Measuring Amps?



  • I am fitting a new heated bed this afternoon. It's a sexy little 200x200 silicone heater (24v 180w (7.5a)). I am getting a little concerend that my PSU (24v 400w) might be getting a little strained, but I can not figure out the correct way of measuring the draw? I have an energy meter that the printer is plugged into which measures the amps and watts but last night while printing it was recording around 0.39a which seems far too low. When you cosider that I have an E3D hotend with 24v 40w heater cartridge (1.66a). Should I be looking at the total wattage drawn through the energy monitor and dividing it by the 24v?

    Current Setup:
    Ultimaker style: X, Y Z, E motors (1.7a)
    E3D 24v Heater Cartridge (24v 40w)
    Duet Wifi
    Silicone Heater Pad (24v 180w)

    Does anyone have any insight?



  • Isn't the energy meter showing the mains AC current? If so, in the UK with 240v mains you would be looking at around 94W being consumed?



  • My worry is whether the measurement on the meter can be used to judge whether the PSU is powerful enough or is there another way?



  • Well, the PSU is not going to be delivering more W than what's flowing into it so as long as less than 400W is going in, you should be OK. Have you checked your power meter using a known load? As the Duet will be continuously switching power to the hotend and bed + steppers & fans the displayed load current should be varying so you need to capture the max load, perhaps it has a max hold feature?



  • I believe it does have a max hold. I will have to try it.

    I will wire in the new bed and test it (with my fingers crossed!).

    Why would it only be 94w? The bed alone would be near double that, or am I missing something? (Probably missing something)



  • Clamp ammeter is the way to go http://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2060353.m570.l1313.TR0.TRC0.H0.Xclamp+ammeter.TRS0&_nkw=clamp+ammeter&_sacat=0 plenty to chose from. These are great for large currents i.e. over 10A where you could not realistically attach a multimeter in series with the load, without blowing fuses/destroying it. I have heard people say they aren't very accurate, but I've measured small currents with the one I have (UNI-T UT211B which is also a true RMS autoranging multimeter, and comes with k-type thermocouple probe so all in all a very useful tool for 3D printing) using the clamp and then compared them to measuring the current in series and have been quite pleased with the accuracy. You have to make sure you only clamp around one of the leads though not both.



  • Where would I clamp that? I assume to the 240v coming into the PSU? Or maybe test everything individually?



  • No, I believe it's used to measure the output current of the PSU, not the input. So it has to be clamped around one of the wires (+ve or -ve but not both) from the PSU to the printer.



  • Yes you would clamp for example the positive (DC) wire from the psu to the printer and see the current draw. You can also clamp the mains input but again, and especially with AC you need to clamp one wire only as the Live and Neutral are opposite phase to one another and if you clamp both you read 0.

    You could then turn the bed on and clamp one of the wires to the bed to see how much it was drawing, then the hotend heater etc.. Do this whilst they are heating (If pwm controlled) so you get an idea of maximum current draw as it will drop off if maintaining the temperature by PWM.



  • So I could check the board input and the bed input. Then combine the 2 I suppose. Excellent, I will have to order one. So is there any reason the current setup with the energy meter on the mains line wouldn't work?

    edit: The ammeter description says it N/A under DC current?



  • You got me thinking about this so I thought I would do some measuring.

    I started with using a power meter between the PSU and the mains supply and it showed complete nonsense, far too high a wattage > 500W !

    I then used a DC clamp meter around one of the leads from the 24V supply to the printer and I got:

    idle (Duet + fans) 0.4A = 9.6W
    Bed (8.4 - 0.4)A = 192W
    v6 (1.9 - 0.4)A = 36W

    So, I guess measuring the power into the PSU is not so reliable after all (power factor?)



  • Power input into the PSU is a perfectly reasonable measure of how much electricity your printer is costing you to run. But loses in the PSU, and Duet board and wiring will mean you aren't really measuring anything useful for diagnosing your problems. If your PSU is meant to be giving you 300w and your total DC current flowing is 15A but its struggling then its a duff PSU.


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