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# Thread: AXBB charge pump fault

1. Ok, bear with me as I'm still learning...

The voltage across the resistor was 0v when off and 4.65v when on

With the relay and resistor connected the voltage was about 1v when on and 14.7 when off (between op1 and -24v)

With the relay disconnected it was 0v both on and off

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2. "The voltage across the resistor was 0v when off and 4.65v when on "

is what I expect between the FET gate & source
when the CHP 01 is switched off and on

"With the relay and resistor connected the voltage was about 1v when on and 14.7 when off (between op1 and -24v)"

implies the FET is leaky and not switching off completely

when working correctly

the FET's ON resistance should be 155 miliohms

as the relay coil resistance is 650 ohms

at 24V DC
the relay coil and 27 ohm resistance should pass around 35.5 mA

the volt drop across the FET when switched on will be

0.155 ohm x 0.0355 A = 0.0055 V = virtually zero on a basic multimeter

either the FET is not switched hard on so the on resistance is higher
or your measurement includes the volt drop across your wiring and contact resistance of the connections

"14.7 when off (between op1 and -24v)"

when the FET is off no current should flow through the 27 ohm resistor and 650 ohm relay coil !

to measure 14.7V at the output
24- 14.7 must be dropped across the total 677 ohms of the relay coil and resistor

the leakage current must be 9.3/677 =0.137 A

so instead of the FET looking like its open circuit when off
its more like a 107 ohm resistor !!

"With the relay disconnected it was 0v both on and off"

is correct
the FET acts as a switch between the -24V/common
and the CHP O1

without the relay being connected
there is nothing between the +24V and CHP O1 to pull the output up to the +24V supply

John

3. So you reckon the fet is borked again? I have stripped the control box back again but cant see any shorts and I'm remaking the control panel, mainly because the wire was too short but also I don't like the layout and realised it would be handy to have jog buttons for the axis as well as the hand held MPG. It also seems the perfect opportunity to start again and eliminate any issues although I've looked and can see no potential for shorts? I'm also going to add a slave axis to the machine. I am happy with it but I can see improvements and want to do it before the machine is working proper.
So back to the Fet...my mpg is wired up with a db15 connector, I wanted to extend the wire longer so tried a male to female db15(monitor) extension cable, it didn't work and the usb controller wasn't recognised in Windows until I removed it. That is the only thing that I can think went wrong....

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4. The SVGA extension cable - there's a couple of no-connection lines in there - perhaps if those aren't wired-through it explains the inability to work. A straight-through 15-w D-type M-F should work without too much drama. Not sure what you mean by the USB controller - unless this is a Windows->pendant interface?, in which case, again, the lack of cabling may possibly explain that (not sure).

The FET behaviour, as described, is borked.

5. Sorry...i should of explained better. The pendant was one of these...
https://rover.ebay.com/rover/0/0/0?m...2F163108997386

The mpg is wired to inputs on the AXBB but the axis and jograte selectors are wired to a usb controller due to the lack of inputs on the AXBB. The svga wire was advertised as fully wired so I assumed that meant each pin was individually wired but I don't think it Is!

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Last edited by CharlieRam; 18-11-2019 at 07:40 PM.

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