Laptop power supply

[oxfordjon]

Hi everybody, hope someone can help with this.

I have a old Toshiba laptop power supply that I want to use for a couple of circuits rated at 12v DC 250ma, the markings on the PSU are as follows - input 100v to 240v AC, then under that it says 0.7A - 0.4A.
Then underneath that it says output 15V - 2A, my question is as the supply is already smoothed to DC 15V can I simply take the DC output through a fixed 12V regulator without having to use any other components and is the total load I can actualy draw 2A ?, by the way I am on the UK so will be using 240V.

Thanks in anticipation

 

[Russlk]

It will work if you use a low dropout (LDO) regulator and use the recommended input and output capacitors. The max current will be 2 amps or the rating of the regulator, whichever is less.

 

[oxfordjon]

Thank you

Thank you very much for your reply, as i am a complete numbskull when it comes to electronics could you give me a little more help ?
I can find LDO regulators on my suppliers web site (www.maplin.co.uk)
the1.5A one would suffice but none are stipulated as being 12V out and do not recommend what capacitors to use, if they did I wouldn't even know how to connect them up !
If you could find the time to check out the Maplin web site could you recommend which LDO VR to use, what capacitors i need and how to hook it up.

your kind help would be very much appreciated.

Regards
John

 

[Russlk]

The L200CV from Maplin will do the job. In the schematic below, the capacitors are the minimum value, you can use larger.

 

[john1]

Hi oxfordjon,

Please bear in mind that a lot of these 'in-line' supplies are not
isolated from the mains.
Some of them actually have their output connections connected to
the electric mains.

When being used for their intended purpose, this may not matter.

But if you use them for something else that they were not made for
then this becomes entirely down to you.

Try to find out if it is an isolated type or not.

John

 

[JimB]

Please bear in mind that a lot of these 'in-line' supplies are not
isolated from the mains.
Some of them actually have their output connections connected to
the electric mains.

I hope not!
We are talking about a supply for a laptop computer here.
The low voltage connector probably has exposed pins.
The laptop will have external connectors, video, RS232, USB etc etc.

If the low voltage supply for the laptop is connected to the mains, the manufacturer is leaving himself open to all kinds of legal problems.

If you have firm knowledge of a PSU which does this, please share it with us, we want to know.


However, having said that, I have a couple of Gateway laptops and if you measure from the DC output connector to earth with a high impedance voltmeter, it reads 120volts. This is due to capacitive coupling between the primary and secondary of the transformer in the PSU. There is no earth connection to the PSU and so there is no earthed faraday screen between primary and secondary.
The 120volts has a high impedance source and is quite safe, at least to a human. Electronic components, CMOS especially, may not agree with that!

If I do the same measurement on my Dell laptop PSUs, I get 0volts. The thing is built properly and is earthed.

JimB

 

[Klaus]

The L200CV from Maplin will do the job. In the schematic below, the capacitors are the minimum value, you can use larger.

Another example of the impracticability of electronic CAD. The resistors you showed are rather odd values.
If I were to make this then I would simply use a 200 Ohm trimpot and connect the wiper to the Vref pin. Adjust pot until the output voltage is what's required.
Klaus

 

[pwillard]

I've seen plenty of laptop power supply "guts" so I can say with reasonable certainty that they are GENERALLY a switching-mode, step down regulator with decent DC smoothing.

I've had decent success with using a generic 3 terminal regulator following the supply in cases like this, since most laptop supplies provide 15-24 volts. I do agree that 15v is a little close to the desired regualted output you need, but try the cheaper version first versus sourcing the LDO you might not already have.

These are usually HIGHER-Current supplies (1.5 amps or more) which makes them slightly more complex than your average wall-wart.

I've seen those shoddy 3 component line-powered supplies and I've only ever seen them used on Cell Phone supplies. (very low current)

Pete