Homebrew power supply

-> ->>Hey,

-

->>Forgive me for the simplicity of this question but I'm rather new to

->>electronics. I want to build a homebrew benchtop power supply and would

->>like a bit of feedback before I try:-

- - -

->>2. What sort of current am I likely to require for simple electronics

->>projects?

->

-> Usually less than 1 Amp. This requirement may change if you're building

-> something with electric motors whough.

->

-Would it be necessary to provide current limiting? I've seen some possible

-examples on circuit schematics of manufacturers data sheets but I'm not

-particularly sure if I need it or not?

Nope. Parts like the LM317 are temp limited. If the current gets too high, the part heats up, then shuts down.

The fuse/circuit breaker I referred to below is also a current limiting device. The current gets too high, then the fuse blows.

-

-I don't plan to use motors at this point and would like to keep the

-project small and simple (low a current as necessary) until I get more

-adept.

Then 1A should be fine for starters.

->>

->>3. Is isolating a power supply from the mains supply difficult?

->

-> Nope. The component is called a transformer. BTW you'll probably want to

-> put a fuse or circuit breaker on your primary of the transformer in case

-> of a short.

->

-250v @ 2amp ok? Should I go for quick blow or surge resistant fuses? My

-mains supply is a pretty stable 230v (UK) and fluctuates little.

Much smaller current. 250V@2A is almost 500W of power. It's very unlikely that you'll dissapate more than 25W of power. So you should be looking for something along the lines of a 250v@0.1A. It can be slow blow as you already have the LM317's thermal limiting as another backup.

-

- >>4. What regulator IC should I consider for a beginner project? I was

->>considering LM137, only on grounds of cost and simplicity - is this a

->>good choice?

->

-> As good as any. Did you want variable voltage? This it's the ticket.

-> Just take care with the max voltage and power dissappation on the part.

-> You'll probably have to heat sink it to draw off the excess heat.

->

-Would a fan be a necessity or just passive heatsinks? Variable voltage is

-a definite requirement.

Depends on the power dissapation. Design for the worst, then double it. If you need a fan at double the worst case scenario, then put one in.

Of course with CPU fans both cheap a plentiful, it may be worth it simply to throw a small one on the part.

BAJ

Hey,

Forgive me for the simplicity of this question but I'm rather new to electronics. I want to build a homebrew benchtop power supply and would like a bit of feedback before I try:-

1. What potential uses does a dual polarity supply have - are they harder to assemble?

1. What sort of current am I likely to require for simple electronics projects?

2. Is isolating a power supply from the mains supply difficult?

1. What regulator IC should I consider for a beginner project? I was considering LM137, only on grounds of cost and simplicity - is this a good choice?

Thanks kindly for your time, Dave

--
David Green (mail.david@dsl.pipex.com)
Hands up for human rights!
http://www.amnesty.org

If you have an alternating voltage, set around a gnd point, the negative value will let you represent those voltages below the 0 point. Especially useful if you're using opamp, then I'd go as far as to say a requirement.

Logic gates (CMOS gates that is) I usually run on 12v, since this is the ultimately most used voltage in electronics. A logic gate requires extremely small amounts of power, like 35mA maximum. Thus, you'd get 30 of them with a 1A PSU. If you are looking at relays, motors and so on, you suddenly need bigger currents, as those pull more. Also, amplifiers and such needs more, but in my expirience, 1A gets you very far.

No. A transformer (not an autotransformer or any other kinda xfmr with shared prim/sec windings) will isolate mains from outgoing voltages.

LM317 is a good choice for having it regulatable. I would also recomend the following:

7805 to provide +5V independent of the adjustable output 7812 to provide +12V independent of the adjustable output 7912 to provide negative 12, especially if you work with operational amplifiers. The +5/+12v I've found very useful, but that can be just me, since I'm working much with pc electronics wich require those voltages. Otherwise, +5V is pretty useless unnless you use TTL logic. CMOS will take anything from 3-18V, so you'd probably end up running cmos logic from the adjustable anyway;)

Good luck:)

Would it be necessary to provide current limiting? I've seen some possible examples on circuit schematics of manufacturers data sheets but I'm not particularly sure if I need it or not?

I don't plan to use motors at this point and would like to keep the project small and simple (low a current as necessary) until I get more adept.

250v @ 2amp ok? Should I go for quick blow or surge resistant fuses? My mains supply is a pretty stable 230v (UK) and fluctuates little. >>4. What regulator IC should I consider for a beginner project? I was

Would a fan be a necessity or just passive heatsinks? Variable voltage is a definite requirement.

As an asides can I actually increase ripple in the DC side of my circuit by adding capacitors across the load of my power supply? If I can't I plan to add a few selected values, mainly for experimentation to see what results I achieve.

Appreciate the response, Dave

--
David Green (mail.david@dsl.pipex.com)
Hands up for human rights!
http://www.amnesty.org

David Green wrote: [...] > How about the mains side? 250v but what amperage? Sorry for the repetitive

Usually, the transformer is marked with a fuse symbol, and a number wich gives the maximum fuse that is to be used on the primary side. Use this size ±10%, and you'll be safe.

[...]

That was to get multiple voltages, and yes, input in paralell, but output as an single point, as it would be a entirely different voltage potential amongst those.

Also, if you want an negative/positive voltage, you'll need an centertapped xfmr, or more effectively a xfmr with two secondary windings.

Good luck with your little project:)

On Mon, 01 Nov 2004 13:28:43 -0500, Byron A Jeff rambled:

How about the mains side? 250v but what amperage? Sorry for the repetitive questions about current but building something running on mains power makes me want to be very sure I've got everything setup correctly.

I'll put a cheap fan in the back of the case and use a finned heatsink - i have some that should fit the package type of the voltage reg.

Vidar mentioned using multiple voltage regs? I presume I would wire these in parallel and they would draw current appropriately when used simultatenously up until the maximum limit, set by the fuse?

Thanks, Dave

--
David Green (mail.david@dsl.pipex.com)
Hands up for human rights!
http://www.amnesty.org

[...]

Yes, this seems perfectly fine :) Also, you will have to rectify. Pass the +ve and -ve trough a rectifier bridge, and take out + and -, and let GND be all alone, do not run that trough any diodes or so...

[...]

I guess you should be able to that. PSU is damn expensive. Something like 300$ for a simple regulatable psu with voltmeter and short protection and such is quite usual...

On Mon, 01 Nov 2004 20:43:54 +0000, Vidar Løkken rambled:

I am looking at a transformer with dual primary and secondary windings - as suggested on the datasheet I was going to jumper the primaries together and then junction the secondaries to create a centre tap, then use the centre tap as the ground reference:-

Something like this:

-------------------- GROUND

-- +240v | | | | ------------- | | | | |

---------------------------- | 0 120 0 120 | | | | | | | | 0 20 0 20 | | |

---------------------------- | | | | | -------------- | | | | | | | | GROUND | +ve (common) -ve rail rail

Then run the variable output and +5 and +12v outputs of the +ve rail, and the -12v negative output of the -ve rail, providing an output of common groud for when I want a +ve supply only.

Is it a good plan? Thanks for the input, hopefully I can build something far cheaper than an off-the shelf supply and learn a lot whilst building it.

Regards, Dave

--
David Green (mail.david@dsl.pipex.com)
Hands up for human rights!
http://www.amnesty.org

Putting a fan it useless, a big heat will make it.

He means this:

____ | | +----+---|7805|--+------ V+ | | |____| | | --- | --- fuse -. ,-------+ | --- | --- )|( | +-+----+ | | | )|( | A A | | | |-' '-+ +------+ | | | | |-. ,-+--+ +------(-+ +-----+-----+----+ )|( | | A A | | | | )|( | | +-+----+ | | | | -' '----|--+ | --- | --- === | | --- _|__ --- GND | | | | | | | +----+---|7905|--+------ V- === |____| GND created by Andy´s ASCII-Circuit v1.22.310103 Beta

Google for "Ian Purdie power supply" (or suplies). He covers that well enough - selecting caps for a specified load current and ripple and stuff like that.

--
Best Regards,
Mike