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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.
Kensington 33197 User Manual
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.
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.
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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 v command inputs. My favorite is the model watt manhal 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: One caveat is they will not run at the full watt 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. 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 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. 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.
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I used turn pots to allow fine adjustment. The 5-pin connector on the schematic is to show the conductor colors in the cable. It does kensinton represent the pin locations on the power supply.
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 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. 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 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.
The Impact of Pullup Resistors Nice, I always wondered if it was the tips that were adjusting “stuff. At the time I only needed one. I’ll map the connector pinout when I have a chance. Just for the record, these supplies are available on-line at reasonable prices. I didn’t mention that this supply also has a 12V DC input, so you could have a variable lab supply in your car.
A diagram showing the pinout of the connector has been added to the article. I took a look at performance and I have to say this is pretty sweet. First, I looked at output voltage vs command voltage. The output voltage equals 5 times the input voltage. For a 5 volt output, supply the command input with 1 volt. For a 12 volt output, supply 2. The relationship is extremely linear with good accuracy.
Kensington 33197 Manuals & User Guides
The measurements shown were made with a load of about mA. This shows that good performance can be achieved driving the supply with a DAC signal from a PIC even if it is open loop no feedback is made of the voltage output.
Next I made a quick measurement of output voltage vs load. Interestingly, the output voltage increases with load. Not awesome performance but not bad considering the cost. Using one of these Kensington supplies as a bench power supply is definitely viable.
Current limiting is not found in cheaper supplies and the possibility of relatively simple computer control beats supplies costing several hundred dollars. Pardon my ineptness, but why do you think this is? I wonder if using some type of closed loop could get you even closer, by using a microcontroller. I do have a couple Microchip 10k digital pots