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How do you check electrical connections before turning on?

Posted by teh.stig 
How do you check electrical connections before turning on?
April 16, 2016 11:16PM

I've just built my first rep rap. I'm to the point where I've connected the wires, but have not plugged in or turned on. Is there a way I could test everything first and if so what's the proper way of doing that? I'm a novice tinkerer, just getting my hands dirty with stuff by building a Prusa from scratch.

Re: How do you check electrical connections before turning on?
April 16, 2016 11:28PM
You test things one at a time. Check each motor, check each end stop, each thermistor. Once each is working, connect everything and power it all up.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: How do you check electrical connections before turning on?
April 16, 2016 11:34PM
Sorry for stupid questions but how exactly do I test it. Can I use a volt/amp meter or something like that, and if so, how do I do that?
Re: How do you check electrical connections before turning on?
April 16, 2016 11:44PM
You connect one of the listed items at a time, power up and check that it is working properly. Of course, you're going to visually inspect wiring as you connect each thing.

I guess you'd power up the controller board without any of the other connections and make sure you can talk to it via USB or make sure the LCD module is working. You won't be able to heat up the bed or the extruder unless the thermistors are working. The extruder motor won't turn unless the extruder is hot. When you connect power to the controller board, make sure you get the polarity right. When you connect/disconnect motors, power down the controller first or you risk blowing up the motor drivers.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Electronic components run on magic smoke. What is important is to keep this smoke inside it. SO if you hear "puff" and see the magic smoke (it smells bad too) escape, it is dead. Happens once out of two when you connect randomly.

Edited 1 time(s). Last edit at 04/18/2016 04:23AM by MKSA.
Re: How do you check electrical connections before turning on?
April 18, 2016 08:16AM
I think this is a good question as this type of electrical testing is second nature to people from the electronics world but not very obvious at first to everyone else.

1) Thermistors, you can see if they're working by whether you are getting a plausible temperature reading on your LCD or computer host program. If you have both a bed and hot end thermistor they should be both reading room temperature within 1-2 degrees C of one another. They are very sensitive so if you handle them they will go up to 30 deg C quite quickly, which is approximately skin temperature. If you warm them up to a known temperature check if it is correct I reference against a kitchen thermometer and an IR thermometer. Within a few degrees C is okay. Do this at temperatures you will be printing at so 185-240 deg C for hot ends and 60-110 deg C for heatbed beds. Its best to do this test with the hot end heater and heat bed heater itself as warming them up by any other method is difficult/

2) Endstops. Run your host program and enter m119. It will show the endstops as triggered or open. Then hold down one endstop (or trigger it in whatever way if its not a simple switch) and check again using m119. If they show triggered when they're physically open its likely the settings in the firmware relating to Endstop logic need to be reverse.The same applies to z probes, in that they are essentially z_min endstops. They should show triggered when the bed is near to the nozzle, and open when it is not.

3) Motors, when you issue a move using the controls on your host the motor (or the axis you are testing) should turn in the direction you asked for - use small moves i.e. 10mm not 100mm in case the axis hits (crashes into) a physical travel limit . If wrong direction then either turn the motor connector to the control board around the other way, or change motor direction in firmware. EDIT- As stated below ALWAYS disconnect power to your controller board before removing a motor connector. Assuming you have only endstops at one end of an axis, that axis will only move in one direction until you home the printer (i.e. move to and trigger the endstop) as the firmware has no idea where the carriage is until it finds an endstop and sets this as 0 or, if using a max endstop - it sets the position as the max_position. As for motor current, the motor should move, and should hold when its energised - i..e after a move, it should not be humming loudly or jittering around. You can set the current by adjusting the potentiometers on the stepper drivers, there is a lot of information on this out there, suffice it to say, be careful they get shorted and damaged very easily. Aim for enough to move the motors properly but not so much that they get very hot. For the standard reprap 1.68A nema 17 motors voltage measured from ground to the potentiometer should read around 0.4v.

4) The bed heater wires (and any high current wires, power supply to ramps, heaters etc..) should be firmly attached, check they wont pull out when things move, fires can start by wires being loosely connected.

5) The extruder motor wont turn unless the extruder is heated, you can temporarily disable this by sending m302, so for testing if you don't have filament loaded use this.

This is only a flavour of the sorts of tests/checks you can do, there is a lot of information out there. I was pretty new to it a year ago and have learned a lot, with still a lot to learn, that's where a lot of the rewards comes from.

Edited 1 time(s). Last edit at 04/18/2016 11:52AM by DjDemonD.

Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Re: How do you check electrical connections before turning on?
April 18, 2016 08:47AM
Thanks for taking a crack at such a helpful answer DJ, I was struggling to think where to start.

For the stepper driver Vref, the suggested value is for a typical A4988, but may be different for any specific driver if the manufacturer changes the sense resistors. The drv8825 mostly use V= current/2, so about 0.85V for a 1.7A motor, but I'm sure there are exceptions for those too. So bottom line, you need the specifications for your particular drivers to know what voltage to set.

Thermistors and heaters can be connected either way around, so polarity isn't critical. Be very careful of the wiring for endstops as it's easy (at least on ramps) to accidently connect the +5V supply directly to ground which will likely destroy the voltage regulator on the mega2560.

Fans do have a polarity, but you're unlikely to break anything if you occidentally hook them up backwards. There is often confusion between the fan used to cool the heatbreak of the hotend and the fan that is for cooling the recently extruded plastic (which is often not supplied with entry level kits). The fan for the extruder/hotend should be running all the time that the hotend is on. Usually it is connected directly to 12V power. The fan for the printed plastic is controlled by software and connects to the printer control board.

The big thing with the stepper motor cables is not to connect or disconnect them with the 12V power connected. Which also means that any connectors in the cables should be locking so that they can't come loose during a print.
Regarding the A4988 based stepper controller, there are in fact more than one type leading to different adjustment values. The original from Pololu able to supply close to 2A, then the Chinese clone, some limited to 1A or 1.5A depending on the clowns making them. One can identify them by the sense resistors used (0.05, 0.2, 0.1 ohm) plus the way they handle microstepping (Allegro data sheet explains all this). I just discovered it as I was having trouble getting the proper torque for the extruder. I designed. My Geetech was supplied with the 1A and luckily I had ordered spare I identified as 1.5A from the pictures and received the 1.5A model.
Re: How do you check electrical connections before turning on?
April 18, 2016 05:37PM
There are some useful tests you can do with a multi-meter before you connect the board to power.

Set your meter to continuity. This is one of the modes in the resistance setting. I like to set the mode to a tone based continuity test. When you touch the probes together you should hear a sound. You can test all sorts of stuff this way before ever connecting power. If you made your own cables you can check continuity from one end to the other to make sure the connectors on either end are making good contact. More importantly you can check for shorts. Touch one probe to your power input on + and one on Gnd. You should not hear anything. If you do hear something then you have a short across the power input lines which will fry your power supply. Do the same with 5V and Gnd. You may have to hunt around for an easy way to connect to 5V (Still power off), but the pins for the endstops would work. In general if you got the board pre-built you should not have these types of issues. If you soldered it together yourself it is more important to check for shorts in the board.

You can check your endstops the same way if they are simple switches. Plug the end stops into the board, (still no power to the board). Test from 5V to Gnd. Hold the endstop switch down and test again. You may need another set of hands to get all of that done at the same time. You should NOT get a sound out of your multi-meter either way. A short across 5V and Gnd when your endstop is pushed indicates improperly wired cables or possibly the cable is just plugged in backwards.

You can check the steppers motors and wiring too. Change the mode back to standard resistance. Measure resistance across both coils with the motor unplugged. Each coil is a pair of adjacent wires in your cable. Easiest place to probe is generally at the end of the cable on the back side of the connector. Resistance should be about the same for each coil and will likely be in the 2 to 8 ohm range depending on which stepper motor you have. If the resistance of one coil is significantly different from the other coil on the same motor then you likely have a bad cable connection or a failing wire. For instance if one coil measured 3.3 ohms and the other measured 5.2 ohms then you probably have a cable problem. The motor might still run but it can run rough. This is also an easy way to find which wires are sharing a coil on a surplus motor you are trying to wire up. There should be no connection between wires that don't share a coil. This will often show up as OL on your meter. (OverLimit) A fun thing to do if you haven't used a multi-meter before is test skin resistance. Pinch the metal part of one probe between two fingers of one hand and the other with your other hand. The reading will be in the 100,000 ohm to 1,000,000 ohm range.

In resistance mode you can also check your thermistors. Resistance across the thermistor will be in the 4,000 to 100,000 ohm range depending on the thermistor and the temperature. Of you see small resistance then you might have a short. This is a useful test to do after assembly of a new hot end or heated bed. Easiest place to connect is likely at the end of the cable on the back of the connector as with the motor cables. Do this power off or with the thermistor unplugged. You could have someone heat the thermistor up with a hair dryer while you measure resistance if you want to see it "working".

If you want you could measure the heated bed resistance. It will be very low. On the order of 1 to 2 ohms. That is what you want.

All of the above tests are done with power off.

With power on you could set the multi-meter to voltage and check your input power and 5V again to make sure you are getting the voltages you expect. Be very careful if you decide to do this. It is often very easy to short 5V to some other pin or to Gnd by accidentally touching both pins at the same time with a probe or by accidentally allowing your probes to touch each other. This will let out the magic smoke. If you don't find any shorts when testing continuity on your powered-off board then you can skip the power-on tests and just start testing functionality as suggested by the other posts above. In general if you don't really know what you are doing, don't mess around with a board that is powered on. It's not just multi-meter probes either, this goes for screwdrivers too. No metal objects should come near your board when power is on.

Edited 1 time(s). Last edit at 04/18/2016 05:38PM by bryanandaimee.
Re: How do you check electrical connections before turning on?
April 21, 2016 08:32PM
I had my first whiff of Magic Smoke today, and far from being intoxicating its a little sobering.
the genie is out of the machine, complete with other pyro fx, and for my next trick...hey presto, sim sala bim
I give you a steaming pile of plastic spaghetti.
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