Kensington Power Supply Hacks

hack-job-200Kensington 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.
Cheers
Dave

70W-Power-Supply-2000

 

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.

schematic

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.

hack-job-2000

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.

pinout


Posted: 5 years 3 months ago by MMcLaren #10428
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Nice find... Thank you Jon...
Posted: 5 years 3 months ago by Jon G #10429
 Jon G's Avatar
Nice, I always wondered if it was the tips that were adjusting "stuff." When Circuit City went out of business they had a huge "sale" and they had hundreds of these things for $12 each. At the time I only needed one. Stupid me.
Posted: 5 years 3 months ago by Jon Chandler #10430
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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.

I'll map the connector pinout when I have a chance.
Posted: 5 years 3 months ago by Jon Chandler #10432
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Just for the record, these supplies are available on-line at reasonable prices. Similar 70 watt supplies are listed for $15 at several sources.

I didn't mention that this supply also has a 12V DC input, so you could have a variable lab supply in your car.
Posted: 5 years 3 months ago by Jon Chandler #10433
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A diagram showing the pinout of the connector has been added to the article.
Posted: 5 years 3 months ago by Jon Chandler #10434
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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 2.4 volts. The relationship is extremely linear with good accuracy. The measurements shown were made with a load of about 470 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. I adjusted for a 5v output at ~470 mA load, and then measured the voltage over the load range. Interestingly, the output voltage increases with load. At a load of ~2.5 amps, the output voltage increased to 5.8 volts. 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.
Posted: 5 years 3 months ago by Jon G #10435
 Jon G's Avatar
Interestingly, the output voltage increases with load.

Pardon my ineptness, but why do you think this is? Pretty awesome otherwise
Posted: 5 years 3 months ago by Jon Chandler #10436
Jon Chandler's Avatar
... Pardon my ineptness, but why do you think this is? ...

Hmmm... thanks for asking the question. It's entirely reasonable to ask.

When I did the above load test, the control voltage was coming from another power supply. I fed the Kensington 1.00 volts to get 5 volts at the output. As the load increased, the voltage increased which I didn't expect.

Then the probable answer came to me. As the load increases, the nominal 5v source of the Kensington supply probably decreases. Instead of feeding the command input 20% of the supply's reference voltage, I was feeding it a fixed level. As the Kensington's reference level decreased with load, the fixed voltage was a larger percentage of the range, resulting in increased output voltage.

To test out the theory, I went back to my first setup shown in the schematic and blown up in the view below. A voltage divider (a pot in this case) will always provide a ratiometric output - the output voltage is some fixed fraction of the input.



I repeated the load test with the pot controlling the voltage the pot is connected to the Kensington's voltage reference output). Again, I adjusted for 5.0 volts at ~470 mA load.



Much better results and more what I had expected with the output voltage decreasing with load. The change across the range is less than 0.2 volts.

To automate control of the voltage output and current limit, a digital pot referenced to the Kensington's reference voltage would provide a better means than using some type of DAC to feed it a fixed voltage level. I don' know how much current may be drawn from the reference voltage pin. It may be possible to power the PIC circuit from it, in which case the PWM-controlled voltage would be ratiometic to the Kensington.
Posted: 5 years 3 months ago by Jon G #7300
 Jon G's Avatar
Very interesting... 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... Maybe you‘d like one? It might even make it there before the weekend. Let me know... Having something like this would be pretty handy, especially if you do all the hard work. :p
Posted: 5 years 3 months ago by majenko #7630
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I didn't mention that this supply also has a 12V DC input, so you could have a variable lab supply in your car.

Nice, I have a bunch of SLA batteries under my desk I use to run my systems when we have a power cut (and also when I need a nice high current clean supply for playing with amplifier circuits and such) and this would make an absolutely perfect BTPSU to run off them. Do you know if these are available in the UK?

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