View Full Version : Review: Oatley Electronics DC-DC converter

18th August 2005, 10:10 AM
I've always run my old Toshiba laptop straight off 12v from the car. I think the power supply is meant to be close to 14v, but it works anyway. A couple of people have asked about ways to run a laptop with higher voltage requirements from a car. After promising Craigus I'd sort something out for his Thinkpad, and then getting a thinkpad myself a few days later - I decided it was time to look into it.

After a bit of looking around, I settled on the Oatley Electronics Kit # 168 - DC to DC converter kit. (http://http://www.oatleyelectronics.com/kits/k168.html) @ $22.50.

This converter will run from about 4 to 24v input, and will supply more than enough current to run any laptop. When building the kit, you determine the output voltage by the number of turns on the transformer and by the values of 2 feedback resistors.

The thinkpads require 16v, so after deciphering the instructions I settled on 2x10 primary turns and 17 secondary turns on the transformer, and 15k ohm total for the feedback resistors.

The kit took me about 30 minutes to build, plus about 15 minutes to wind the transformer. The transformer only took so long because the instructions are a bit vague. The kit instructions (http://www.oatleyelectronics.com/kits/Notes/k168.jpg) don't quite explain process as clearly as they could, but all the details given are correct.

Good points:

Very easy to assemble
Wide range of input and output voltages
Capable of delivering up to 4 amps
very stable voltage output

Now the bad points:

Holes for diodes and terminal block need enlarging
No mention anywhere of where to connect the input power
Vague instructions

The kit fired up fine first go, with 15.5v output from 12.9v input. I then connected it to a 'flat' battery, and it still delivered 15.5v from 9.5v input.
I think the output is 0.5v lower than I was aiming for because I used a 12k and a 2k feedback resistor instead of a 15k. I'll swap it out some time and see if that bumps the voltage up.

Overall, so far I'd highly recommend this kit, providing you have basic soldering iron skills. I'd rate the assembly difficulty as 1.5/5 (easy)

18th August 2005, 11:11 AM


this sure beats buying an exy targus style thing and apparently can pump 50w easy and up to 100w with heatsinking... wow

18th August 2005, 12:59 PM
good find festy. nice and cheap.

This might be a good cheap 12V rail for people running carPC's that need a solid 12V supply from the input rail that can be anywhere from 8V during starting to 15V engine running?

Will it operate in this mode? If i configure one for 12V output will it operate as a true buck-boost converter?

Might have to get myself one to test. My laptop is sick of its inverter anyways ;)

18th August 2005, 01:27 PM
just ordered 2 kits. after some more reading it looks like good gear.

should be able to bin my ****ty inverter in no time ;)

18th August 2005, 01:37 PM
The key bits of the instructions that could be improved are:
- Transformer - primary winding is 2x10 turns, secondary winding is as per the graph (ie 17 for 16v)
- feedback resistor - RA and RB are in series, so RA+RB = resistance from the graph
- 12v input is not labeled on the PCB - there's 2 extra holes near the transformer, it's pretty obvious which is posative once you look at the circuit.

vincentvega, I doubt you'd need the above advice, but it would have saved me a bit of head scratching...

18th August 2005, 02:52 PM
All advice graciously accepted festy. Thanks mate. Should make assembly nice and straight forward.

I was actually wondering how many turns to use on the primary. Where did you get 10 from?

18th August 2005, 03:01 PM
The printed instructions I got were slightly different to the on-line ones. Not much different, but there was an example table of windings/feedback resistor values from memory. Would have been even more head scratching without that :)

31st August 2005, 12:53 PM
Just finished building my converter. Winding the transformer is a pain in the ***. I dont know if it was supposed to be included or not but i didnt get a bobbin to wind the coils on, which made things a little awkward. In the end I found that finding the right size pens to pre-wind on is the key.

Apart from that though im very happy with it. For people with more conventional laptops:

Target voltage: 18.5V
Primary windings: 10
Secondary winding: 21
Feedback resistors: 16K + 2.7K = 18.7K

No load output voltage = 19.26V
seems to happily regulate for input voltage from 10V to 24V

As festy found a few things are annoying:
The holes for the diodes, screw terminal and mosfets need to be drilled out.
The pin spacing of the mosfets is wrong
The instructions leave alot of info out.. (festy's instructions cover most of the "tricks")

All in all though it's a ****** good regulator for 22 bucks. Next job is to fold up an aluminium case for it which will double as the heatsink. Should be able to pull 3 or 4 amps out of it no problems then.

2nd September 2005, 10:34 AM

Might sound like a silly question but the power point provides AC and the car provided DC. Does this mean that on the board you need to polarity correct or does the converter cover the change from DC to appears AC, or does the PSU in the computer convert the AC to DC?

More thinking about installing a carputer rather than a laptop.


2nd September 2005, 10:45 AM
Try reading that back to yourself. Im totally confused...

In your house:
Power point is 240V AC
Computer power supply converts 240V AC to a variety of DC voltages that the computer needs to run.

In the car:
Car battery is around 12V DC
Converter takes 12V DC in and converts it to 19V DC that the laptop requires.

Does that answer your question?

Powering a PC in your car is alot more involved than powering a laptop. I would recommend you use an old laptop for your first carPC until you learn your way around a bit...

have a look at the forums on www.mp3car.com theres a whole section of FAQ's dedicated to people like yourself who are new to the game.

2nd September 2005, 11:58 AM

Sorry if it was a little confusing, I should have put in the house its AC and in the car its DC. I wasn't sure what the computer "runs" on, but you have clarified the situation.

I asked this as AC the electrons flow back and forth, and DC the flow like in a hose. IMHO that why plugs for some AC powered radios for eg they dont have a set way to plug in at the back of the radio, but then again a computer does...

Looking at the PSU on an old computer at home it states input 230V at 50Hz (ignoring the 110 for america) ie AC. Output for the 3 rails 12V, 5V and 3.3 volts but no mention of the AC/DC.

Yes I could run an inverter to convert the 12V DC (Which is more like 14.4 when the car is actually running and charging the battery) then allow the computer to convert that back to the 12V, 5V and 3.3V rails. To me this seems redundant if I can run 3 of these kits, one for each rail.

We currently use a nice lap top to run OZI. I would rather run a basic machine with a touch screen LCD instead. Hidden away nice and safe. I can get hold of an old PIV micro for this and prevent damage to our laptop.


I hope this was a little clearer.

2nd September 2005, 08:31 PM
For the price of 3 kits, you'd probably be better off buying a proper DC-DC ATX power supply. While an Opus one is ~$300, there are much cheaper brands. Talk to Craig, he's just put one is his car.

3rd September 2005, 12:29 PM
$300 thay would make it the most expensive part of the system. :( excluding the touchscreen LCD

I'm watching what Craig is doing with great interest.

19th September 2005, 05:27 PM
$39 from supercheap auto. Checked it out with a cro and not too bad, nasty, but not too bad.

19th September 2005, 05:35 PM
$39 from supercheap auto. Checked it out with a cro and not too bad, nasty, but not too bad.

$39 sounds reasonable!
How many amps? When you checked it with the CRO, was there a lot of ripple on the output?
Would you power YOUR laptop with it?


11th October 2005, 01:15 PM
I finally got around to building a case for my power supply. Instead of folding up the entire case, i decided to just make an aluminium lid for a standard ABS box.

Basically i replaced the plastic lid with an aluminium one to act as a heatsink, and then folded up some small brackets to thermally connect the mosfets to the lid. Due to the design of the circuit, the mosfets have to be isolated from the heatsink using mica insulators and plastic washers. Thermal paste was also used at each joint.




The orange connector is just a 4 way plug in screw terminal that i had lying around at work. I decided to go with a plug in connector so that I can plug in connectors to suit different laptops, and different inputs as needed.

The finished product:


I have a complete set of drawings of the metal parts if anyone is interested.