I recently purchased a variable-voltage, variable-frequency power supply (which was designed for driving capacitive loads) and an extra transformer for experimentation. Before I purchased the power supply, in response to my email questioning certain aspects of the power supply the company sent me a schematic of the unit. The transformer also came with an example plan for a Jacob's ladder, with a schematic and a fairly detailed description of the circuit. The two aforementioned schematics are very similar, with a few minor difference in the details, including the fact that the secondary of the power supply's transformer is endpoint-grounded to the AC electrical outlet earth ground via the metal chassis but the plan for the Jacob's ladder calls for a center-tap-grounded secondary.
By the way, before purchasing, I did question the safety of grounding the high-voltage, high-frequency secondary through the electrical outlet instead of to a separate earth ground. My question went unanswered. I also asked about the availability of datasheets for the company's products, because the product information on the company's website leaves out a lot of information, and the engineer(?) to whom I spoke assured me that the products came with datasheets--when I received the shipment, I had no datasheets. The schematic for the power supply covers two or three different products (designed for different purposes) and defines the primary and secondary turns for three different transformers but only gives the open- and short- circuit inductance for one of the transformers, which isn't the transformer of my unit. As you might imagine, I'm quite frustrated by the missing details.
CIRCUIT DESCRIPTION* (The same for both circuits.)
The primary (Lp) of the output transformer (T1) is driven by a pair of IR450 MOSFETs (Q1, Q2) in a half-bridge configuration driven by an IR2153 self-oscillating half-bridge driver. The primary of the transformer has one end (TP2) connected between Q1 and Q2 and the other end (TP3) connected between two 1.5 microfarad(?) capacitors (C5 and C6), which are between the positive and negative DC rails. Pin 6 of the IR2153 connects between TP2 and Q2. The .0015 microfarad(?) capacitor (C7) and 10/3 resistor (R8) in parallel to Lp and between Lp and TP2/TP3, to paraphrase the circuit description, slow down the transition time of the switched pulses across Q1 and Q2, with the resultant time constant limiting the dv/dt rate, which could cause premature turn-on at the wrong switch, "creating a catastrophic fault mode." C5 and C6 "provide a voltage midpoint and produce the necessary storage energy to maintain the voltage level of the individual pulses.
Since the secondary (Ls) of the power supply is endpoint grounded to earth, it leaves only the power output lead for connection to the load, and, except for experiments with single-ended loads, the other end of the load would be connected to earth.
- Especially considering that the company is willing to send a schematic of their product to a prospective buyer without a request for a schematic and that the assembly is clearly visible through the clear plastic case of the unit, a partial description of the circuit for discussion should be FAIR USE.
MY QUESTIONS
- In his description/sales-pitch of the unit, the engineer(?)/ saleperson insisted that the [capacitive] load across the secondary is in series with the inductor Ls, so the voltage is dependent on the Q of the secondary circuit. Granted, IIRC, in the effective circuit of a transformer each coil presents a series inductance and a parallel inductance. However, because the load shares common power and ground with Ls, isn't the secondary effectively a parallel RLC circuit?
- I see a parallel RLC circuit consisting of C7, R8 and Ls. If I adequately described the circuit, would this statement be accurate?
- Every electronics reference I have found treats transformers and RLC circuits separately. Granted, most references deal with theoretical ideal components in their lessons on the theory (unless the coil of the transformer is the only theoretical inductance, eg. in descriptions of tank circuits). However, even in concrete experimental examples, with step-down transformers providing the AC power source to (R)LC circuits with separate coils, the example measurements leave the inductances of the transformer out of the equation.
I did some calculations for a few RLC circuits, each with a capacitor (C2) and an inductor (L2) powered by the seondary (Ls) of a transformer. If Ls is relatively large the difference between the resonant frequency of C2 and L2 and the resonant frequency of C2 and Lst (ie, 1/(1/L2 + 1/Ls)) is much smaller than if Ls is relatively small.
Why would an electronic reference leave Ls out of the equation, especially if an experimental example about power factors and resonance in LC circuits specifically includes a transformer to power the experimental example?
- Back to the power supply schematic, is there any reason I couldn't choose the resonant frequency of the primary LC with an inductor L1 and a resistor Rx (matching the time constant of C7 and R9) parallel to Lp and parallel to C7--R9?
(Why would I want to do that? I can think of a few reasons. First, according to my calculations, if I understand it all correctly, raising the resonant frequency of the primary with an extra parallel inductor will raise the reactance of Lp, effectively reducing the VA across the primary. Since I don't have a datasheet or VA rating for the transformer and don't look forward to dealing with the company for the answers, VA could be important, especially on the secondary coil. Also, the unit was engineered for capacitive loads falling within a specific range of capacitances that might not work for my experiments. Furthermore, it might help me with impedance matching between the primary and secondary.)
- If in addition to adding L1 to the primary I add an inductor L2 likewise across the secondary circuit to match the resonant frequency of the primary circuit, does a difference in the inductive reactances of Lp and Ls draw power? Or is resonant impedance matching between L1, C1 and L2, C2 more important?
- The manner in which I phrased Number 5 might be confusing. To paraphrase the question, do I want to match the resonant frequencies of Lpt, C1 and Lst, C2 or the resonant frequencies of L1, C1 and L2, C2?
- I don't have inductance values for the 80T:2500T transformer in the power supply, but I do have a multimeter with a built-in inductance meter. I don't think I can measure the primary or secondary inductances for frequency-reactance calculations without revoming the transformer from the unit. Right?
- Are frequency-impedance calculations based on open-circuit or short- circuit inductance measurements or neither? (I can't seem to find the answer to this one online or in my references.) The relationship between open- and short-circuit specs would lead me to believe that calculation would be based on the short-circuit inductance, but I don't want to guess.
- I can't quite wrap my head around how exactly the half bridge works. As described, would C5 and C6 add a DC bias? Or would the power lead swing from +V to -V with respect to ground?
Thanks in advance for the answers to any or all of these questions.