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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 19th Sep 2019, 2:59 pm   #1
Diabolical Artificer
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Default Buck/Boost regulator IC.

I've had a little pcb in my draw for a while and powered it up to check working voltages. The board AFAIK, is a little flashing light type thing to music using a indicator valve as the display. I put 12v DC in and was very surprised to measure around 270v on some pins of the valve holder.

I was looking for a simple way to power a guitar pedal that uses an ECC82. This after building the "Valvecaster", see attached, a while back and also looking at this similar project - http://www.dogstar.dantimax.dk/tubestuf/mctube.htm This sort of HV efficient lightweight PSU if able to provide a clean power source would be ideal for low powered valve projects.

As far as I can see it uses a MC34063A regulator IC to get the needed HT, but looking at the datasheet I can't see any OP voltage mentioned or for the life me figure out how with a few external components it "generates" this higher voltage.

Looking at the board all there is in way of PSU components is a few caps (timing etc) an inductor and an IRF740 mosfet. I presume the oscillator in the chip switches the fet to produce HV AC, which is then rectified.

Any light shined on the subject would be appreciated. Andy.
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Old 19th Sep 2019, 3:28 pm   #2
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Default Re: Buck/Boost regulator IC.

Boost converters are pretty simple - they exploit the back-emf effect of an inductor to get an output voltage that is higher than the input. Start with the basic circuit (inductor, switch, diode) and understand how that might work before worrying about what the control chip does.

https://en.wikipedia.org/wiki/Boost_converter

The reason the output voltage isn't explained on the IC's datasheet is simple: it's up to the designer. Think of that IC as being a bit like the 723, which is a linear regulator "kit" that can be used in a wide number of ways, depending on how you hook it all up.

The switch (as seen on the basic block diagram) is a transistor inside that IC. That has a maximum voltage rating of around 40V, so that puts a limit on how much output voltage you can achieve with just the IC. But if you add an external switch, then that sets the limit. You mention the IRF740 - that's what it's doing.

In short, that MOSFET is being turned on and off at many 10s of kHz by the IC. The duty cycle (the on time compared to the off time) is varied to regulate the voltage. It's magic, but understand how it works, and you'll understand the basics of all modern switched-mode power supplies
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Old 20th Sep 2019, 12:27 am   #3
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Default Re: Buck/Boost regulator IC.

This should give you a pretty good idea of what is happening, also includes a calculator to design for your own (HV) output voltage.

https://www.changpuak.ch/electronics...ly_MC34063.php
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Old 20th Sep 2019, 6:36 am   #4
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Default Re: Buck/Boost regulator IC.

Thanks both, couldn't understand how this one worked without a step up tfmr, inductor EMF explains it. Will look in to this further.

Andy.
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Old 21st Sep 2019, 8:40 pm   #5
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Default Re: Buck/Boost regulator IC.

TV line output stages use the inductive back-emf pulse created when the scan current abruptly reverses for flyback to create a high voltage pulse AND it uses a step-up transformer turns ratio to magnify the voltage even further.

So you can have a boost converter making an increased voltage with just a single inductor, but you can use the same thing with a transformer to provide isolation and the turns ratio can be used to step the voltage up further, or can be used to step the voltage back down. They also get called flyback converters.

A derivative of the boost is the SEPIC which has the ability to output a voltage from less than the input voltage, to more than it, and it's smoothly adjustable.

You've just opened the door on a whole family of converters. Once you understand the basic boost, you get several others for free.

David
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Old 22nd Sep 2019, 6:03 pm   #6
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Default Re: Buck/Boost regulator IC.

In solar garden lights, the control chip pulses an inductor with the LED wired as its anti-kickback diode.
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Old 22nd Sep 2019, 6:21 pm   #7
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Default Re: Buck/Boost regulator IC.

Also, have a look at the "Joule Thief" - https://en.wikipedia.org/wiki/Joule_thief

It's a great example of how to get a high(er) voltage from an all-but-dead battery. I've built plenty of these over the years - germanium transistors, with their really low saturation-voltage - are great in such an application where you're maybe only starting off with a Volt of 'HT' from your dying battery.
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