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# Homebrew power supply

B

#### Byron A Jeff

Jan 1, 1970
0
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?

Good for circuits that do audio. Also circuits that use opamps may or may
not need a dual polarity supply.
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.
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.
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.

BAJ

D

#### David Green

Jan 1, 1970
0
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?

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

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

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?

Thanks kindly for your time,
Dave

V

#### Vidar Løkken

Jan 1, 1970
0
David said:
1. What potential uses does a dual polarity supply have - are they harder
to assemble?

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

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

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

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

Good luck

D

#### David Green

Jan 1, 1970
0
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?

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
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.
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.

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

B

#### Byron A Jeff

Jan 1, 1970
0
-> In article <[email protected]>,
->>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:-
-
-<snip>
-
->>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

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. [email protected] 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 [email protected] 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

V

#### Vidar Løkken

Jan 1, 1970
0
David Green wrote:
[...]
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.
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.

[...]
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, setby
the fuse?

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

D

#### David Green

Jan 1, 1970
0
-> In article <[email protected]>,
->>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:-
-
-<snip>
Much smaller current. [email protected] 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 [email protected] It can be slow blow as you
already have the LM317's thermal limiting as another backup.
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.
Of course with CPU fans both cheap a plentiful, it may be worth it
simply to throw a small one on the part.
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

V

#### Vidar Løkken

Jan 1, 1970
0
David said:
]

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.

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...

[...]
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.

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... D #### David Green Jan 1, 1970 0 David Green wrote: [...] 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? 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. 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. Good luck with your little project 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 J #### Jack// ani Jan 1, 1970 0 Would a fan be a necessity or just passive heatsinks? Variable voltage is Putting a fan it useless, a big heat will make it. A #### Active8 Jan 1, 1970 0 On Mon, 01 Nov 2004 21:52:51 GMT, Vidar Løkken wrote: 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... He means this: ____ | | +----+---|7805|--+------ V+ | | |____| | | --- | --- fuse -. ,-------+ | --- | --- )|( | +-+----+ | | | )|( | A A | | | |-' '-+ +------+ | | | | |-. ,-+--+ +------(-+ +-----+-----+----+ )|( | | A A | | | | )|( | | +-+----+ | | | | -' '----|--+ | --- | --- === | | --- _|__ --- GND | | | | | | | +----+---|7905|--+------ V- === |____| GND created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de 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. [...] 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. 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...
Regards,
Dave

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