Meanwell Power Supply AC Ground and V- Connection

  • Hi,

    In a printer I'm building, I'm using a Meanwell RSP-500-24. The datasheet can be found here:

    In that datasheet, the block diagram seems to indicate that V- is connected to AC earth/ground. However, when I test with a multimeter, I find no continuity between any of the V- pins and the AC ground pin. This is with no power connected to the PSU inputs.

    Am I misinterpreting the block diagram for the PSU? Should I connect a wire from V- to AC GND? The frame of my printer, and the shields of the stepper cables will all be grounded to A/C GND.

    I'm not an electronics expert, and I'm trying to make the safest/best approach possible for this project. Any advice would be appreciated! Thanks.

  • Are you talking about that capacitor connection?

  • @sigxcpu this is what I'm referring to:


  • Yes. That is a capacitor. Maybe some noise supression thing. That is not a direct connection to ground.

  • The capacitor on the ground leg will allow AC to pass but not DC which is what is coming from your meter.

  • Thank you both for the information.

    So, the question remains for me: is this setup adequate in terms of safety? Would an extra, direct link from AC GND to -V be advisable?

  • Some swear by the direct link. There doesn't appear to be anything easily to hand that confirms things definitively. In my recent searches I found that this is prescribed by code for some types of devices but not others - including some scientific instruments.

    Checking schematics helps. If they have shorted the pads around screw holes to ground then you may as well do it at the psu. The Ormerod 2's external 5V regulator did this, meaning that without a ground and dc -v link any DC +v short to the chassis/ground (such as heater plate shorting to aluminimum heat spreader, through the bolts and bearings to the chassis) would then try to make it's way back to the psu terminal through the external regulator board. (Not necessarily the reason this connection was toasted on a board I recently spoke about on the forum.)

    I always ground as much as possible of the chassis to earth which also includes earth bonding the stepper motors. I struggled to get or test a good bond with aluminium extrusion but realise normal multimeters aren't a great test for this.

    If you do bond -v and earth use the largest connections on the earth bonding as practical so the majority of noise and fault currents go direct back to the psu rather than effecting the electronics.

    Edit: The other big ones from the safety point of view is fusing and earthing. Where practical add fuses (or reset-able circuit breakers/trips). Main candidates are on the +V feeds to the hot end heater cartridge and heated bed. If possible check to ensure you have a good earth connection for your machine. If your in an old house that has never/not recently been safety checked that may be worth getting that done, and avoid long extensions or daisy chaining extension leads where possible. I run my machines after a mains plug in surge protector and RCD trip.

    Edit 2: This is no religious matter where by I will stick to no link in the face of thorough evidence to the contrary. I'm not saying that I will always avoid a tie between the -V and ground (and will add it in situations like Ormerod 2) and may well someday understand it better! 😄

  • @doctrucker Thanks again, especially for such a thorough answer.

    My only dilemma here is that I'm suspicious that the capacitor link to ground would be sufficient to protect against AC shorting to the DC circuit, and that by adding a separate direct link I'd be creating a very small ground loop, and I do not know if there are any negative effects from this.

    What do you think? Just put the link and call it a day? I hate doing things without knowing why I'm doing them.

  • My understanding is it is not about the ac shorting to the dc circuit but more a potential difference building up between the dc rail and ground. Although you are measuring the output of the dc rail to its own 0v pin giving you 12 or 24v (what ever your power supply is rated to), due to the fact it is not grounded to earth, its has the ability to actually be a lot higher than that with respect to ground. It could be 30v or even higher. If for example the dc circuit is isolated from ground but then touches, that is when it becomes a problem. Sensitive electronics such as sensors etc do not like it.

    That is my understanding of it. I am not a sparkie, but am in the pumping and irrigation industry where we have sensors (loop powered pressure and transducers etcm) on 24v power supplies that a) do silly things because of it or b) blow up in rare situations. By binding the 0v to ground it avoids these issues.

    Correct me if I am wrong but I believe that is what our sparkie said the reasoning behind it.

    Kind Regards,


  • @samlogan87 A very compelling argument! Thank you for that.

    I will add the ground link!

  • I should have also said the control board as well as the sensors.



  • administrators

    I recommend that you ground the negative PSU output for safety. The only drawback I am aware of is that if you connect the USB port to a PC, you create a ground loop and that can be troublesome. See the wiki page on USB ground loops.

  • Just to clarify @dc42, do you recommend putting a wire link between GND and V-, or e.g., a 1 MOhm bleed-off resistor, or something else (like a diode or varistor)?

    For those who want to read some more on this topic, I think this is a good read:

  • Thanks for the information. As I read the first detailed post I was thinking a resistor rather than plain join might be best. Only other arguments I've seen before as whereone of the you-tubers shock themselves, but that was due to a bad mains earth, not lack of DC - AC connection.

  • I also do lots of other DIY electronics.

    For audio amplifiers, and circuits where ground loops can cause some very undesirable things, particularly since there are a lot of places that can cause ground loops in audio equipment, it seems to be pretty standard practice to have an isolator between the DC ground and actual earth.

    I have built a bunch of circuits from Eliott Sound Productions, pre-amplifiers, signal processors, and audio amplifiers. I made quite the sound system with his circuits.
    From Eliott Sound Productions website
    In this schematic, you can see in the bottom, the parts used for the loop isolator. D1, D2, R1 and C1 are all in parallel providing a nominal break between earth and the DC ground. This one is for a dual amplifier supply, but shows that the AC earth should have at least a modicum of isolation from the DC ground. This is also true for single-rail supplies. Failing to do this can result in audible noise in audio circuits.

    If you omit D1 and D2, this will still work as an isolator, however should a large potential build up, you might end up burning R1, which would normally be a 0.25W device. The diodes protect R1 by acting as a dead short for voltages above the Vf of the diode, typically a bit under a half volt.

    The Meanwell diagram seems to only use C1. This is probably adequate if there is no electrical fault elsewhere, but we're doing this in case there IS a fault elsewhere.

  • That looks great, thanks for sharing. Can you give the bill of materials for that?

    I've been meaning to draft a power board for dropping down 12V to 5V so I can loose one of three power supplies on my vapour build! That could sit on the same board.

    12v - fans, safety relays, interlocks & indicators.
    24v - (controlled by duet & interlocks) steppers & heat.
    5v - Controllers.

  • @doctrucker The parts arent critical for a loop isolator like that. I think I just grabbed some 1N4004 diodes, a 1/4 watt resistor and a ceramic disc capacitor. I just breadboarded it in the PSU housing and hot glued it after soldering the earth lead to the 14 gauge solid core wire.

  • administrators

    @doctrucker said in Meanwell Power Supply AC Ground and V- Connection:

    That looks great, thanks for sharing. Can you give the bill of materials for that?

    I've been meaning to draft a power board for dropping down 12V to 5V so I can loose one of three power supplies on my vapour build! That could sit on the same board.

    12v - fans, safety relays, interlocks & indicators.
    24v - (controlled by duet & interlocks) steppers & heat.
    5v - Controllers.

    What will you be powering from +5V? If the power requirement is no more than about 1A then the Duet can provide that power.

  • @dc42 I didn't want the power to the controller or fans cut when I interlocked the heaters and steppers.

    Having thought about it more I'm wondering of it is necessary to have the steppers on the robust interlock and just rely on the Duet to stop the steppers.

    I'm aware of risk assessment tables that jelp establish the performance level of a system but are there any tables that relate to the kinetic energy of a moving blunt object and the level of injury that could cause?

  • administrators

    No I am not aware of any such tables.

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