Problem with step up voltage regulator LM2577 ADJ

[Lambo3]

I'm trying to build an adjustable voltage regulator to put out ~3 to 14 volts. I purchased a kit with PCB based on LM2577T-ADJ regulator. The original intention of the circuit was to simply regulate +5V to 12V.

I'm guessing this will be an elementary question for many of you... I took a couple electronics courses in high school, so I know enough to follow a schematic, but that's about it.

...so, here I am, modifying the circuit, and frying parts.

This schematic is more or less what the kit is based on:
**broken link removed**

More details on this page:
https://www.hobby-hour.com/electronics/lm2577-switching-regulator.php

It pretty closely. I wanted it to be adjustable. So I modified resistors R1 and R2, according to this formula:

VOUT = 1.23V (1 + R1/R2)

R1 was originally 2K, and R2 an 18K. Testing the breadboard this worked well to produce 12V. I replaced R1 with a 1K resistor, and R2 with a 10k potentiometer and an inline 1K resistor to which gave me 1K-11K ohms. Plugging this into the formula, it should technically regulate 2.23V to 14.76V.

When I finished it all up, it actually worked well, but the range was more like 5.5-14.5 give or take. That is fine for my purposes so I wasn't worried. The input source is 4AA batteries at 6V.

On the multimeter, this all worked well. I'm using this to power a 12V cpu fan. It operated well at first, but when I ran it at a higher voltage it ended up toasting the inductor. I happen to have a spare inductor, so it I soldered that one in, and it toasted it as well as soon as I cranked up the potentiometer.

I'm sure there is some math I'm not doing, and perhaps dropping from 2K to 1K on feedback resistor is creating some problems?

The PCB:
**broken link removed**

Thanks for any input.

Scott

 

[panic mode]

first of all you cannot use boost configuration to get lower voltage. if your input is 5V, your output has to be greater than that.
second, your inductor is not correct for this circuit. LM2577 can work for up to 3A. according to link and description on PCB, it is 0.8A max at 12V. i would be impressed if one in picture can handle more than 250mA. google any of the part numbers for inductor listed in your link and you will see something like this
https://www.electro-tech-online.com/custompdfs/2012/10/P635-1.pdf
huge difference - those are 3A inductors with thick copper wire and a massive ferrite core, not a little inductor shown on PCB.

check datasheet for sizing of the inductor:
https://www.electro-tech-online.com/custompdfs/2012/10/uc2577-adj-1.pdf

smoke is the ultimate teacher, you don't get to experience this using simulator.

 

[Lambo3]

first of all you cannot use boost configuration to get lower voltage. if your input is 5V, your output has to be greater than that.
second, your inductor is not correct for this circuit. LM2577 can work for up to 3A. according to link and description on PCB, it is 0.8A max at 12V. i would be impressed if one in picture can handle more than 250mA. google any of the part numbers for inductor listed in your link and you will see something like this

That does make sense... what confused me is that the spec sheet for the LM2577 requires a minimum input voltage of 4.5, but says it's output can product from 1.5V to 15V. I guess it doesn't matter, because I really don't need it to produce less than 6 volts.

https://www.electro-tech-online.com/custompdfs/2012/10/P635-1.pdf
huge difference - those are 3A inductors with thick copper wire and a massive ferrite core, not a little inductor shown on PCB.

check datasheet for sizing of the inductor:
https://www.electro-tech-online.com/custompdfs/2012/10/uc2577-adj-1.pdf

smoke is the ultimate teacher, you don't get to experience this using simulator.

I was wondering if that was the case... I'll order up some 3A inductors, and see how that treats me. I'm also getting some 12K switched pots, and I'll run 2.2k in R2, and a 10K inline with the 12k pot in R1. That should attain a range of 6.8-13.5 volts. Pretty acceptable for what I'm doing. I'm using 1/4 resistors, is that acceptable?

Yes, the smoke was definitely the signal to educate me... only problem is a guy like me will run out of parts quick, with my trial and error ways.

Thanks for the info, very helpful.

 

[panic mode]

what confused me is that the spec sheet for the LM2577 requires a minimum input voltage of 4.5, but says it's output can product from 1.5V to 15V.

traditional switchers can be configured to work in different modes by swapping location of main three components (switch, inductor and diode). this allows output that is higher or lower than input voltage or even opposite polarity. but when configuration is SEPIC, it should be able to produce output that is lower, equal or greater than input.
https://en.wikipedia.org/wiki/Single-ended_primary-inductor_converter

 

[audioguru]

My cheap solar garden lights use a little inductor like that to boost the voltage of a single cell Ni-Cad battery to 3.5V to light a 20mA LED.
The peak current in the inductor is about 70mA.

 

[panic mode]

so i'm not the only one to open everything i can get my hands on...?

 

[chemelec]

Your Inductor has TOO LOW of a Current Rating.
Probably Less than 100mA.

 

[Smoketester]

I am doing a similar project with lm2585, the first thing I am trying to.do is oversize everything just a bit.so I hopefully don't let the smoke out of anything. You may try that also.