How to Connect lm393 Across Capacitor for DIY Induction Heating?

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Discussion Overview

The discussion revolves around the design and implementation of a DIY induction heating circuit, specifically focusing on the use of an LM393 voltage comparator to measure capacitor voltage in relation to inverter voltage. Participants explore issues related to phase locking in a PLL configuration and the challenges of capturing voltage signals without an inductor in the circuit.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Sina describes their setup involving an H-bridge and expresses difficulty with maintaining a 90-degree phase lock between inverter voltage and capacitor voltage.
  • Sina seeks advice on how to connect the LM393 across the capacitor while protecting its inputs from excessive voltage.
  • Sina mentions using a shunt resistor to measure inverter current phase angle but notes that the voltage shape is unexpected and does not accurately reflect the phase angle.
  • Sina expresses interest in using phase comparator 2 of the 4046 to achieve zero angle locking but acknowledges the need to adjust the capacitor phase by 90 degrees first.
  • Imsmoth suggests a resource that includes a tutorial on PLL circuits relevant to induction heating, indicating that it addresses capacitor voltage concerns.
  • Sina responds that while the suggested resource is helpful, their circuit differs in using an H-bridge without a matching inductor, complicating the capture of capacitor voltage.
  • Sina offers to provide a schematic for better clarity on their confusion regarding the circuit design.

Areas of Agreement / Disagreement

Participants have not reached a consensus, as there are differing circuit configurations and approaches to measuring capacitor voltage. Sina's specific challenges and requirements remain unresolved.

Contextual Notes

Participants have not fully explored the implications of using an H-bridge versus a half-bridge configuration, nor have they clarified the effects of omitting a matching inductor on the circuit's performance.

sina_stuck
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Hi there,
I've been interested in this induction heating thing and would like to build one. Its quite a long time searching the net and experimenting to make it work. I've been stuck with the pll issue. My configuration is Hbridge which is fed from a simple power supply(not split) so the lowest point is ground. The two signals I try to maintain a phase lock of 90 degrees are the Inverter voltage and tank cap voltage. The inverter voltage is fed from pin 4(VCO out) to pin 3(COMP in) of 4046 as inverter voltage representative. But in order to get a square wave as a representative of cap voltage I'm trying to use a voltage comparator lm393.

My question is how to connect lm393 across the cap and meanwhile protecting its inputs from excessive voltage that appear across the cap?

I tried to put a shunt resistor in series with LC tank and use the voltage drop across it as an indicator of inverter current phase angle but the voltage across it is very odd in its shape and if converted to square wave does not show the phase angle of the inverter current.

Also I am very interested to use the phase comparator 2 of the 4046, this phase comparator tries to lock zero angle between its input signals which is alright if I was able to use the voltage across the shunt resistor. but in this case I should change the phase angle of the cap 90 degrees and then try to lock zero angle between it and the inverter voltage.

Any help, suggestions and schematics would be very welcomed. Please let me know if I should send schematics or if my explanations are not complete also I can be available for a chat at any time if this would be an easier way for anyone to help. Thanks in Advance.
Best Regards
Sina
 
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If you go to http://inductionheatertutorial.com
you will find an extensive tutorial on what you are looking for. The unit uses a PLL circuit. Your question with regard to the capacitor voltage is also addressed. Look at the circuit in the middle (2kw unit).
 
Thanks imsmoth for this address. I have already read it but the difference is two things between his and mine. first he uses a halfbridge while I use an hbridge. secondly he puts a matching inductor between inverter and tank while I do not plane to put one. what i seek now is how to capture cap voltage when there is no inductor in between assuming the same LC tank circuit.
thanks again
sina
 
Hi,
I thought nothing would be better than visually express my point of confusion. Here I post a schematic and tried to clarify my question. please have a look an let me know if I should provide more details.
Thank you again
sina
 

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