### Author Topic: How to figure out the power scheme of a PCB?  (Read 1028 times)

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#### pcee

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##### How to figure out the power scheme of a PCB?
« on: November 16, 2021, 10:40:48 pm »
I have a PCB that takes 12V DC input and converts that to 3.3V (along with using 12V for some components). I'd like to understand how this board is converting the 12V to 3.3V and what components it's using to do so.

It's got a bunch of SMD components with dubious labels, so it's not immediately obvious to me what most of the parts are. (There's one well-marked 78M09 voltage regulator, but that produces 9V.)

Where should I begin? I've measured voltage around a bunch of components, and found some traces with 3.3V, but I'm not sure whether they're supplying power to the adjacent component, or whether the component is converting and supplying the voltage. I haven't had much luck identifying most of the SMD components apart from resistors and capacitors. I can infer some likely SOT23 diodes and transistors, but I don't know their characteristics. And then there's a mystery SOT23 marked "18H" that has 3.3V on pin 1, 12V on pin 2, and 0V on pin3.

I'm sure there's a good approach for this, I just have no idea what it is. I'd appreciate any help or pointers!

#### Manul

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##### Re: How to figure out the power scheme of a PCB?
« Reply #1 on: November 16, 2021, 11:24:46 pm »
The process is pretty much how you think it is. If you are trying to find where and how 12V to 3.3V is being converted, look for components or circuit blocks which have these voltages as input and output. The rest is experience and pattern recognition making the process quicker and more intuitive. Just spend more time looking at different PCB's, practice indentifying components. Nothing will help you more. SOT23 marked 18H could be TLV76033DBZ. A 3.3V linear voltage regulator indeed.

#### pcee

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##### Re: How to figure out the power scheme of a PCB?
« Reply #2 on: November 16, 2021, 11:51:05 pm »
Thank you! To that I have two follow-up questions:

1. How can I tell what's input vs. output if I don't know what the component is?

2. How did you find that TLV76033DBZ? That's almost certainly what it is. I searched high and low for 18H but voltage regulators never came up!

#### Manul

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##### Re: How to figure out the power scheme of a PCB?
« Reply #3 on: November 17, 2021, 12:23:37 am »
1. Just like in science, you have hypothesis, then you collect more data and prove or disprove it. Yes, you may not know what's input and output, but both of these voltages present near each other is a hypothesis worth checking. On the other hand, you often know. Like in your case, you know that 12V is the primary voltage. You can also just trace all the circuit points where it goes, creating something like a "flow diagram".

2. I googled sot23 18H. No kidding. So next time do a better job. Also the key words are "smd code book". You can find many pdf's and websites with databases of SMD components markings. That migh be a tedious job, because these databases and books are incomplete, you need to go through many of them. Also google searches. Try to include package name, guess component function, for example "voltage regulator" or "op amp", word "marking". Sometimes you get results that way. Go through what you can find, also look images. Searching is a form of art these days.

I will also note, that there are mostly three types of voltage converters you will find. Linear regulator, switching converter and rarely switched capacitor converter. Study these. On a typical device PCB you will notice switching converters by inductor or transformer. Transformer usually means it is isolated converter. Most low voltage converters will be buck or boost topology, non isolated. You can recognize them easily if you are familiar with them. They can be used for both small and high currents. Linear regulators are usually used for small currents, low power. Switched capacitor converter are a bit exotic, but sometimes used for low currents, or as negative voltage generators. There are much more to say here, but idea is that you need to collect experience by involving yourself.

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#### pcee

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##### Re: How to figure out the power scheme of a PCB?
« Reply #4 on: December 10, 2021, 01:20:32 am »
I googled sot23 18H. No kidding. So next time do a better job.

Ha! It turns out my choice of search engine was my undoing. Note to future self: try multiple search engines!

Smf