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  1. #51
    Doddy

    something like cnc4pc's C4 charge pump I made reference to in post 12

    John

  2. #52
    Guess what...its gone and done it again I don't understand what is causing it! I had the 27ohm resistor directly on the charge pump output then through two e-stop switches and onto to 24v- on the relay. The relay 24+ goes direct to the 24v output from the PSU . I also have the diode across the coil of the relay

    It is behaving slightly different this time though in that the relay doesn't switch on until the charge pump is active. The problem comes when the reset or estop is tripped. The charge pump led switches off but the relay stays active ( the relay led stays lit but dimly when the charge pump output should be inactive?)
    I thought It was going too well I had done a couple of trial parts and was working on reducing backlash and and alignment when I noticed the estop didn't cut the power, I am wondering if it's the actual relay causing it

    Sent from my SM-G920F using Tapatalk

  3. #53
    Doddy's Avatar
    Lives in Preston, United Kingdom. Last Activity: 5 Hours Ago Has been a member for 6-7 years. Has a total post count of 805. Received thanks 129 times, giving thanks to others 34 times. Referred 1 members to the community.
    Quote Originally Posted by CharlieRam View Post
    It is behaving slightly different this time though in that the relay doesn't switch on until the charge pump is active. The problem comes when the reset or estop is tripped. The charge pump led switches off but the relay stays active ( the relay led stays lit but dimly when the charge pump output should be inactive?)
    So the LED on the AXBB is off, but the relay remains energised and another LED on the relay is on/dim?

    That suggests the FET is conducting, when it should be off. No disrespect to John but worth a visual around the replacement FET and a wipe down with IPA if there's any flux residue, but I doubt that will resolve anything (the gate is tied to ground via resistor so can't really believe any parasitics will be able to draw the gate high). Worth measuring the voltage on the gate of the FET (report here) and the voltage ACROSS the relay coil when it should be off - my guess is you'll have a few volts there and if the voltage on the gate is essentially zero then, with the circuit as shown, it can only be the FET. Replace again. If it fails again then replace with a bigger bugger. Or haywire the 74HC14 output to the adjacent FET and use that output instead.

  4. #54
    Quote Originally Posted by Doddy View Post
    So the LED on the AXBB is off, but the relay remains energised and another LED on the relay is on/dim?
    Exactly that...the relay has a green status led in its case. It is dim when the charge output is off but brighter whenever the charge output is on. This is different to the last time because that time the output was on constantly.



    Sent from my SM-G920F using Tapatalk

  5. #55
    Hi Charlie

    I am surprised the the charge pump output has failed again

    I would of thought the combination of the diode added across the relay coil to control the back emf
    and the 27 ohm resistor to limit the current if the relay was ever short circuited
    would of protected the FET

    Can you measure the voltage between the CHP O1 and -24V terminal

    when the output should be ON and OFF

    and if possible the voltage across the 10K resistor on the PCB

    knowing the voltage between the 74HC14 GND pin 7 and output from pin 2
    will help

    I would expect it to be 0V when OFF and +3 to +5V when ON

    from the FET data
    Gate Threshold Voltage = min 1.5V to max 2.5 V

    John
    Last edited by john swift; 15-11-2019 at 11:51 PM. Reason: add ref to Gate Threshold Voltage

  6. #56
    Quote Originally Posted by john swift View Post
    Hi Charlie

    I am surprised the the charge pump output has failed again

    I would of thought the combination of the diode added across the relay coil to control the back emf
    and the 27 ohm resistor to limit the current if the relay was ever short circuited
    would of protected the FET

    Can you measure the voltage between the CHP O1 and -24V terminal

    when the output should be ON and OFF

    and if possible the voltage across the 10K resistor on the PCB

    knowing the voltage between the 74HC14 GND pin 7 and output from pin 2
    will help

    I would expect it to be 0V when OFF and +3 to +5V when ON

    from the FET data
    Gate Threshold Voltage = min 1.5V to max 2.5 V

    John
    Sorry for the late reply, so I managed to check a couple of voltages before the battery died on my volunteer again...really should remember to turn it off! It reads 9v when off and 24v when on when the relay is connected. but I had a brainwave and it works so unless you say it's a bad idea I'm going to stick with it...
    The charge pump only activates the relay when the software is active and reset state is inactive, the problem was that it wouldn't then shutoff on reset or estop so I have used an opto isolated relay on another output which is the true state of reset and settable in uccnc. This means that the relay isn't active unless the software is active and reset is not. It also protects the AXBB against any other issue causing the charge pump failure..

    Sent from my SM-G920F using Tapatalk

  7. #57
    Doddy's Avatar
    Lives in Preston, United Kingdom. Last Activity: 5 Hours Ago Has been a member for 6-7 years. Has a total post count of 805. Received thanks 129 times, giving thanks to others 34 times. Referred 1 members to the community.
    EDIT: Ignore below the line below for now, I've re-read your answer and am confused.

    I understand you're measuring the OP1 output with respect to the negative supply to the 24V PSU?

    The OP1 is a low-side switched output. That means that if the relay is ON, the output voltage should be near zero. If the relay is to be OFF, then the OP1 output should rise (through the pull-up by the relay) towards 24V.


    Relay....OP1=Expected.....Reported
    ..ON..........0V..............24V
    .OFF.........24V...............9V


    Can I ask you to confirm the reported values? My thoughts, below, were with ON/OFF voltages swapped.

    ----------------------------------------------
    --- Bit below here was original reply
    -----------------------------------------------

    What you're saying is that you're using the charge-pump as the initial "Computer is now on" signal.

    It doesn't safeguard against the computer going doo-laa-lee after the event, but it avoids a spurious start-up spin-up. Not that I believe you're likely to get that with the AXBB - it's more a concern for random start-up on PPs.

    A couple of things to consider:-

    * You don't have the intended watchdog cut-out protection intended by the charge-pump. Safety is compromised *1
    * You're running a board with a failure evident. You cannot predict at this time how this failure will behave - it might remain, or fail completely. *2
    * The FET is partially conducting. I'm uncertain if the 9V is across the relay coil, or the relay coil plus 27R resistor. I'm going to randomly guess that you include the resistor, and together with a randomly selected relay coil resistance that you have a total of 750R resistance (feels about right and it makes the maths easier). The problem here is that the current flow is voltage-across-relay-when-off (24-9V / 750R) is 20mA holding current on the relay when the output is set to 9V. That means that you're dissipating (P=V*I) 9x0.02 = 180mW in that FET in its failed state. The max power dissipation in a 2n7002 (don't know if that's the device used to replace the original) is 200mW, derated above 25C, so you might find that the FET subsequently fails differently. Or if the assumption about coil resistance (and therefore current, and therefore power) is inaccurate - then adjust the numbers accordingly. You're likely to see the FET fail over.


    *1: But then, most home-grown won't have a charge-pump anyway.
    *2: This output would, at best fail to drive the charge-pump, and the machine will fail-safe, or at worst intermittently fail to drive the charge-pump - random shutdowns. Somewhere in the middle it never deactivates the charge-pump and you lose the charge-pump function - see (*1)


    My thoughts: I don't think that you are getting any benefit above the controlled output that you have from the reset-discrete output. You may end up with the machine intermittently or hard-failing in the case that the watchdog fails. Likely this will never happen mid-cut, but each time you shut the machine/software down you're risking the machine hard-failing. This uncertainty would lead me personally to either :-
    1) Repair the board again in one of several ways previously discussed.
    - or -
    2) Remove the charge-pump from the system.

    EDIT:

    It's still worth performing the voltage measurement advised earlier - the voltage-across-the-10k (what I meant by the gate voltage, Vgs) is intended to understand if the FET itself is at fault. If the Vgs is near 0 you expect the FET to be "off" - Resistance Drain-to-Source (Rds) is very very high. If the Vgs is above Gate Threshold Voltage it should be in the (near) fully "on" state, Rds = very very low. The problem lies that between Vgs=0 and Vgs = 3V then the FET operates in it's linear(-ish) mode, where the Rds is a function of Vgs. By understanding if Vgs (voltage across 10k) is a definite 0/3V then we know if the device driving the FET (74HC14) is operating correctly and the FET has failed. If the voltage is somewhat lower than 3V then the FET might be behaving as expected.
    Last edited by Doddy; 17-11-2019 at 08:03 AM.

  8. #58
    Morning, I will be nipping out soon and I'll have a look for a battery for my voltmeter...its a tiny A23 size and hopefully i'll be able to report back later.

    Just to clarify....
    voltage between 24V- and output one with charge pump active on and off. (Relay and resistor connected or disconnected?)
    And voltage across resistor circled, again with charge pump on and off.
    Click image for larger version. 

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  9. #59
    Doddy's Avatar
    Lives in Preston, United Kingdom. Last Activity: 5 Hours Ago Has been a member for 6-7 years. Has a total post count of 805. Received thanks 129 times, giving thanks to others 34 times. Referred 1 members to the community.
    Sorry, been up to my elbow in a Myford since 9am. Anyway, V(OP1) with Relay / Resistor connected. And, yes, across the resistor marked 01C (one side is ground, the other is the output from the 74HC14), the signal that drives the FET On/Off. I expect that to be either <0.5V / > 4.5V - assuming the local logic (HC14) is supplied at 5V.

  10. #60
    yes
    that's the resistor

    Click image for larger version. 

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    unless the FET has gone low resistance between the gate & source

    the voltage across the 10K resistor should be the same as the voltage across the FET gate & source

    as the static FET gate current is virtually zero ( leakage current 100 nA )
    the volt drop you measure across the 220 ohm will be zero

    you will find the via (plated through hole ) in the copper trace connecting the 10K resistor and FET source is the – 24V supply - making it an easy test point

    with a damaged FET
    a fault current with the relay connected could leak out of the gate and alter the gate voltage you measure


    John

    PS

    the FET was replaced by the original part

    NTR5198NL from FARNELL

    https://uk.farnell.com/on-semiconduc...ngdom%2Fsearch

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