- Published: Monday, 23 April 2012
- Written by Jon Chandler
Kensington has produced several models of "Universal Power Supplies" for use with laptop computers and various other pieces of gear. These power supplies use a special tip which controls the voltage and current for the connected device. It may surprise you that one of these power supplies can make an excellent bench power supply, with voltage adjustable from 0 - 24 volts at up to 6 amps. Additionally, the current can be limited, either for safety (so a short doesn't blow up your widget) or to use as a constant current supply (no matter what the load, the current will remain constant). Constant current supplies are perfect for charging batteries or characterising LEDs.
I found one of these supplies at the local thrift store for $6 and couldn't pass it up. Since the supply is adjustable and the voltage and current are controlled by the small tip adapters, I knew the process couldn't be too difficult and I guessed it would be controlled by resistors in the tip.
I got lucky. A little time with Google found exactly the information I needed on the dorkbotpdx forum. Thanks goes out to Dave, who wrote:
I've been using a Kensington universal power adapter as a bench power supply for some time. Kensington makes several model that are very easy to hack. You can control output voltage and current with simple 0-5v command inputs.
My favorite is the model 33197 120watt adapter:
Inputs: 120VAC, 240VAC or 12-16VDC!
Output: 0-24VDC at 0-6.5A
I've tried this on several models. The hack is the same for all as long as they use the same replaceable tips normally used to adapt to different computers. The tip normally determines the output voltage and current limit.
If you cut off the output cable you will find these 5 conductors:
White = 5v reference voltage (output)
Blue = voltage output adjust (0-5Vin = 0-24Vout)
Red = current limit adjust (0-5Vin = 0-6.5Aout)
Yellow = power output
Shield = Ground
One caveat is they will not run at the full 120watt for long before they start shutting down from over heating. That problem can dealt with using a heat sink and fan or just run at less power.
Models tested: 33197, 33196, 38004, 33234 (70W), 33194 (70W)
You can sometimes get one cheap on ebay. I hope this will be useful to
you for your project.
So the voltage and current are controlled by 0-5 volt signals on two lines. Conveniently, 5v is also available on the connector. The easiest way to control the power supply would be to use a couple pots configured as voltage dividers.
Simple! the schematic above shows the needed connections. The values of the pots aren't critical, since they are being used as voltage dividers. I used two ten-turn pots I had to test this out, one of 4.7kΩ, the other 10kΩ. Anything between 1k & 10kΩ should work great as long as they are linear taper. I used 10-turn pots to allow fine adjustment.
Note: The 5-pin connector on the schematic is to show the conductor colors in the cable. It does not represent the pin locations on the power supply. See illustration below.
This was truly a hack job just to test out the theory. I'm happy to say the supply works as expected. Adjustable over a 0 - 24 volt range. When current exceeds the current limit setting, the current remains constant with the voltage changing as needed to maintain the current. Not bad for six bucks and a couple pots!
The first enhancement I could make would be to put it in an enclosure with my TAP-20 current monitor. Voltage and current displayed, over a 0 - 24 volt range at up to 6 amps. A typical bench supply only puts out 3 amps maximum and would cost a couple hundred dollars.
The next enhancement would be to control the voltage and current limits with a micro. Graham's tutorial shows how to generate 0 - 5 volts using PWM and a filter. A USB-enabled PIC and you'd be set. A computer-controllable supply!
Kensington supplies are available in a number of models. Dave lists some compatible models above. Any that use the same type adapter tip should operate the same way.
Igo makes some similar DC converters. I suspect they work along the same lines but I haven't tried it out...yet.
The illustration below shows the pinout looking into the connector on the power supply if you're not so lucky to have a cable with it.