AXBB charge pump fault

[john swift]

Hi Doddy

I see you changed your mind
dont worry
your original reply just pointed out we use the technique that we are happy with

I have to admit unless I am under instructions to maintain a lead free status
I use 60/40 solder and add some rework flux

If lead tin solder is good enough for use in pacemakers and other mission-critical equipment
its good enough for me

John

[Doddy]

If lead tin solder is good enough for use in pacemakers and other mission-critical equipment
its good enough for me

John

Similarly, gifted with an industry that embraces the old 60/40.

Right, just tried a quick experiment - I have some 2N7002's in the shed so hooked one up to a signal generator set to 1Hz (though I ramped up to 35Hz out of boredom).


Basic circuit...



...using a practice board for SMDs as a breadboard...



So, this ran for 5 minutes without fail.

I then cut the diode (observed in image) to see if the back EMF would pop the transistor. I ran that for 5 minutes at upto 35Hz, with no failure.

Out of interest, I wet my fingers and placed across the relay coil - this calibrated method of measuring EMF confirmed that there was OUCH! volts of back-emf. Recovering the diode did quench the back emf, as expected.

I had expected the 2n7002 to fail due to the excessive VDS, but for the duration that I ran this it did not. The OUCH! test did confirm the expected high EMF present on the drain of the transistor.

So, CharlieRAM - you asked what could cause this to happen again... although inconclusive I'd recommend placing a diode, e.g. 1n4004 across the relay coil, cathode to +24V, anode to the AXBB board output. If you want a couple of diodes and a couple of 2n7002s thrown into the post let me know.

I'd recommend this for any output driving a relay or other inductive load.