Hi Itsu,
I corrected a little the circuit to have a better response, see the attached asc file. I reduced the signal generator output voltage to 10mV (at f=3868 kHz), I chose the
series DC resistance of L1 to be 0.1 Ohm what you measured with the Ohm meter, I chose the ESR of the 100 pF capacitor also to 0.1 Ohm (a good quality capacitor should have
an ESR < 0.1 Ohm).
Note that with 500 mV generator input the resonant frequency of L1C2 also changes, this change can be considered normal I think, driving voltage levels change
semiconductor parameters.
Note also: if we choose 100 mV input for the generator, the resonant frequency changes to 3878 kHz to get maximum resonant voltage across either L1 or C2
as per the simulator.
Regarding where to measure the generator voltage: I think across the 10 Ohm emitter resistor and the scope be in AC coupling to get rid of the DC component.
OR try to measure directly across L1 with 2 probes in differential mode (probes alligator clips floating).
Here is the simulator display of the voltage across the 10 Ohm, (1st pic), the AC wave rides about on 10.312 VDC, if I use the cursor for this V(n003) AC voltage and I
substract the bottom peak value from the top peak value, I get 10.314413 - 10.311302 = 3.111 mV, (this seems to remain from the 10 mV generator input). I blew up the
display for the V(n003) AC emitter voltage vertically for clarity, cursor shows the bottom peak value (riding on the DC value) which is 10.311302.
The 2nd picture (blown up horizontally) shows the direct generator input voltage (20 mVpp in red color) and the voltage across the L1 coil (3.79 Vpp in green color).
So the Q of the L1 coil would be 3790 mV / 20 mV = 189.5 and considering the same ESR of 0.1 Ohm I used in the simulation for the 100 pF, we would need to halve this,
getting about Q = 94.7 for the LC circuit WHEN we use the direct generator input voltage.
Using the AC emitter voltage which is only 3.111 mVpp in this simulation, the calculated Q would be higher, i.e. 3790 mV / 3.111 mV = 1218 and halving this would give
Q = 609
Considering the inductive reactance of L1 which is around XL1 = 406 Ohm at 3868 kHz and considering its 0.1 Ohm DC resistance, the calculated Q from these data would be
Q = 406 / 0.1 = 4060 in theory. The core loss and the virtual emitter resistance surely reduces the real Q and it should be (much) higher than 7 to 11 or so values.
Hopefully the measurements would reveal this.
Gyula
Gyula,
Thanks for the info.
What strikes my from the start is that you are able to produce 3 different Q values on this one circuit depending on how / where you measure: Q = 94.7, Q = 609 and Q = 4060.
This sound very odd to me.
Anyway, i can reproduce your values in the sim, but when i try it on my real circuit, things are very different.
Firstly, the 10mV (20mVpp) input from the FG is so low, my scope has problems showing it, see the blue trace (23.7mVpp) in the screenshot below.
But also this low input voltage seems problematic for the rest of the circuit as the voltage across the 100pF capacitor is still very low (274mVpp, see yellow trace).
When using my differential probe across the coil L1, it not only influences the resonance frequency (3620kHz), but it also shows a low value as 285mVpp, see purple trace:
In this situation with 20V input, there is 1.12A running through my 10.4 Ohm emitter resistor which get hot quick.
I have to increase the FG input to at least 150mVpp to get some readable signals, but still the Q will be around the familiar level of 11:
Further increase of the input (500mVpp) lowers again the Q, see here:
I indeed think the 2N3055 has problems with the used frequency, and also the probes seem to have again an impact on the Q as well.
I will see what i have in the junk box, would 2SC5200 do? I have some.
Itsu
Author
Topic: Bucking Coils (Read 2307 times)
