DC to AC inverter circuit not working  - Page 1
Forum section: electronics / beginners

author: mkvl
author_rank: Contributor
timestamp: June 03, 2023, 10:57:43 pm
content: HI, I have build a DC to AC inverter as in the picture below but no matter what I did of testing different components value and transformers it doesn't work, the problem is that when I test the voltage of transformer output the multimeter shows high voltage 237v or even 400v and there's the noises in the coil but it can't even power up a 20v light pulb.the components:the power source: 10dcvthe two mosfets codes: 2sc2625the transistors codes: 2n5551thank you.

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 03, 2023, 11:23:34 pm
content: Quote from: mkvl on June 03, 2023, 10:57:43 pmThose are NPN transistors. They are NOT mosfets and that's probably why your circuit doesn't work.

author: Benta
author_rank: Super Contributor
timestamp: June 03, 2023, 11:24:14 pm
content: IRF630 is a MOSFET. 2SC2625 is a BJT. Like running diesel in a petrol car.

author: mkvl
author_rank: Contributor
timestamp: June 03, 2023, 11:45:21 pm
content: Yes you're right, I just forget, I used it because this circuit is driving me crazy so I thought to try these transistors, but I have tested the circuit with many mosfets like 2SK3115, 21n50c3, K1529, k2519....... and nothing works.

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 04, 2023, 12:12:20 am
content: Do you have a scope? If yes, then scope the waveform on the gates of the FETs. Should be a peak-to-peak square-wave between apx 0V and 10V.If not, you may be able to use your multimeter to guesstimate the AC voltage on the gates of the FETs. Switched to AC it should read 5Vrms. Switched to DC it should read 5V average DC.First, check the AC voltage across the 10VDC supply. It should be zero. If it reads almost 20VAC then you multimeter is no good for that test and gives a false AC reading in the presence of DC.

author: Manul
author_rank: Super Contributor
timestamp: June 04, 2023, 12:25:41 am
content: Quote from: mkvl on June 03, 2023, 10:57:43 pmAnd then you list a bunch of components which are not in the picture. Sorry, but it is hard to guess what is the problem if we are not sure on which components you used. Maybe you used different transistors, maybe different resistors, capacitors. What separates a good circuit from bad circuit are often fine tuned details. Generally it is not advisable to substitute components without real deep understanding of the details. I see people do this mistake and then suffer.

author: mkvl
author_rank: Contributor
timestamp: June 04, 2023, 01:04:33 am
content: I don't have a scope, but I have tested the transformer 10v it reads zero.

author: Psi
author_rank: Super Contributor
timestamp: June 04, 2023, 01:24:53 am
content: If I had to guess I think you have already blown up your two mosfets, or whatever you're using to pull the transformer wires to GND.There is nothing to limit the current when they turn on. They are essentially a short-circuit across your battery through the transformer.So you are relying on a perfectly turned circuit so the switching perfectly matches the transformer and the fets turn on then off at a rate that perfectly transfers energy into the transformer. If this is wrong the fets stay on too long and become a short circuit and immediately blow up.I would replace/test the two fets then start with two 4 ohm resistors. One per side of the transformer input wires.

author: mkvl
author_rank: Contributor
timestamp: June 04, 2023, 03:07:46 am
content: I have checked the mosfets I used and it still fine, and I'm sure that mosfets are not staying on too long, also I have changed capacitors and resistors value many, many times, now I'm using 1u capacitors and I have even used 103m capacitors, and 10k resistors to the transistors base, 470 ohm resistors to the mosfets gate.

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 04, 2023, 03:22:16 pm
content: What type of transformer are you using? Is it a 24Vcenter tapped50/60Hz transformer? (12v-0-12v)

author: mkvl
author_rank: Contributor
timestamp: June 05, 2023, 10:16:48 am
content: I'm using a 220v, 50/60 Hz primary as secondary, and I have made the primary coil my self 80turns - 80turns, 10v-0-10v, also this is not the only transformer I used, I tried with several transformers and nothing worked.Thank you for replying, I'm confused and struggling with this issue.

author: Psi
author_rank: Super Contributor
timestamp: June 05, 2023, 11:50:29 am
content: Without a scope its a guessing game where the issue is.Even one of those cheap 1MSPS $20 scopes would help track this down.I expect this circuit to be a lot more likely to blow the mosfets due to overcurrent into the transformer than to run with a very low output power.What are you using to power the circuit and are you sure it can handle the high surge current that this thing will draw?Maybe try something with a bit more grunt. RC lipo cell, SLA, car battery etc..Just be prepared for a bang and mosfet chunks to ricochet around the room.eg safety glassesTry set your DMM to min/max mode, connect it across the 12V input then run your circuit.See what the min voltage is.One other question, as i have seen people make this mistake before.Do you have these 4 way joins connected correctly on your circuit.Sometimes people miss-interpret them as 2x two way joins passing over each other because they do not notice the dot mark.See image below.

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 05, 2023, 03:18:52 pm
content: Quote from: mkvl on June 05, 2023, 10:16:48 amHave you tested this transformer in "reverse"?ie: Inputted 220V on the original primary winding and measured 10v-0-10vas well as20V without the center tap on the secondary? And then drawn a few amps as well while measuring the voltages...

author: mkvl
author_rank: Contributor
timestamp: June 05, 2023, 10:58:57 pm
content: Yes the 4 wares that you showed in the pic are connected correctly, also I tested the circuit by using LEDs and the switching seems work fine, it maybe that the current delivered by the power source is high like you said, because I hear noise coming from the step up transformer, but I'm not sure of what cause the noises. Thanks for helping.

author: mkvl
author_rank: Contributor
timestamp: June 05, 2023, 11:05:59 pm
content: You mean to connect the primary of the step up transformer to 220v then measuring the voltage of the secondary that I made without the middle wire, 0?.

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 05, 2023, 11:15:19 pm
content: Quote from: mkvl on June 05, 2023, 11:05:59 pmYes:

author: mkvl
author_rank: Contributor
timestamp: June 05, 2023, 11:18:13 pm
content: I have connected the step up transformer to the 220v main source, the multi-meter shows 21v output of the two winding I made and 11v from one winding and 10v from the other winding.

author: boB
author_rank: Frequent Contributor
timestamp: June 05, 2023, 11:25:34 pm
content: You will need an oscilloscope to really tell what is going on.My guess is that if the FET gates are swinging voltage, that their duty cycle is very short on time.That would give you some output voltage but no real output power available.What frequency were you expecting for this to switch at ?boB

author: james_s
author_rank: Super Contributor
timestamp: June 05, 2023, 11:44:36 pm
content: The circuit is a classic multivibrator, an easy way to see what's going on is to slow it down to where you can use LEDs instead of the transformer and watch the actual switching with your eyes. Try replacing the capacitors with something like 470uF and see what happens. Again you must disconnect the transformer or it will saturate and behave almost like a short circuit.

author: mkvl
author_rank: Contributor
timestamp: June 05, 2023, 11:52:53 pm
content: But I have changed the value of the components many, many times to make the circle much longer of time, at first I made the circuit exactly as in diagrams available in internet but when it didn't work I started changing components, I tried everything with no result, I'm struggling with this circuit for long time, it's driving crazy, in internet like diagrams videos it seems very easy, but with me it insist of not working I just don't know what's the problem.

author: james_s
author_rank: Super Contributor
timestamp: June 05, 2023, 11:55:11 pm
content: Well it's not a very good circuit in the first place, it's going to be a very poor inverter with low efficiency and a nasty square wave output, but it shouldn't be hard to make it work. Without some kind of oscilloscope you are flying blind though.

author: mkvl
author_rank: Contributor
timestamp: June 05, 2023, 11:55:24 pm
content: Yes, I always connect first the LEDs and it work fine, the switching work fine.

author: mkvl
author_rank: Contributor
timestamp: June 06, 2023, 12:22:22 am
content: oscilloscope isn't available where I live and isn't cheap, also electronics is just a hobby for me so I'm not willing to pay for an expensive oscilloscope just for a hobby in this sh.tty economic situation.

author: CaptDon
author_rank: Super Contributor
timestamp: June 06, 2023, 01:20:16 pm
content: The Miller Capacitance effect is probably killing the ability to self oscillate. Usually this circuit would get the feedback from the transformer winding and not a multivibrator self contained circuit driving mosfets. The 12k resistors would normally go to the junction of the transformer winding where the mosfet drain is connected. You may need to add a few components and the transformer will only function in this circuit if the circuit timing elements generate a frequency high enough to get enough inductive reactance to sustain the feedback, possibly a few hundred hertz. Then the downside is any load on the secondary will disturb the entire circuit operation. This circuit was done many years ago with bipolar transistors to create 12vdc to 120vac semi-sinewave inverters for campers, motorhomes and boats. Also Swan Electronics used a similar circuit for their automotive 12vdc inverters to power the popular 'Swan 350' transceiver. Cornell Dublier used the circuit in several of their power inverters up to about 500 watts. Now everyone uses digital frequency generation to drive the power mosfets. Most of those old inverters relied on some capacitance across the transformer to make the transformer somewhat self resonant at the desired oscillation frequency. This helped sustain oscillation and kept the overall frequency in a more narrow range regardless of load.

author: james_s
author_rank: Super Contributor
timestamp: June 06, 2023, 06:02:35 pm
content: Self resonant is not suitable if the goal is to produce mains frequency power though. I doubt Miller capacitance is enough to prevent it from working at 50 or 60Hz, that's normally a problem you see in the 10s of kHz range.

author: CaptDon
author_rank: Super Contributor
timestamp: June 06, 2023, 07:57:43 pm
content: The O.P.'s over-simplified circuit has so many shortcomings. If he ever gets it running it will be like inventing the first square wheel. The G/S capacitance may also be swamping the ability to self oscillate. He could probably leave out the multivibrator transistors, redesign the gate circuit and cross-couple the gates to the push-pull transformer. It will sort of find its own 'happy place' frequency of operation determined to a large part by the transformer itself. Sadly, the mosfets will be in the linear range of operation longer than we may like and make some heat and probably the output wave will be closer to sine unlike what it would be with multivibrator squarewave drive. Also, with his multivibrator based circuit if the timing elements are a bit off it will leave flux buildup in the transformer core with possible saturation or huge current spikes reflected into the mosfets.

author: mkvl
author_rank: Contributor
timestamp: June 08, 2023, 01:39:33 pm
content: Hi, thank you for helping, I would like to know what's the solution for this circuit to work? You and some guys have mentioned some things that maybe cause it not to work, like the miller capacitance, so I'm wondering what's the solution. Also I'm confused because I saw many videos which this inverter circuit work fine without any extra components or circuits, just the multi-vebrator connected to transformer, also I appreciate if you can tell me that if I used a digital signal generator to guide mosfets will solve the problem?.

author: james_s
author_rank: Super Contributor
timestamp: June 09, 2023, 12:39:27 am
content: If the circuit itself is known to be a working design then you have only a few possible issues. Wiring error, poor layout, or defective/counterfeit components.

author: mkvl
author_rank: Contributor
timestamp: June 09, 2023, 12:50:59 am
content: The wiring is fine, I tried anything to make it work, I tried with different transformers, transistors, mosfets, capacitors...., I tried different values, no result.

author: MMMarco
author_rank: Regular Contributor
timestamp: June 09, 2023, 01:20:55 am
content: Quote from: CaptDon on June 06, 2023, 07:57:43 pmI'm sorry, I really had to laugh out loud when I read that - wanted you to know