorize that the Joule Thief should charge the capacitor up to its maximum spike voltage and you will have a voltage in terms of Direct Current (DC) that you can, therefore, measure using your multi meter. This only would work if you expect the voltage coming out of your Joule Thief to be no more than 600 volts as most consumer grade multi meter is capped at measuring DC voltages at no more than 600 volts. If you expect that your joule theif would generate more than 600 volts DC and would like to prepare for that possibility, then you will need a capacitor rated for higher voltages, like 1,200 volts DC at 0.1 uF (or less uF) and a multi meter rated for the maximum voltage measurements in terms of DC for your target voltage up to 1,200 volts in this example, you can also use the resistor trick to stick with your 600 volts max measurement multi meter to measure greater than 600 volts DC (in this case, you should google up how to use a multi meter to measure very very high DC voltages, and how to interpret its output readings correctly).Note about 0.1 uF capacitance value: The less capacitance the faster a joule thief can charge it up and the less likely that you will experience not being able to charge up to the maximum voltage. A Rectifying diode might be needed to ensure the highest voltage retention on the capacitor as possible. Experiment at your own findings with different configurations until you get the most optimal results.

You can't measure Joule Thief output using a multi meter for the following reason: Multi meters are configured to measure AC voltages at 60 cycles per second, a Joule Thief, while still being AC, well I wouldn't say AC because in reality its Pulsed DC at a high frequency, that's why you can heat a whistling noise depending on your Joule Thief configuration. A multi meter is not going to be of any use for you to perform Joule Thief based measurements in DC or AC mode. If you would like to measure the irregular wave forms of a Joule Thief to determine their maximum up most voltage spike to get an accurate and correct pulsating voltage readings you will need an oscilloscope.

I forgot to add that there could be a work around that I haven't tested or confirmed for accuracy, its my theory that you can experiment with to allow you to measure the voltage using your multi meter. For that, you will need a high voltage capacitor with capacitance as less as possible, for example: 600 volts at 0.1 uF or less then connect the capacitor to the output of the joule thief. I theorize that the Joule Thief should charge the capacitor up to its maximum spike voltage and you will have a voltage in terms of Direct Current (DC) that you can, therefore, measure using your multi meter. This only would work if you expect the voltage coming out of your Joule Thief to be no more than 600 volts as most consumer grade multi meter is capped at measuring DC voltages at no more than 6...see more »I forgot to add that there could be a work around that I haven't tested or confirmed for accuracy, its my theory that you can experiment with to allow you to measure the voltage using your multi meter. For that, you will need a high voltage capacitor with capacitance as less as possible, for example: 600 volts at 0.1 uF or less then connect the capacitor to the output of the joule thief. I theorize that the Joule Thief should charge the capacitor up to its maximum spike voltage and you will have a voltage in terms of Direct Current (DC) that you can, therefore, measure using your multi meter. This only would work if you expect the voltage coming out of your Joule Thief to be no more than 600 volts as most consumer grade multi meter is capped at measuring DC voltages at no more than 600 volts. If you expect that your joule theif would generate more than 600 volts DC and would like to prepare for that possibility, then you will need a capacitor rated for higher voltages, like 1,200 volts DC at 0.1 uF (or less uF) and a multi meter rated for the maximum voltage measurements in terms of DC for your target voltage up to 1,200 volts in this example, you can also use the resistor trick to stick with your 600 volts max measurement multi meter to measure greater than 600 volts DC (in this case, you should google up how to use a multi meter to measure very very high DC voltages, and how to interpret its output readings correctly).Note about 0.1 uF capacitance value: The less capacitance the faster a joule thief can charge it up and the less likely that you will experience not being able to charge up to the maximum voltage. A Rectifying diode might be needed to ensure the highest voltage retention on the capacitor as possible. Experiment at your own findings with different configurations until you get the most optimal results.

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