This is an inverter I'm preparing to use with EL tape. For now the load on the inverter is just a small speaker, R1. This will be replaced with a step-up transformer, a wall wart run backwards with an impedance of just 1-2Ω.
I wanted to use only N-channel MOSFETs for the switching, so I put a voltage doubler into the circuit to give me a "reservoir" of 2Vcc to drive the high-side gate. The high side gate is held at the doubled voltage except when Q1 turns on and pulls it low whenever the low-side switch is turned on.
In the schematic, U1 is an LM358 op-amp making two square waves. U1a sets the variable inverter frequency in the 1kHz range. U1b is a higher frequency (20kHz? I forget what I chose) and drives the voltage doubler. The sim shows 1N5818 diodes but I'm using similar SB140 Schottky 1A diodes. [correction - they are 1N5817]
Everything works fine in simulation. The voltage on C5 builds to 2Vcc in a few milliseconds and never drops more than a fraction. I've built the circuit on a breadboard and it "works" in the sense that the speaker buzzes at a frequency that varies with VR1. But I wanted to measure some voltages and be sure things are working as expected before adding the step-up transformer. Here are some observations with my cheapo Cen-Tech multimeter:
How can there be such a high AC voltage, compared to the DC voltage, appearing on the top of C5? Is this all just an artifact of my meter? Before you say yes, remember that the buzzer sounds identical if C5 is removed.
I wanted to use only N-channel MOSFETs for the switching, so I put a voltage doubler into the circuit to give me a "reservoir" of 2Vcc to drive the high-side gate. The high side gate is held at the doubled voltage except when Q1 turns on and pulls it low whenever the low-side switch is turned on.
In the schematic, U1 is an LM358 op-amp making two square waves. U1a sets the variable inverter frequency in the 1kHz range. U1b is a higher frequency (20kHz? I forget what I chose) and drives the voltage doubler. The sim shows 1N5818 diodes but I'm using similar SB140 Schottky 1A diodes. [correction - they are 1N5817]
Everything works fine in simulation. The voltage on C5 builds to 2Vcc in a few milliseconds and never drops more than a fraction. I've built the circuit on a breadboard and it "works" in the sense that the speaker buzzes at a frequency that varies with VR1. But I wanted to measure some voltages and be sure things are working as expected before adding the step-up transformer. Here are some observations with my cheapo Cen-Tech multimeter:
• The DC (AC) voltage out of U1b is 6V (12V)
• The DC (AC) voltage out of C4 is 12V (24V)
• The DC (AC) voltage on C5 is 12V (24V)
• The speaker buzzing stops if R9 is removed, but C5 voltage is unaffected
• Changing or removing C5 has no effect on the buzzing
• Attaching the LED indicator to the source of the high-side makes it glow less brightly than Vcc.
• Both diodes test OK; open in one direction and 145mV in the other.
My conclusion from these observations is that something is wrong with the voltage doubler. The DC voltage is not building on C5. I speculate that, when turned on, the high-side switch is pulsing on very briefly with each pulse of the doubler. I can't hear the high frequency of the doubler, so I'm not completely sure.• The DC (AC) voltage out of C4 is 12V (24V)
• The DC (AC) voltage on C5 is 12V (24V)
• The speaker buzzing stops if R9 is removed, but C5 voltage is unaffected
• Changing or removing C5 has no effect on the buzzing
• Attaching the LED indicator to the source of the high-side makes it glow less brightly than Vcc.
• Both diodes test OK; open in one direction and 145mV in the other.
How can there be such a high AC voltage, compared to the DC voltage, appearing on the top of C5? Is this all just an artifact of my meter? Before you say yes, remember that the buzzer sounds identical if C5 is removed.
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