2012-11-27 Says TN0702 “Not Good Enough”

The Youtuber going by the name of Sanjev21 has read my comment about using the TN0702 low gate threshold MOSFET for a Joule Thief that will start and run below 0.6V.  He replied that this has a Rds(on)of 4 ohms at 2 volts, which is “too high”. The problem is that he is using a regular junction transistor that will not start at voltages below 0.4V, which makes it necessary to use a pushbutton to get it running at very low voltages.   Using a pushbtton or anything that requires human intervention to get it running is unacceptable in my opinion; it should start by itself.

Think Outside Of The Box

This Youtuber has too narrow minded to think outside of the box.  I can get the circuit to start and run at very low voltages and I don’t even need a transistor to do it.  It involves using technology that is way over a hundred years old, invented by a painter and used extensively by the railroads back then.  It was also used back in the early days of car radios to boost 6V to hundreds of volts.  Another hint is that it can be very noisy when it is running.

Very Low Voltage Means Very High Current

Remember that when a circuit has to operate at very low voltages, it puts very high demands on the circuit; the resistance of even a short piece of wire can be detrimental.  Circuit board traces must be very heavy to accommodate the very high currents.  The current passing through the transistors may be hundreds of times higher than in a simple Joule Thief.  So it’s difficult to get high performance from these circuits.

But back to the TN0702.  In my blog I gave some figures on how well this MOSFET did in a JT circuit.  It does better than the junction transistors I used – and I used some very high current transistors.  Also two or more of them can be put in parallel to reduce the on resistance and get more current to light the LED.  Or this circuit can be used to boost the voltage up high enough to drive the gate of a very high conductance MOSFET such as the IRL3302 .  More about this in my blog here.  The MOSFETs are capable of handling many times more current than junction transistors.  Also, they take less gate current than junction transistors.  A 2N3055 for example, might have a current gain of only 30 at 15 amps, its maximum current.  That means the base must be driven by more than a half amp – 1.5 amps or so – to get the 2N3055 to go into saturation.  The MOSFET gate capacitance needs to be driven but not at such high currents.  And the MOSFET can handle much more than 15 amps.

Back to experimenting…

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