DC-AC inverter using 555 - Page 1
Forum section: electronics / projects

author: Rob Lay
author_rank: Newbie
timestamp: August 05, 2021, 06:15:39 am
content: Im working on a project that needs an AC signal and I am quite new to all the world of electrical engineering so i thought i would run my design for a dc-ac converter by someone who actually knows what they're doing before i buy the parts and put it together. I want to use a 555 in astable mode as an oscilator to drive a H bridge. (schematic link below)https://crcit.net/c/144c68d804e043588c64ef2863234b4dthe general idea is by using both n and p chanel mosphets together a high signal will cause one pair of mosphets to turn on causing current to flow one way while a low signal will cause the other pair to turn on causing current to flow the other way making AC. I just want to know if this will work, the frequency of the signal isnt really important i just need AC so i can use a step up transformer in the next stage of my circuit.thanks heaps

author: WattsThat
author_rank: Frequent Contributor
timestamp: August 06, 2021, 03:54:03 am
content: That is not an astable 555 configuration and the RC values shown would provide about 0.00125 Hertz if you connected pin 7. Your bridge configuration is incorrect as well, the top two devices should be P channels, the bottom two N. But, it needs far more help than just that.In general, power electronics is not something that can be achieved by a novice. It would be best to explain the scope of what you’re attempting to do in more detail rather than just jumping right to your proposed solution. Without details, the best that anyone can do is to say that circuit is not going to work. To state things bluntly, it is destined to go poof, crack, pow, boom or some combination of those outcomes.

author: bdunham7
author_rank: Super Contributor
timestamp: August 06, 2021, 04:15:50 am
content: What frequency, power and voltage levels are you trying to design for? The overall idea will work, but you're way short on the details to make it work.You need to look up correct diagrams and instructions for 555 timers (all over the web), then you should probably select a transformer and complete the design to fit that. Depending on your power levels and tolerance for a little complexity, you might try an all-NMOS H-Bridge. You could even supply the high-side gate drivers with a charge pump run by another 555, or use a 556 dual timer IC.

author: TurboTom
author_rank: Super Contributor
timestamp: August 06, 2021, 07:40:15 am
content: Welcome to the forum, Rob!The previous contributions are correct -- it would be best to provide more information on the project you're working on, especially expected frequency and power of the DC/AC converter.A barebone 555 as the oscillator in such an application that requires a highly symmetric drive to the H-Bridge in order to keep the load DC-free, is a bad idea (if you want to prevent it from magnetic saturation). A standard 555 design won't produce a symmetrical output signal (you also need to connect the discharge line to the R/C node by a resistive current limiter to make it oscillate).You can still use a 555 to drive a flip-flop-divider to produce two opposite-phase square signals of 50% duty cycle and opposite polarity. Then add two high/low side gate drivers with dead time to prevent shoot-through and four beefy N-Channel mosfets and you should be fine.If you need any kind of regulation loop, you better directly go for a dedicatedPWM controller like the SG3425, this will cause still enough but way less headache than the "jellybean approach". To drive an H-Bridge, you'll still need the gate drivers. But you may use a split primary in the transformer instead, then a low-side drive will be sufficient (see the example circuit of fig. 8 in the datasheet).I wish you success!

author: Rob Lay
author_rank: Newbie
timestamp: August 06, 2021, 08:15:44 am
content: Thanks everyone, considering what you guys have said i have decided to do something else to solve my problem so dont worry about replying to this.thanks for trying to help this seems like a really nice community

author: viperidae
author_rank: Frequent Contributor
timestamp: August 06, 2021, 10:28:21 pm
content: Correct me if I'm wrong, but that 555 will oscillate with no connection to pin 7.The output pin, 3, is what charges and discharges the capacitor and allows the 555 to run at 50% duty cycle.

author: WattsThat
author_rank: Frequent Contributor
timestamp: August 07, 2021, 01:08:28 am
content: Quote from: viperidae on August 06, 2021, 10:28:21 pmPin 7 is the discharge pin. What happens when the cap charges but never discharges?An exact 50% duty cycle requires some additional effort.

author: TurboTom
author_rank: Super Contributor
timestamp: August 07, 2021, 07:49:18 am
content: Quote from: viperidae on August 06, 2021, 10:28:21 pmIndeed, you are correct! I completely missed this detail in the OP's schematic. The capacitor gets charged and discharged via the output! This means, the circuit will oscillate and the signal will be (more or less) symmetrical, depending on the accuracy of the 1/3 : 2/3 Vcc Trigger and Threshold levels and possibly voltage drops in the output stage (the "traditional" 555 internal schematic shows an NPN darlington as the high side driver, resulting in at least 1.2V drop while the low side is driven by a common-emitter NPN, that, if saturated, has neglible drop).Well spotted!

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2021, 12:43:41 pm
content: Quote from: TurboTom on August 07, 2021, 07:49:18 amThe asymmetrical output voltage drop of the 555 means the circuit will not produce a duty cycle of 50%.Driving the MOSFETs like that will result in a considerable amount of shoot-through. A series resistor, or inductor could be used to limit the shoot-through.Here's another circuit which uses BJTs to form a self-oscillating H-bridge. It also has considerable shoot-through, but the hFEof the transistors limits it somewhat.