creating a power supply

[cvjoy]

I want to create a power supply since off the shelf ones are real expensive
3 or 5 amps
0-30v
AC(230) to DC(0-30v)
it should not be fixed as said above it should be selectable
sufficient for projects relating to robotics and electronics.

if IC can be used then fine but please do gimme a schematic if possible.

 

[Rbeckett]

Need a little more info. such as polarity and a little better description of intended use to really nail down a specific circuit. Mr Bill has a bunch of PS drawings if you will do a little seaech on the site. Expense is relative, how much do you want to spend? 3 amps will be cheaper than 5, but do you really need 3 amp or would one amp on more than one output serve you better. Jus a few thoughts to help us help you better. I built an Elenco with the same specs with multiple outlets and positive and negative polarity for about 50 USD. For my purposes that was very cheap, but for yours it could be too cheap or too expensive, that sorta depends on you and your intended purpose. Hope this helps.
Bob

 

[MikeMl]

There are hundreds of schematics out there on the net. There a couple on the "Circuits" section of this web site.

Building one is totally dependent on finding the right transformer. Look at a few schematics to see what transformers they specify. Then start looking for the availability of suitable transformers.

Another consideration is cooling. If you start with a transformer-rectifier-big filter capacitor, followed by a linear regulator, to get 3A at 1V out, you will need to get rid of (35-1)*3 =~100W of heat. This requires a huge heatsink, and maybe even a fan.

This is why most new supplies of these specs are switchers, not linear supplies.

 

[cvjoy]

I am ready to for the heating part it will be outside and so installing any fan is not much of a problem.

i searched the net but the only thing i got was like 3-5V or 8-11V or ... something of a limited range using ic's and selection of a transformer accordingly.. etc..
eg. see the link **broken link removed**

what i want is voltage selection in range of 0-30v and enough Amp for robotics (so 1A-3A would be sufficient) [variable voltage power supply] but less complicated.

i have a 5A transformer in a power supply which had a analog stick which read 1.5V,3.2V,4.6..... that stick i suppose is a potentiometer connected to a bridgerectifier with a capctr. in center then 2 leads from it plus and minus.

that powersupply mysteriously has stopped working. :-(

 

[cvjoy]

why can't i see my posts here
but i can see it elsewhere
is it that 2nd and 3rd posts don't have value?

 

[Mr RB]

All 5 posts look to be complete to me.

You can easily make a regulated 0-30v 0-3A PSU by using a LM317 and a large pass transistor (on a big heatsink). And just a couple of resistors and caps and a pot. The circuit is usually shown in the LM317 datasheet.

 

[4pyros]

if IC can be used then fine but please do gimme a schematic if possible.

You will not find that on a IC.

 

[cvjoy]

that post took a real lot of time to get printed here

Can u help me bulid it by attaching a correct/tested schematic/ckt diag. and any precautions if any.
I am no expert like you i need some amount of spoonfeeding (hehehehhehhe )

the datasheet i hav has small parts of ckt's so implementing them in a design needs some expert advice.
hope Mr. RB could provide for some.

 

[Mr RB]

Post the LM317 circuit you got from the datasheet (that has the extra transistor) and people can help you with it.

 

[cvjoy]

here is how my old power supply looks like
attached img: mobile uploads so might lack bit of clarity

 

[MikeMl]

It looks like you have a transformer with multiple taps on the secondary, which is a very valuable starting point for a regulated test bench supply. It is very useful to be able to select a tap to control the unregulated voltage upstream of an IC regulator. This will keep the power dissipation in the IC regulator to a more reasonable level compared to always starting with an unregulated voltage of ~35V. The tap should be selected so that at the min point in the ripple across the filter capacitor, the voltage into the IC regulator is just above the supply output voltage + the dropout voltage of the IC regulator (typically ~2.2V). The downside of this arrangement is that to set the supply to some specific output voltage, you will have to manipulate two knobs, rather than one.

 

[ronv]

There are always 3 problems with these power supplies.

1- It is much simpler to make it work from 2 volts to 30 volts instad of 0 to 30. Do you really need 0?

2- To get 30 volts you need a transformer of about 36 volts or so. This is not to common. 24 volts is more common but then the maximum output will be only 21 - 22 volts. You can get a 24 volt 2 amp transformer in the US for about $12. For a 36 volt 4 amps about $42. Yours has enough current but not enough voltage to meet your spec.

3- To drop 28 volts at 3 amps is a lot of heat. Maybe requiring several pass transistors and a huge heatsink

 

[cvjoy]

That power supply in the given image
for your kind info is not at all and in any way connected to a IC or transistor

only to a pot, a bridge rectifier(4 diodes and a cap), then taken out directly
there is a link in one of my above posts which shows a ckt. diag and schematic i am currently trying to follow that design .

also image 1 will give you approx. idea about the available voltages.
That is the one misbehaving powersupply i talked about earlier(wires are intact).