2011-01-07 JonnyDavro’s Joule Thief, Power Xfmr Drives CFL

I was watching JonnyDavro’s Joule Thief driving a power transformer, which is used in reverse to step up the voltage to drive a CFL tube.  He shows the schematic, and later says that the unlabeled transistor is a BD139.

I added a comment, saying “What amazes me is that you’re getting these high voltages at points that are very sensitive to HV. The emitter to base junction is rated for a max of 5V (reverse), yet you haven’t fried the transistor yet. Especially considering that the maximum power for a BD139 is only 8 watts! I would put a LED across the emitter to base, cathode to base, just to protect the junction from excessive reverse voltage.”

I said this because the voltage across the emitter to base must never exceed 5 volts in the reverse direction.  I explain why this important in my earlier blog.  It is important.  Two ways of protecting the junction, as I explain below.  A diode can be put in series with the base, so that the base current must pass through both in the forward direction.  If the current tries to pass in the reverse direction, both the diode and E-B junction will be reversed and the diode will block it up to the point where its reverse voltage is exceeded, which could be a hundred volts or more for some diodes.  The one drawback with doing this is that the forward voltage across both junction and diode is 1.2V or more, which is too high for a Joule Thief with a supply of only 1.5V.

The other way to protect the junction is to put a diode in parallel with the E-B junction, so that (for NPN transistor) the cathode or banded end of the diode is connected to the base, and the anode is connected to the emitter.  If the negative voltage across the E-B junction exceeds 0.6V, the diode will conduct and shunt the current away from the E-B junction.  The disadvantage of this is that the diode adds a small amount of capacitance to the junction.  But this does not affect the Joule Thief, so we will use this diode to protect the junction.  We can change the diode to a red LED to let us know when and how much the reverse voltage is by how bright it lights up, so it serves a dual purpose.

Using a diode in one of these ways to protect the transistor will prevent damage, and it is important.  When a transistor is used as a switch, another fault that can occur is the collector becoming forward biased.  For a NPN transistor, that means the collector goes negative in relation to the emitter.  In some power MOSFETs there is a high current diode connected between source and drain, with cathode connected to the drain (for N channel).  This conducts if the voltage reverses polarity.  A diode can be put across a power transistor in a similar manner to protect it from reverse polarity.

In the vlog he said that the relay was chattering.  I commented that the relay is telling him that it doesn’t like AC.  He replied that it was a 12VDC relay and that no harm will come to it. I agree as long as the high voltage across its coil doesn’t arc over at some point.  What I heard was the relay chattering – it was not a periodic noise like a buzz.  This may be telling him something.  One would think that if the circuit is oscillating, the sound would be a buzz, but if the sound was a chatter, something else was happening, something erratic.  It could be a quirk of that relay, or it could be something in the circuit that was not intended to happen.  Anyway, it’s just a thought.