555 timer flyback driver circuit problem

[Fluffyboii]

Hi,
I was trying to make an op amp circuit work from 5V USB power and lack of dual power supply was making it very difficult as always. One way of solving the problem would be using 7660 IC as Nigel suggested. And it seems like a brilliant solution. But I already made an order for electronic parts form an online warehouse and I don't want to make a new order to buy a single IC and pay shipping costs.
So I was thinking of making some kind of isolated boost converter with the stuff I have at hand. There are multiple ways of getting some kind of AC waveform and feeding it to a transformer to increase its voltage then rectify it. I have decided to use a 555 timer to generate high frequency pulses. Then I could split the AC I get from transformer to get positive and negative voltages as I wanted. I also had a small flyback transformer left from an kit I failed to build some time ago so at worst case scenario it would be used with that. First circuit that comes when Googled is the one at the bottom.

I made it and test it. The frequency was about 1Khz which was too low for driving a flyback but I gave it a go anyway. It consumes about 2A at 3V when flyback transformer is connected because I limited the current so it is not sustainable at 12V. I checked the waveforms at the BJT and it seems like circuit sends some chonky high duty rate pulses to the BJT and it gets inverted to small duty cycle pulses for the Mosfet to do the switching. My choice of mosfet here is IRF630 which I believe should function similarly. The problem is the waveform 555 generates and what shows up at the base of the BJT is quite different. For some reason there is a huge voltage drop at the base of the BJT. In turn it shows up as a very small voltage at collector and at the Mosfet's gate. And I believe because of that reason Mosfet doesn't really do any switching and instead stays open all the time causing huge current to flow. I am putting oscilloscope photos to bottom so you can see how small the voltage gets at the base and collector.
I believed that the 1K resistor was too high and switched it with 100 ohms instead. I also decreased the timing capacitor with 10nF increasing the switching to about 100Khz. This did not change anything. I basically switched to this circuit:

Sooo what I am exactly doing wrong here. I believe 555 part is functioning, BJT is connected right and I did not mixed up the Mosfet connections as well this time. Should I put some series resistance with the gate do decrease current. I actually got some high voltage randomly at the output when I tested it but that was one time only and my 555 blow up later. Now I am sure everything is connected right. If I can get this to work I will use a transformer with small turn ratio and use a bridge rectifier to get my positive and negative rails. If the output signal is full of garbage ripples I will use linear converters to cut it off. Are there any similar oscillator circuits to get a quick dirty AC wave. I doubt lack of feedback will be an issue since my load will be few op amps pulling less than 10mA at total.

The more I think about it stupider it feels that I went with this instead of doing anything else.

 

[audioguru]

Fluffy, I cannot find your post that said the complete part number of your 555 but I remember it is not a normal NE555.
Maybe its specs are different to an NE555. What is it?

C2 is 3.3uF with no polarity shown. Try 100uF with its positive wire at the output of the 555.

 

[Fluffyboii]

Fluffy, I cannot find your post that said the complete part number of your 555 but I remember it is not a normal NE555.
Maybe its specs are different to an NE555. What is it?

C2 is 3.3uF with no polarity shown. Try 100uF with its positive wire at the output of the 555.

It was writing NE555 on it but I am not sure if there was anything after it. I will try changing 3.3uF and check what was the exact model of 555 I have. I have few different NE555 ICs from different sellers. I also have TLC555 but I did not use it since Mosfet version has less output current as far as I can remember and it is expensive so did not wanted to risk destroying it. Maybe it simply can not output much current and I need to buffer its output.

It is not really important since I now have two 9V batteries I can directly use but will still check.

 

[Fluffyboii]

Where you stand now Fluffboii, what's the problem?

Sorry I noticed your message now. The problem is that 555 negative voltage converter creates a negative voltage much lower compared to the positive line. For example for 4.6V it gives -3.5V at max and drops significantly when there is a load attached, down to -2.7 with single op amp attached which is bit concerning.

 

[Fluffyboii]

Fluffy, I cannot find your post that said the complete part number of your 555 but I remember it is not a normal NE555.
Maybe its specs are different to an NE555. What is it?

C2 is 3.3uF with no polarity shown. Try 100uF with its positive wire at the output of the 555.

It is CHN NE555N. I think these are the new ones I bought after I purged some of them by accident thinking they were defective while the schematic I was using just made it run at %98 duty cycle or something like that.

Circuit outputs -11.3V at 12V input. Putting 100uF makes it -11.34V.
Using another NE555 made by Ti makes it -11.38V.
I guess this is normal behavior.

 

[Lightium]

The flyback in the schematics is very

555 negative voltage converter

That's very different then what is in the schematics. just saying.

 

[Fluffyboii]

The flyback in the schematics is very

That's very different then what is in the schematics. just saying.

My bad for having discussion about two different circuits under one post. I was talking about the one alec_t posted at #6

 

[alec_t]

The circuit I posted was not intended to provide more than a few mA, nor can it provide a negative voltage as great as the positive supply voltage.

 

[audioguru]

The old 555 is not rail to rail so its output voltage high is 1.3V less than the positive supply when the load current is low.
The 1N914 diodes have a 0.7V loss but Schottky diodes can be used that have a -.25V loss when the load current is low.

 

[Fluffyboii]

The old 555 is not rail to rail so its output voltage high is 1.3V less than the positive supply when the load current is low.
The 1N914 diodes have a 0.7V loss but Schottky diodes can be used that have a -.25V loss when the load current is low.

Yes it checks out. Would it benefit from using the CMOS version of 555 IC or using a bjt or mosfet connected to the output to get a rail to rail pulse. I am already using Schottky diodes.

 

[audioguru]

A Cmos 555 has a low output current.
You need an ICL7660 or a Maxim higher power similar product.

 

[Fluffyboii]

A Cmos 555 has a low output current.
You need an ICL7660 or a Maxim higher power similar product.

I will definitely get few ICL7660's in my next electronic component purchase. Seems like it makes life easier. Seems like adding the same charge pump to an existing boost converter is also an option.

 

[rjenkinsgb]

A CMOS 40106 may work for this, in place of the 555?

eg. Use one inverter with a resistor for feedback and a cap from input to ground, a standard CMOS schmitt trigger oscillator.

Then connect the other five inverters in parallel, with the input from the oscillator.

The output would then be capable of about 5mA at 5V or ~20mA at 12V, still keeping a voltage swing fairly near the supply voltage.

That would feed the capacitor & diodes to give the negative voltage.

A 7660 or equivalent is simpler, but you may have a 40106 in your parts already?

 

[Fluffyboii]

A CMOS 40106 may work for this, in place of the 555?

eg. Use one inverter with a resistor for feedback and a cap from input to ground, a standard CMOS schmitt trigger oscillator.

Then connect the other five inverters in parallel, with the input from the oscillator.

The output would then be capable of about 5mA at 5V or ~20mA at 12V, still keeping a voltage swing fairly near the supply voltage.

That would feed the capacitor & diodes to give the negative voltage.

A 7660 or equivalent is simpler, but you may have a 40106 in your parts already?

I was planning on using 4049 or something like that to buffer the hell out of the 555 output but yes using 40106 may make more sense since I got many CD40xx ICs from synth stuff including the 40106 and it would be an one chip solution.

Something like this I assume would work. What would be an ideal frequency. I guess going too high doesn't make sense since I will filter all noise and stuff above 100Hz.

 

[Fluffyboii]

It somehow reads bit more than the supply voltage at the output of the CD40106 with no load. Under load of 2 LEDs with 1K series resistor it drops to 10.7V from 12V I think it is still better since op amps will pull less current than a LED. 1V higher compared with 555 circuit at same conditions and load.

I think I did not wire all of the outputs together on breadboard since it is confusing cable mess so it should be even better when I connect all of them.

Never mind I think I connected all of them right. But still probably parastics are causing clock to shift into weird shapes and get delayed between gates.

I think I killed my new multimeter while trying to measure the voltage of the boost converter I made at the circuit at #1. I changed the flyback with a small torroid transformer I wound that had low turn ratio, probably still caused a large transient voltage. Now it only reads positive voltages and shows larger wrong negative voltage when connected in reverse for 20V scale and it seems like continuity mode is glitched as well. But 200V reads right.

Interestingly I used the high voltage AC option and got inconsistent voltage reading then tried rectifying it with a single diode and capacitor to hold the peak voltage and still got inconsistent voltages but those scales are fine. It is just cheap garbage I guess.

Damn it this is the 5th multimeter I got. Looks like the original one resurrected from its wrong voltage reading state, it use to read like 0.7V at 12V. IDK why I am so unlucky with this stuff.

 

[Fluffyboii]

My boy died again. I cleaned its contacts so this is no logic behavior. Video proofs the nonsense. It started going into that state and out by its own until permamently got stuck like that.