Reduce output Voltage of Power Supply

[Robertc65]

I have a bunch of old Compaq Laptop power supplies that output 18.5V at 2.7 Amps. I would like to be able to use these power supplies to power items that require a lower voltage. For example I have a small camera that requires 12Volts @ 200ma. Is there a simple way to (maybe a resistor across the output of the supply) reduce the output voltage from 18V to 12V. I'm sure in time I will come up with other uses for these power supplies that may require other output voltages say 5V or 3.6V.

Thanks in advance

 

[Lou1]

Hi Robert

I think it is perfectly possible to reduce the ouput voltage of these supplies to 12V, but you won't succeed by overloading them with a bleeding resistor. What you have to do is to tamper with the voltage feedback servo loop. I don't know specifically these supplies, but you should find the feedback circuit on the secondary side, probably feeding an optocoupler linking the secondary to the primary. The regulating circuit takes a sample of the output voltage with a resistive divider; try to locate it and reduce the value of the resistor going to the supply output (not the one to the ground). This should reduce the voltage. Make a first try by lowering it of 20%; this won't destroy anything and will let you know if you have found the right spot. If it's OK find the value that gives you 12V. If there's no change, try somewhere else. Maybe there is already a trimmer on this spot.
All this will work for 12V but probably not for much lower voltages; you'll be too far off the design values.
Lou

 

[Robertc65]

I understand what you are saying. The switching supply will try to maintain the 18V output depending on the load. What if I were to connect two say 1K resitors in series across the output leads then connect the + side of the camera to the junction of the two resistors and the - side to GND. Would the camera then see 9Volts?

 

[Kevin Weddle]

Yeah you can do that. It is a shameful waste of power but to get good regulation, set the resistors to 10 ohm. The only problem with that is the voltage is not now regulated. But you could add a regulator to the mix also. Unfortunately though you will have a lot of ripple still in the input. You can even use a capacitor on the input to reduce the ripple. You might find that the amount of ripple you get from this configuration is unacceptable.

 

[ante1]

Hi Robert,

Everything that Lou said makes perfectly sense. But if you are not sure what you are doing, please be careful. A PSU like this has capacitors inside which has several hundred volts perhaps for minutes after you have disconnected it! Can you try to get schematics for it and we will try to help you, even a close-up photo of the circuit and one from the PCB solder side might help!

 

[Robertc65]

Thanks to the both of you for the detailed response. Doing this on the outside of the supply just doesn't sound like a wise plan. I'll see if I can dig up a schematic. I didn't realize that a switching supply was so sensitive. Taking this Compaq supply put of the mix, would the voltage divider circuit work on the output of a simple non-switching supply using say an LM7812. Could I reduce the 12V output of this circuit to 6V using two of the same vaule resitors (1K or so) in series across the output. I'm just trying to understand because I have three shoe boxes full of assorted non-switching, basically wall wart type supplies I would like to use for various projects. Unfortunatly most of the are 12 Volts and higher.

 

[audioguru2]

Hi Robert,
I just bought a bunch of 5V/2A wall-wart switching supplies for only 2 bucks each! Their label says, "Output: 3.0-36VDC. Preset for 5V".
So they probably have a voltage control inside. Maybe your supplies have a voltage control too.

Forget about a resistor voltage divider. Ohm's Law says that a 1K resistor will pass a maximum current of only 12mA from a 12V source and with that much current, the ouput voltage from the resistor is reduced to zero.

You could use an LM317 variable voltage regulator IC that will get very hot with a high load current plus a high differential voltage between its input and output.

 

[ante1]

Hi Audioguru,

Nice bargain 3 to 36VDC switcher for 2 bucks! I wonder if you can get 2A over the entire range though? If you can, congratulations to a very nice PSU! Normally switcher like these for laptops and such do not have that big range maybe 14 to 21 volts or so but with a schematic sometimes you can see some possibilities. I have changed the secondary winding on some switchers with perfect result even though some other pieces also had to be changed. In some rare cases I have encountered a switcher that totally lost regulation when unloaded but they have been

 

[audioguru2]

Hi Ante,
My bargain switchers are tiny little things that are rated at only
15W output. No air vents.
I'm going to hack one apart to see where to drill a hole on the others for access to the voltage-adjust trimpot.
I measured one and it has a 5.1V output without a load, and exactly 5.0V at 1.85A (2.7 ohms load). Pretty good load regulation. It got warm, not hot.
I can't remember whose name was on the crate, Philips or another big company. Made in Thailand.
They have a good output cord and connector, nice ratings label and are approved everywhere, have a long model number and says, "I.T.E. Power Supply". Is that a manufacturer?

I got some more 5.5V switching wall-warts that have nice folding mains prongs, from my daughter's old Cannon printers.
When the printers ran out of ink, she bought new printers that were cheaper with all the rebate coupons.

I just took the vibrator motor out of her old cell phone that got run over by a few hundred cars. I was wondering what it was, wrapped in a blue rubber sleeve. I have never seen such a small electric motor, less than 1cm long by 3mm diameter. The unbalancing weight it swings around is half its size. It goes crazy with 1.5V, I can hardly hold it. If I can get the weight off and it doesn't smoke too much with 3V, I'll make a tiny lithium coin cell powered indoor helicopter. ;D

 

[Robertc65]

Thanks again for the replies. Based on the advise given I'll shelf the voltage divider idea. If I find a schematic for the Compaq supply I'll post it.

Thanks

Rob