Homemade 555 buck converter only works with a certain supply

Thread Starter

Tyler Simmons

Joined Mar 30, 2019
22
Hi. I have a problem with my buck converter that I made using tutorials and schematics online to help. I have used several different voltage supplies to test the circuit but only one of them works. It is a 12v supply that works and I have tried other 12v supplies but they do not work properly! The output voltage is fully adjustable from 1.8v to 12v while using the one 12v input, but for all other voltage supplies I use the output voltage is either not adjustable or only adjustable between about 2v range.
Here is a schematic of my circuit.
http://tinyurl.com/y6n9m687

I am not an engineer or fluent with circuits, I am just playing with electronics at home. This is my first time asking a question about my circuits. I'm sorry if I have omitted any needed information.
 

Thread Starter

Tyler Simmons

Joined Mar 30, 2019
22
You did.
Can't really help without knowing the characteristics of the supply that works and those that don't, particularly maximum current rating.
the one that works is 12v 0.75A output. It a lower current rating than any of the other voltage supplies i have used.
 

crutschow

Joined Mar 14, 2008
34,285
Monitor the output current of the various supplies when the circuit is operating to see how they might vary.
Why type of supplies are these?
 

Thread Starter

Tyler Simmons

Joined Mar 30, 2019
22
Some of the supplies are wall warts some are transformers rectified to dc and also a dc supply made from an xbox power supply. I did find another one that works, but I have to change the resistor values and potentiometer that I use on the 555 circuit. I feel like I am missing or not understanding something. I am under the impression that I can make a "universal" buck converter for a given range of input voltages.
 

Thread Starter

Tyler Simmons

Joined Mar 30, 2019
22
All but one of the supplies can hold their max current, and the current drops when the voltage drops like it should right?

Most of the supplies seem to work better with the buck converter with a load which I dont understand.

But my mosfet gets very hot when held at 1 amp and starts to crackle soon after that. I have a small heat sink on it too. here is a link to the mosfet I am using. Is it possible maybe I chose a bad part for this circuit?

https://alltransistors.com/pdfview.php?doc=fqb30n06ltm.pdf&dire=_fairchild_semi
 

LesJones

Joined Jan 8, 2017
4,174
The schematic in the link from post #1 does not have any decoupling capacitors on the supply input. I suggest you add an electrolytic capacitor of a few hundred uF in parallel with a 100 nF ceramic capacitor as close as possible to the 555 IC. The mosfet should not get that hot if it switching correctly. The type of inductor used is also quite important. The core must not saturate. post a picture showing the inductor and we may be able to tell you if it looks suitable for the application.

Les.
 

Thread Starter

Tyler Simmons

Joined Mar 30, 2019
22
.The inductor on it currently is one I made :) i have many that I can try though.
https://imgur.com/a/LBRNxz2

I need to read more about inductors and how to calculate saturation current. This one measures 2.4mH on my gm328 tester. Im pretty sure it's a ferrite core.

I tried a decoupling cap in parallel with the 100nF as suggested but the output voltage is either max or min to my supply voltage.there is no in between voltage level. I tried this with the same load resistors and also with a fan that will slow and speed up without the capacitor, but the fan just turns on high or off with the decoupling cap.in place.
 

Thread Starter

Tyler Simmons

Joined Mar 30, 2019
22
It turns out that the mosfet I am using is faulty fake or just the wrong part to use. Or maybe I dont know how to use mosfets :p but I popped one of these in there https://pdf1.alldatasheet.com/datasheet-pdf/view/30498/TOSHIBA/2SD2539.html(salvaged from an old tv). and the pwm signal works flawlessly to change the output voltage with all supplies! Can anyone help me understand this? is there a way to test the mosfet to tell if it is fake? The specs for the mosfet check out on my transistor tester (i think).
 
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