I found that my modified Joule Thief circuits were running at the frequency of 550 kHz, or at the bottom of the AM broadcast band. This is the highest frequency that I have settled on as a satisfactory frequency for a Joule Thief. I have built ones that have gone as high as a few MHz, but they were just to see how high the JT could go and still function.
What is important is that when the frequency gets above a few hundred kHz, then the circuit becomes more dependent on the stray resistances, capacitances and inductances of the wiring and PC board traces.
People often assemble a JT on a proto board or with clip leads. The stray capacitance between the contacts inside of the board will cause the circuit to have a different operation than if it was tack soldered together. And using the clip leads to assemble the circuit adds an excessive amount of stray capacitance and inductance so that at the higher frequencies, the circuit may become unstable so that it operates erratically or quits altogether when the clip leads are moved. The proto board also has more stray capacitance than a PC board, and the circuit may work differently than if it is on a PC board or tack soldered together.
The most damaging of these to a Joule Thief is the stray resistance. The voltage is so low that any stray resistance becomes much more detrimental to the circuit. When a circuit is running at 12 volts, a small resistance might be tolerated. But when a circuit is operating at 1.2 volts, that same small resistance becomes ten times more influential on the circuit and ten times more detrimental – it can ruin the performance. The solder joint is a very low resistance connection; the contact in a proto board and the clip lead are not low, their resistance affects the circuit much more. And this is especially true if the proto board’s contacts are old and dirty.
Some circuits (not the conventional JT) require a tuned circuit with a capacitor and inductor in parallel. These two components have a very high circulating current when they are working properly. The hams would use silver plating to reduce the resistance and increase the Q or quality factor. Using a proto board to hold the cap and coil adds resistance to the circuit, which reduces the Q or quality factor, and this makes the tuned circuit’s performance lower than if the cap and coil were soldered together. The penalty is even greater with clip leads (see note below).
If the experimenter is going to build any circuit where the frequencies are more than a few hundred kilohertz, then he or she should learn how to use good RF construction practices. Keep leads short. Make good mechanical connections to the leads before soldering. Make good solder joints. And use good quality components for the critical circuits where RF is highest.
One can download this 256 page manual on RF and microwave grounding.
Note: I bought a hundred mini alligator clips online, in red and black. I made my own clip leads with wire that is only four inches or 100mm long, so the clip leads have a minimum impact on the circuit. They are supposed to be temporary, and not the way to build something. If you want to hold leads together, you should buy a hundred Fahnestock clips to hold the component leads together with no extra wire. At least they don’t have so much capacitance as the proto boards have.