2016-10-04 Joule Thief Powers Field Strength Meter

There are many circuits that are used infrequently that are powered by the usual 9 volt battery. One of these happens to be a field strength meter (FSM), which is used for detecting the power output of a transmitter such as the one a radio amateur uses. Some FSMs are very simple and use no battery. I chose this circuit because it uses the 9 volt battery, takes very little current, and does not care if the power supply is noisy and unregulated. And it is turned on only for short periods, very intermittently.

A standard Joule Thief (pic in this link must be clicked more than once) is used with a few modifications . It is powered by a single 1.5 volt AA or AAA cell. This has the advantages that it is smaller and much cheaper than a 9V battery.

The first modification is the coil. Since the voltage will be higher, the coil will have a different number of turns for each winding. The typical Joule Thief might have ten turns for each winding. This will be changed so that the main winding for the LED will have 16 turns and the feedback winding to the resistor will have 8 turns or half as many turns as the main winding. This allows the LED voltage to be twice as high without exceeding the transistor’s base voltage.

The Joule Thief’s pulsating output has to be rectified and filtered. We replace the LED with a 1N4148 diode. The anode (the end without the band) is connected to the collector and coil. But the cathode (banded end) is connected to the positive lead of a 22 uF, 16 volt electrolytic capacitor. The negative lead of the capacitor is connected to the negative of the battery. The diode rectifies the pulses and the 22 uF filters out most of the pulses, so we now have unregulated DC.

The next change is to connect a zener diode across the capacitor. We connect the cathode (band end) of the zener to the capacitor’s positive lead. We choose a 6.8 volt zener diode, and connect a red LED (2 volts) in series with the zener to get a total of 8.8 volts. You could also use a 6.2 volt zener and a 3.2 volt blue or white LED. The cathode (flat spot) of the LED goes to common negative, like it was in the original Joule Thief. The anode of the LED is connected to the anode of the zener diode. You can use any NPN transistor in place of the zener. Connect the emitter to the capacitor’s positive, and the base and collector together and to the common negative. The emitter to base junction will act like a zener. But the voltage may be anywhere from 6 to 9 volts, so measure it to see if it’s okay. If you use a transistor for this, it may have its current gain severely damaged, so I recommend cutting the leads off and putting it in the trash.

The Joule Thief uses a 1k resistor, but the resistor can be increased to save power. It can be 2.2k or maybe more. First let’s do a few calculations. The standard Joule Thief puts out about 20 milliamps at 3.3 volts to the LED. That’s about 66 milliwatts. The FSM uses about 1 mA for the 10k trimpot, and 0.25 mA for the FET and meter. That equals about 1.25 mA max at 9 volts, or about 11 milliwatts total. So if we set the Joule Thief to generate 30 milliwatts total, we can have 11 mW for the FSM and 19 mW for the zener and LED, to show that the power is on. The resistor depends on the current gain of the transistor, which may vary widely.

The FSM is used during transmission, not while receiving. If you have it on during receiving, you might hear some noise at some frequencies while tuning. I think this might be a good warning indicator that you left the FSM on. However if you use a pushbutton switch or you just don’t want to hear the noise, connect a ceramic capacitor, such as a 0.01 uF (“103”) across the 22 uF. If you still hear noise, use some ferrite beads on the leads from the battery holder, and the leads from the Joule Thief to the FSM. If the whole circuit is inside of an aluminum enclosure then it should stop the RF noise from getting out.

Another change to the FSM. The on/off switch should be connected between the AA or AAA cell and the Joule Thief.

I have used another circuit similar to this to power a cheap DMM, and it does the job okay. It had a regulator that turned down the circuit when demand went down. This Joule Thief circuit draws the same battery current no matter what the load is. But the FSM is used very infrequently so it shouldn’t make any difference in the battery life. If this circuit is used for frequently used equipment, then a rechargeable battery can be used. If it will sit for long periods without use, the Eneloop rechargeable cells will hold their charge for a year or more. Since rechargeables put out 1.25 volt instead of 1.5 volt, the Joule Thief’s resistor might have to be lowered a bit.

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