Note (from yesterday’s blog):
I have seen many good Joule Thief projects mistakenly use the 1N4003 series rectifier, which is made for 50 or 60 Hz, and has a slow recovery time. I have put a 1N4003 in parallel with a 1N4148, and found that the Joule Thief’s performance often drops. This is because it was not made to do a good job of rectification at 50 or 60 kHz, which is a thousand times higher frequency. It should not be used at frequencies higher than audio frequencies*.
If you can buy some, use the 1N5817 Schottky 1 Amp rectifier instead. But 1N5817s are not found at the average electronics store, so an alternate choice is to use 2 or more 1N4148 diodes on parallel (see below for maximum ratings). Putting 2 or more in parallel gives more current handling capability and less voltage drop, and less wasted power. I seldom see any projects with 2 or more of these or any other diodes in parallel, even though they are dirt cheap and are available at any Radio Shack store.
Another Schottky diode for currents up to a half amp is the BAT85. But it’s even less common than the 1N5817.
The 1N4148 is rated at a maximum of 75 volts reverse voltage, and 75 milliamps forward current. But the manufacturers often have their own substitute diode. For example, Vishay Siliconix calls it a V4148, and Fairchild Semi calls it a F4148. These substitutes may have a higher forward current, higher peak forward current, higher reverse voltage, and lower reverse recovery time than the 1N4148 specifications call for. Essentially you’re getting a higher performance compared to the original 1N4148. But it’s still not capable of handling a full amp, like the 1N5817.
Another common diode number given for projects is the 1N914. Long ago in projects using diodes, the specifications of the 1N914 were essential for diodes, but in the decades since, the advances in technology have made diodes that surpass those specifications, one being the 1N4148. But the two numbers became synonymous and often the parts list called for “1N914/1N4148” in an either/or manner – one would work equally as well as the other. So makers often sold the same diode, but labeled differently depending on what the buyer wanted. Their specifications were similar enough to freely substitute one for the other. Their specifications were so similar enough that Fairchild put these diodes all on the same datasheet.
But even though these diodes are “high conductance” that doesn’t mean that they can meet the demanding needs of the Joule Thief, where currents are much higher than 100 mA. That’s why the 1N5817 or even the 1N5820 (3 amp diode) are commonly used for switching power supplies.
* There is an exception to that. If you are in need of a high capacitance diode to use as a varicap or variable capacitor, then the 1N4003 series may be able to do the job. It may have several tens of picofarads of junction capacitance when reverse biased, and of course that changes with the reverse bias voltage, which is how the capacitance is varied. Sometimes varicaps are mistakenly called varactors. A varactor is used not for a variable capacitor, but in a frequency conversion circuit.