Using a resistor in series with the LED or supply
The reason for using the 1 ohm current sensing resistor between the LED cathode and negative is that it has lower resistance than the meter. The meter’s internal resistance varies depending on the range setting so one can’t depend on getting consistent readings between ranges. One ohm is one ohm, and the number of milliamps is always equal to the millivolts on the DMM. And a 1 ohm resistor costs a few cents and can be left in the circuit. and the meter leads connected to it only when necessary.
One other important thing: some may find they can buy a 1 ohm wirewound power resistor. The resistive element is a winding and has a small amount of inductance, which may or may not affect the circuit. It’s best not to use this kind of resistor. Instead get the regular carbon film resistor, which has almost no inductance.
I Made My Own
But 1 ohm resistors are uncommon, so before I bought a bunch, I wound a short length of very fine wire onto a regular resistor and wrapped the fine wire around the leads and soldered them to the leads. I’m not saying what the wire was because some of you wouldn’t be able to find AWG so you would use an equivalent SWG or metric size instead. The copper wire tables tell how many ohms per thousand feet, which is the same as how many milliohms per foot, or in other countries milliohms per meter. Some adhesive tape or clear nail polish will hold the wire in place. I chose enough wire to give 1000 milliohms, and a resistor big enough to hold a few feet.
But the supply current was much greater than the LED current; something close to or more than 100 milliamps. So I chose to reduce the length of wire to give a half ohm or even 1/10 ohm. I just had to multiply the millivolts by the factor of two or ten to get the current. The reason was that 1 ohm would give excessive voltage drop. If the supply current was 100 mA, then the 1 ohm resistor would have 1/10 volt across it, and the supply voltage at the JT would be only 1.4 volts. That’s 7 percent of the supply voltage – too much. So using a 1/10 ohm resistor would reduce the error to less than 1 percent.
Coil Wire
I’ve been winding most of my JT coils with fine wire, usually 30 AWG but I’ve also tried finer (32 or 34 AWG) on some smaller cores. I usually wind more than two (bifilar) windings, so that I can tie three of the four together for the primary. The remaining single winding is the feedback winding. This works well, and the length of the wire on smaller cores is less than a foot or 300 mm. I measured the inductance of these and they are typically anywhere from 150 microhenrys to 600 uH – that is plenty for a JT. The secret of the coil’s inductance is described here (scroll down to Stored Energy), and in my blog (scroll down to The math).
