How can we measure the voltage at the coil of a Slayer Exciter?

Question

How can we measure the voltage at the coil of a Slayer Exciter?

We have built a simple 9V Slayer Exciter for a school project. We are examining the conservation of energy at the transformer. The Slayer Exciter works fine: when held in front of it, a TL-light gives light. We use a BD-139 transistor. But we haven’t succeeded at measuring the voltage of the coils.

Measuring with a multimeter give incorrect voltages, like 0V or less than 1V. So, we thought that maybe a voltage divider would work. We connected it to the top of the larger coil and to the ground of the battery. We connect an oscilloscope to the smallest resistor, at the start and at the end of a resistor. It shows a sinus. However, If we put the resistor out of the voltage divider, the oscilloscope shows the same sinus. Without the resistor! If we put one cable from the oscilloscope to the voltage divider out, it also gives the same result. If we measure with two unconnected cables at the oscilloscope it gives a weaker signal. We believe that the electromagnetic field is causing a problem to the oscilloscope and the multimeter.

Our questions are:

1. How can we measure the voltage at the coil of a Slayer Exciter, the primary and the secondary?
2. Why is there no current going through the top of the bigger coil to the voltage divider?
3. How can we stop the Slayer Exciter from creating an Electromagnetic field? The transistor creates an AC current, but how can we know what the frequency is of that current? Is it a specific property of the transistor?
4. We have tested different transistors, all of them NPN. But the BD-139 was the only transistor working. The others got too hot. What are the reasons why they don’t work?

We have built the Slayer Exciter through the following scheme:

Answer 1

1. To measure voltage at the secondary coil correctly you need a device with very high resistance, about 100 MOhm or even 1 GOhm. So, when you create a divider use 100 MOhm resistor for oscilloscope input withput divider (it will probably have 1 MOhm input resistance), or 1 GOhm resistor for oscilloscope with 1:10 probe (it will have 10 MOhm input resistance). In both cases you'll get 1:100 divider with proper input resistance. Using AC voltmeter is not a good idea because most cheap AC voltmeters work correctly only with low frequency signals (less than 1 kHz) and frequency of your generator is much higher, I guess. Voltage at primay coil may be measured by direct connection of oscilloscope to it's ends.
2. I don't think that there is no current flow to the divider. You can calculate it according the Ohm's law, using R1 value of divider.
3. You can measure the frequency using oscilloscope. Measure the period T and calculate frequency as F=1/T. This freq is a property of transformer - resonance frequency of tank, consisting of secondary coil and it's stary capacitance. You can't stop this circuit to generate electro-magnetic field, because it is it's primary function. Take into acount, that any alternating current produces alternating megnetic field, and alternating magnetic field produces alternating electric field.
4. In this circuit transistor works in active mode and have to dissipate considerable power, so 2222A is bad choice and BD139 is much better. You can try to tune base resistor, trading output power versus transistor heating.

^{simulate this circuit – Schematic created using CircuitLab}

EDIT: The divider circuit should meet at least two criteria: 1.It should attenuate input signal by factor of about 100 (because the transformer have about 1:100 turn ratio). 2. It should have at least 100 MOhm input resistance (to minmize it's influence on secondary voltage level). Divider, consisting of 100 MOhm and 1 MOhm resistors meets both criteria. Ideal oscilloscope has input with infinite impedance, so when you connect it to the divider where will be no influence. But typical real oscilloscope has input, which equivalent circuit presented on the drawing (parallel connection of 1 MOhm and 10 pF, this values probably printed near scope input connector). The input Rosc plays role of second divider resistor and you needn't add another one. Oscilloscope input IS the second resistor of divider. If you want to get 1:1000 attenuation you may either use 1 GOhm at divider 'high side' and continue to use scope input as 1 MOhm at 'low side', or continue to use 100 MOhm at 'high side' with 100 kOhm at 'low side'. In the last case you neet to place 100 kOhm 'low' resistor in parallel to oscilloscope input.

Answer 2

These two photos in comment are very informative!

The circuit generates at about 2 MHz frequency. It's quite high, so there is the need to take into account electro-magnetic radiation and reception. Main rule: keep area of any loop with high frequency current as small as possible. Magnetic field radiation is proportional to that area. The same is true for oscilloscope connection circuit. It have to be shielded (to be protected from electric-field) and must have as low loop area as possible. For your setup at least two points must be fixed: 1. Use coaxial cable to connect oscilloscope to the circuit. Two separated wires (black and red, with that huge distance between them) – is inappropriate oscilloscope connection method. To test amplitude of signal induced directly on oscilloscope/cable, measure signal at both disconnected cable input (as you do on second picture) and short-connected cable input. This amplitude must be as small as possible. 2. Reduce area of primary coil loop. Twist first winding wires on all their path from coil to breadboard. Use decoupling capacitor (parallel ceramic C2+ electrolytic C3) to bypass HF current near T1. Try to keep wire lengths from these caps as small as possible.

Some other ideas: 1. Try to insert ferrite rod into the coil. This will increase coupling between windings and reduce both working frequency and magnetic field radiation. Try to increase first winding turn amount (for example to 10...30). 2. Try to measure voltage across first winding, connecting OSC ground to '+power' wire.

Answer 3

It's right decision to solder the circuit! It should be much reliable now.

According to you experiments, I think that electro-magnetic coupling between circuit and probe still exists. To estimate it's value, try two experiments.

1. Leave both ends of oscilloscope cable unconnected and observe amplitude. Move the cable around circuit and watch amplitude change, try different geometric configurations.
2. Connect both ends of cable to each other (make short circuit). Again, move the cable and observe amplitude. Try to make loop of maximum area. At the opposite, try to twist both ends before connecting them.

Consider each wire in the probe and in the circuit as receiving and transmitting antennas. After these experiments you can get estimation of non-conductive coupling strength, which interfere with conductive coupling while you are trying to measure secondary coil voltage.

To minimize magnetic field radiation, all loops of significant current must have as small area as possible. Te only such loop in the circuit is primary coil loop, which consists of collector-emitter path of Q1 and of battery. The function of capacitors is to exclude battery from this loop. High frequency (HF) current will flow through the capacitors, not through battery. To further reduce area of HF loop, twist the ends of primary coil (red wires) on their path to Q1. Place them and C2, C3, Q1 as close to each other, as possible. You can measure radiation level using closed loop of oscilloscope cable, as mentioned above.

When you measure secondary voltage, connect GND clip to 'minus' pole of battery to make conductive path of scope's input current. The length of coax cable is not so important (coax configuration is intrinsically immune to radiation). But the length and configuration of non-coax part of cable (red and black wires) is important. So, the best way is to make your own specific coax cable with very short non-coax part (about 1 cm). At least, twist non-coax part of existing cable to maximize it's HF radiation immunity.

The goal is to significantly reduce 'radiation' path of signal, to make possible accurate measurement of signal via conductive path.