# Designing a circuit - where to start?

#### StrongPenguin

Joined Jun 9, 2018
233
Ok. When I tried to play around with digital gate circuits, I just took an output, and worked from there, knowing nothing about the wonders of Kmaps and Boole.

Now I want to design a voltage regulator to my ATX power supply, from 0V to 12V (or 24V, if that is possible..)

I know there are about a million articles on voltage regulators, but I am more looking for the board design process, with the purpose of building and designing a regulator my self.

Where do I start? I have a (two, actually..) 741 op-amp, could I use it for this purpose?

#### dl324

Joined Mar 30, 2015
10,513
Ok. When I tried to play around with digital gate circuits, I just took an output, and worked from there, knowing nothing about the wonders of Kmaps and Boole.
You don't always need to generate truth tables and solve Kmaps to design circuits.

For example, if you wanted to design a 2 digit BCD up/down counter, you could just connect a couple BCD counters and 7 segment decoders with a clock and some switches to control up/down and, probably, reset. No truth tables necessary.
Now I want to design a voltage regulator to my ATX power supply, from 0V to 12V (or 24V, if that is possible..)

I know there are about a million articles on voltage regulators, but I am more looking for the board design process, with the purpose of building and designing a regulator my self.
Since no ATX supplies provide more than 12V, you'd want to look into a buck/boost switching regulator. Since the ATX supply has a negative voltage available, getting 0V is straightforward.
Where do I start? I have a (two, actually..) 741 op-amp, could I use it for this purpose?
As many will point out, the 741 is an old design and there are many newer designs that are easier to use. That being said, the 741 was the first opamp I used and I didn't, and still don't, find it particularly difficult to use. You just need to understand it's limitations.

#### crutschow

Joined Mar 14, 2008
24,716
The simplest is to use a low-dropout linear regulator to go from 0V to near 12V.
If you want 24V you will need a switching boost regulator.
What maximum current do you want?

If you want a simple design I would not use a 741 circuit.

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#### Hymie

Joined Mar 30, 2018
798

#### StrongPenguin

Joined Jun 9, 2018
233
@dl324 I think I will just keep it simple, at say from 0 to 12 V. Do you have another suggestion for an IC i could use? I could also give the 741 a try, but I've never designed a circuit before.

@crutschow Ok, interesting. I will look into that.

@Hymie That's pretty neat. Too bad I've cut the cables and made the housing.

#### dl324

Joined Mar 30, 2015
10,513
I think I will just keep it simple, at say from 0 to 12 V. Do you have another suggestion for an IC i could use? I could also give the 741 a try, but I've never designed a circuit before.
For 0-12V, you could use a linear regulator and modify the 12V rail of the ATX supply to give you something higher. How high depends on the dropout voltage of the regulator.

If you're want high current at a low voltage, you probably want a switching regulator.

#### Hymie

Joined Mar 30, 2018
798
Here is a circuit that will allow you to vary the output from 0-12V using only 2 components.

The pot (VR1) can be almost any value with the transistor Q1 being a darlington with a very high gain.

Bear in mind that the transistor Q1 power dissipation will be the volt drop multiplied by the current, and so may require suitable heat-sinking.

#### ebeowulf17

Joined Aug 12, 2014
3,274
Here is a circuit that will allow you to vary the output from 0-12V using only 2 components.

The pot (VR1) can be almost any value with the transistor Q1 being a darlington with a very high gain.

Bear in mind that the transistor Q1 power dissipation will be the volt drop multiplied by the current, and so may require suitable heat-sinking.
View attachment 157702
Depends on the maximum required output voltage and the need for regulation.

This circuit can only output a maximum of input voltage minus Vbe, so if input is 12V, then max output is 10.5 - 10.8V, maybe less. The maybe is because Vbe varies as a function of current, so for any given setting, the output voltage will be lower when current draw is higher.

Depending on the application, this may all be fine, but it's good to know the limits of a circuit before using it. I wouldn't consider this circuit a regulator, but it might be enough for what that thread starter wants.

#### crutschow

Joined Mar 14, 2008
24,716
The circuit in post #7 also has no short circuit protection, so could zap a transistor if the output is accidentally shorted.

If you use a Sziklai pair (complementary Darlington) as shown below, you can have a maximum output of one base-emitter drop below the input (<1V drop), not two drops as from a standard Darlington.

The PNP must be a power transistor but the NPN an be a standard small transistor.

#### Audioguru

Joined Dec 20, 2007
11,249
Where should you start the design? First decide the spec's.
Do you want good voltage regulation from a buffered voltage regulator IC or lousy voltage regulation from one of these Mickey Mouse extremely simple circuits that have been shown?

#### crutschow

Joined Mar 14, 2008
24,716
one of these Mickey Mouse extremely simple circuits
Are you sure it wasn't your nephew Donald Duck?

#### StrongPenguin

Joined Jun 9, 2018
233
I will try to be more specific. Forget what I said about higher voltages, 0 to 12V (not 10) is fine.

What I had in mind was if someone could list the parts I need for my regulator, no circuit drawing. I am more interested in breaking my head trying to stitch together this circuit from the parts list, than getting a finished diagram ready to solder.

Something like "you need an XXXX op-amp, two NPN transistors, five zeners, five resistors" and so on. Just like assigning homework, only you know what the ingredients are. This way, I will learn something too about analyzing circuits, which I am not good at. At all.

Of course I want the thing to be stable, and it doesn't have to handle high amps at a low voltage.

#### ebeowulf17

Joined Aug 12, 2014
3,274
Forget what I said about higher voltages, 0 to 12V (not 10) is fine.
It's essentially impossible to make a regulator with no voltage loss (dropout voltage.) If you put 12V in, you get less than 12V out. That said, using a MOSFET output stage instead of BJTs, you can get relatively close, depending on what amperage you need. Looking at one common MOSFET datasheet, you could get roughly:
11.994V at 100mA
11.94V at 1A
11.4V at 10A

Of course I want the thing to be stable, and it doesn't have to handle high amps at a low voltage.
Aside from the max voltage limits, this is an interesting idea. I'd never considered making my own regulator - well designed regulator ICs do almost all the work for you, and they're cheaper and smaller than anything you can design yourself, so there's not much reason to. Nevertheless, as an educational tool, it would be a good exercise, so I'm going to try to design one from scratch, without copying from regulator IC datasheets, and see how I do. If I come up with anything promising, I'll share the parts list so that you can try to figure it out.

Cheers!

#### Dodgydave

Joined Jun 22, 2012
9,079
I will try to be more specific. Forget what I said about higher voltages, 0 to 12V (not 10) is fine.

What I had in mind was if someone could list the parts I need for my regulator, no circuit drawing. I am more interested in breaking my head trying to stitch together this circuit from the parts list, than getting a finished diagram ready to solder.

Something like "you need an XXXX op-amp, two NPN transistors, five zeners, five resistors" and so on. Just like assigning homework, only you know what the ingredients are. This way, I will learn something too about analyzing circuits, which I am not good at. At all.

Of course I want the thing to be stable, and it doesn't have to handle high amps at a low voltage.
1x LM317 regulator, 1 x 120R resistor, 1 x 1K variable resistor , 2 x 100uF 16V capacitors.

That will give you a voltage of 1.25 to 11.5V, if you want more current over 1Amp, then use a Tip42 slave power transistor.

#### Janis59

Joined Aug 21, 2017
1,148
RE:""Where do I start?""

My method is so: at first look how this problem is solved by other man. Look Google-pictures, look Google articles, look russian web pictures (them often solves the things in rather distant way, which not obligeously is bad). Then make a choice and with rapid technologies make a one experimental sample (just the thick paper "board" and wires). Take the oscilloscope and find the weak points, try to re-made that parts in own way. Use extensively Spice modelling capabilities. If need, re-work all the main idea behind in better manner. But never start the brand new thematic for yourself from the very ground zero. Such jump will never be 'tiger jump' while the pound of salt will net be aten.

#### crutschow

Joined Mar 14, 2008
24,716
That will give you a voltage of 1.25 to 11.5V
I believe you slipped a volt in your mental math.
The maximum small load output of an LM317 will be about 10.5V with 12V in.

#### StrongPenguin

Joined Jun 9, 2018
233
@Janis59 This sounds like something I would do if I had a specific problem, but I am more looking for circuit design.

@Dodgydave Thanks for the list! But if it only gets up to 10,5V then I'm not sure if I can use it for an end game lab bench. However, I really wanna try this just for the education.

@ebeowulf17 Which MOSFET are you going with? I am assuming there are a gazillion different types.

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#### crutschow

Joined Mar 14, 2008
24,716
But if it only gets up to 10,5V then I'm not sure if I can use it for an end game lab bench. However, I really wanna try this just for the education.
Then best you can likely do with a simple circuit is to use the transistor output configuration in post #9 in the circuit in post #7.
That will get an output adjustable from 0V minimum to about 11.2-11.3V maximum.

But it would be a lot simpler to use an LT3081 which will go from 0V to about 11.5V with an adjustable current limit.

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#### dl324

Joined Mar 30, 2015
10,513
You're asking for us to give you a list of parts to make a voltage regulator we have in mind. That's going to be an exercise in frustration...

This is a classic, probably the first voltage regulator I built:

No attribution because it's just an image from the internet and I couldn't find any...

Look at the configuration, try to understand what each component does, then try to come up with some reasonable component values.

#### StrongPenguin

Joined Jun 9, 2018
233
@crutschow I will try the transistor setups mentioned above, as I have no experience working with them. So this should be fun.

@dl324 Thanks for the circuit! This was also a good twist, knowing the circuit but no values. Just so that I understand it correctly; the potmeter is the one in the middle, while the above and below are separate resistors (say R4 and R5..)?

Thanks for the help, gals/guys!

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