Author Topic: How to identify input, output of DC-DC converter--blind?  (Read 508 times)

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Offline iXodTopic starter

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How to identify input, output of DC-DC converter--blind?
« on: July 24, 2020, 07:47:05 pm »
I've got an old car DC-DC converter for a laptop, 12v in, 24 out. I've long since cut off the cables and was going to recycle it but now could use it.

The PCB is well-potted inside an extruded aluminum "tunnel" so access isn't practical. The cable stubs are identical so I can't identify which had a cigar-lighter (remember those?) plug and which had the barrel connector.

Using the diode test measurement (and swapping DMM probe polarity) on these two 2-wire cables, I get:
* 0.8v / 0.5v
* 0.6v / increasing to infinity

Is the one with 0.6v / infinity measuring the output rectifiers and filter caps?

How can I verify which is which?

Thanks.
- - -
EDIT: "Just buy another" isn't an option. This is a project, a challenge. I have this and want to use it.
« Last Edit: July 24, 2020, 11:35:17 pm by iXod »
 

Offline Pawelr98

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Re: How to identify input, output of DC-DC converter--blind?
« Reply #1 on: July 25, 2020, 02:36:43 am »
It's a boost converter so you can just try to apply a current limited voltage source on either side.

If you connect it to output then it should just charge the output cap, nothing more.
When connected to input it should just start up.

It would be that simple, provided you know which one is positive and negative.
Which is simple to check. Negative is common so it should measure as a short when probed between input and output.

If you look at the schematic of a boost converter with a mosfet switch then it becomes fairly obvious, how should the DMM react.
Output should just go to high resistance over time as the output diode is blocking current from flowing back.
On input it will be mosfet diode drop at reverse bias and in other direction it's an unknown really, depending on the circuit itself.
 

Offline iXodTopic starter

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Re: How to identify input, output of DC-DC converter--blind?
« Reply #2 on: July 25, 2020, 05:00:12 pm »
Thank you Pawelr, that worked.

Using current-limited PS, connected first to what turned out to be the output and got a tripped current limit immediately. Reversed polarity and ran up to 12v with no indication, and no voltage appearing on the other pair of wires (which turned out to be he input).

Applied the 12v to the input and magically(!) I have 24v on the output.

Thanks again for helping think out this solution.
 


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