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Dragonfly:
Thanks Bacon for the clarification. So the situation is you are providing an input voltage and the output voltage is always a slightly lower than the input.  That is probably due to the diode's voltage drop in the boost circuit which leads me to believe the problem could be the Mosfet (the part labeled D4184).  It looks like station240 provided the right advice for troubleshooting.


--- Quote from: station240 on May 09, 2016, 07:04:16 am ---I'd first start by testing the D4184 mosfet.
Measure the voltage on the gate pin, and if your meter has the Hz option, the frequency.
Datasheet for the mosfet here
http://www.aosmd.com/pdfs/datasheet/AOD4184A.pdf

You could also have blown the diode, it's got two diode symbols like this >|
If your multimeter has a diode check function, look up how to use it.

--- End quote ---
Dragonfly:
From what you describe I don't think it is the diode (SD1040CS) but let's eliminate that to be sure. With the board powered OFF and your meter in diode mode, measure the forward voltage drop across the diode by placing the meter's positive probe on one of the small legs (either of the outside ones, not the short stubby pin in the middle) and the meter's negative (COM) probe on the large flat part near the output connector.  It should read something like 0.3V or less.  Then reverse the leads of your meter (positive probe on the fat part and the negative probe on one of the short legs).  You should meter should show OL or something similar to indicate an open circuit.

If that passes, then check the frequency on the gate of the Mosfet again.  To do that power up the boost board, put your meter in frequency mode, connect the negative probe to the "Input -" of the module (which should be the circuit's common ground) and put your positive lead on the Mosfet's gate pin.  Do you read a frequency?
Marco:
His meter might not be able to count the gate signal, I think it's ~500 kHz and the meter tops out at 400.
station240:
I've got a DC-DC converter here with the same D4184A mosfet, which blew due to bad design.
Also if you have the same 11.8V on the Gate and Source pins, the mosfet is blown.
The gate is an isolated floating input, clearly it's gone short circuit.

The switching action of the mosfet is what makes a Boost converter live up to it's name.
See this diagram.

paulhm81:
Hello

First of all take a look at Dave's video so you can understand how a boost converter works!

and this one


You can test the diode with you meter. Put it in diode mode and check between pins 1-2 and 3-2 both ways. Put the red probe on 1 and black on 2 and the other way around. It shoud show a voltage drop one way and nothing the other way. If it shows you a voltage drop both ways then it's bad or if it doesn't show a voltage drop in any direction it's bad again. You should really check the mosfet as it's the most probable part to have blown. If you replace the mosfet it should work if the IC that controls the gate of the mosfet isn't blown aswell. The LTC1871 (if it's what you have) is the brains of this board. It turns the mosfet on and off to step up the voltage. If the mosfet is shorted, it will deliver the input directly to the output minus the drop on the diode that you removed witch would have a voltage drop of around 200mV.

It's clear that your mosfet is not working but if it's the mosfet or the ltc1871 or both is unclear. 

Take a look at this to check the mosfet and find it's datasheet to know what the pins are!

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