AC and DC common ground; is it needed?

Hello,

Going back to basics here, as I’ve come across a problem which I can’t seen to rectify and maybe I am missing the obvious.

I made a solder hot plate thingy for soldering SMD, using this link:

Components;
23VAC to 5VDC psu - 5V input to the Nano, everything else is running from 3.3v, supplied via Nano
Solid State Relay
Oled 128x64
Nano
MAX6675 module
K type thermos
Iron base - 230VAC, supplied via psu and the SSR

The purpose is to solder smd components. It consists of two displays, one being a temp which the user selects using the pot and another screen which shows whether the device is ‘off’ ‘preheat’ and ‘reflow’ and shows the current temp from the thermo. So, one oled is used with two different screen, showing which state it is in or another screen comes up when the pot is adjusted.

A push button is pressed and the relay is powered, letting 230VAC power the iron, this is until the temp reaches 150C and then the button is pressed when ready, with the temp being raised to the set point. As the screen is flickering between displays, the button can not be selected.

Originally I thought my issue was solely the thermocouple, as when the CS connection is removed, the display stopped switching between screens. The thermocouple outer is braided metal, which I thought may be the problem. So I wrapped the outer with heat resistant tape, as when the braided metal initially touched the iron, there was static, but only upon movement of the thermo. Obviously, the end of the thermo is directly touching the iron, which didn’t cause static and it is reading fairly accurately and settles.

Further fault finding led me to checking all connections, which appeared to be fine. The I discovered if I remove the 5V supply from the psu and power the Nano via USB, the oled settles and it all works perfectly.

So, this must be a ground issue? I tried a common ground between the electronics and the psu and the iron but nothing changes, still the same issue. I tried grounding the sheath of the thermo and still the same problem.

I could just use separated supplies and never worry again but this is bugging me and I’m not sure what else to try and I don’t like the idea of a common supply without some sort of protection, such as a optocoupler, maybe?

Anybody shed any light? Thank you

Why is everything connected to arduino 3.3v? The arduino 3.3v can supply a measly 150 miliamps of current. You need an external 3.3v regulator for this project. You need something like a LD33CV linear regulator, assuming that it doesnt need current above 5-600 miliamps. be sure to use a small heatsink and 10uF filter capacitors for stability

I’m guessing the iron is being regulated by PWM. If you viewed this on an oscilloscope (be careful with grounding issues) my guess it’s causing load drops which is causing your oled to reset. You need clean power sources for your digital circuitry that’s isolated from the resistive and inductive load.

kaseftamjid: Why is everything connected to arduino 3.3v? The arduino 3.3v can supply a measly 150 miliamps of current. You need an external 3.3v regulator for this project. You need something like a LD33CV linear regulator, assuming that it doesnt need current above 5-600 miliamps. be sure to use a small heatsink and 10uF filter capacitors for stability

I should have been clearer. The OLED display, MAX6675, potentiometer and button are connected to the 3.3V of the Nano. The Nano triggers the signal to the SSR. I've not checked what the current being pulled is, I should have checked but I'll do that tonight.

Thank you for the advice on the linear regulator, I'll bear that in mind, until I check the current and then I'll probably do down that route.

wolframore: I’m guessing the iron is being regulated by PWM. If you viewed this on an oscilloscope (be careful with grounding issues) my guess it’s causing load drops which is causing your oled to reset. You need clean power sources for your digital circuitry that’s isolated from the resistive and inductive load.

I'm not sure if the Nano is resetting, although that does make sense but there is never a blank screen, as it would if I pressed the reset button but just switching between two different displays. Thinking about that as I type, then you may be right there, is it resetting constantly...

Yes, using a separate source for the electronics, then it works as it should.

I mentioned this to a few guys at work and one of them suggested using shielded wires for the 5V input, as they're near the 240VAC wires. This also sounds like a plausible explanation but I'll have to find some of that stuff at in the week.

For the circuit to function you don't need any more common grounds than you already have. However, you have mains involved, with the possibility of the mains finding its way into the low voltage circuitry under fault conditions. I would connect mains earth to the 0V (what you are calling 'ground'*) for safety reasons.

Have you read Common ground and why you need one ?

  • There is much confusion over the meaning of 0V, ground and earth. Possibly because in many circuits they are the same thing, possibly because I suspect the same words mean slightly different things in different countries or to different people. I am considering adding something to cover this in the common ground tutorial, but the problem is I can't get it straight in my head, and I don't think anyone else can.

Hi, What is your 230Vac to 5V Supply?

Thanks.. Tom... :)

Likely not your issue but I don't like the thermocouple boards that don't have any filtering on the T/C input lines. I've been using this board for T/C TC board

Since the circuit works better with USB power than with the external supply I'll assume the 3.3V nano supply is not being overtaxed.

Keep in mind, although your mains to 5V supply is "isolated" there is still some capacitance between the mains and the 5V output. So some noise can still make it through the supply.

You did not say what would normally cause the display to change state. Is it movement of the Pot? If so, how much movement is req'd to change the display state? Perhaps your display switch algorithm is too sensitive.

In addition to what Perry suggested I would Isolate the T/C from the iron plate. You mentioned you had some heat resistant tape. I think a layer of that would be beneficial. Don't worry about any thermal drop across the tape, unless you have a Huge thermocouple the difference will be less that the error in the T/C reading.

You also need some filtering on the switch input to the nano and the Pot input to the Nano. I would think a 2k to 5k in series with a 0.1 µF cap to ground near the nano would do.

You showed an SSR operating the heater. Can we assume the SSR has zero crossing capability? This would mean the heater is not controlled by PWM but controlled by the on / off function of the SSR and control loop.

PerryBebbington:

For the circuit to function you don't need any more common grounds than you already have. However, you have mains involved, with the possibility of the mains finding its way into the low voltage circuitry under fault conditions. I would connect mains earth to the 0V (what you are calling 'ground'*) for safety reasons.

Have you read Common ground and why you need one ?

  • There is much confusion over the meaning of 0V, ground and earth. Possibly because in many circuits they are the same thing, possibly because I suspect the same words mean slightly different things in different countries or to different people. I am considering adding something to cover this in the common ground tutorial, but the problem is I can't get it straight in my head, and I don't think anyone else can.

Yes, I called it ground, just for simplicity, as that's what a lot of people would refer to it as but I'll go for the reference. It is a tricky subject, as I went on the 18th Edition Wiring Regs (UK) course recently and it seems even seasoned electricians/technicians have a difference of opinion. All of the electronics GND terminals are connected to the -V terminal of the psu. Nothing is connected to the earth/ground of the AC side, although for the purpose of fault finding, I did try this.

I did read that thread, very informative, although I didn't get the answer which I was looking for; AC and DC common ground. I think an article explaining the difference between 0V, gnd and earth would be well received.

TomGeorge:
Hi,
What is your 230Vac to 5V Supply?

Thanks.. Tom... :slight_smile:

It's just a generic power supply, which I bought from AliExpress. 230Vac to 5Vdc, with a trim function. I've not removed the housing but I can see that there are some decent sized capacitors, which could be the issue.

JohnRob:
Likely not your issue but I don't like the thermocouple boards that don't have any filtering on the T/C input lines. I've been using this board for T/C TC board

Since the circuit works better with USB power than with the external supply I'll assume the 3.3V nano supply is not being overtaxed.

Keep in mind, although your mains to 5V supply is "isolated" there is still some capacitance between the mains and the 5V output. So some noise can still make it through the supply.

You did not say what would normally cause the display to change state. Is it movement of the Pot? If so, how much movement is req'd to change the display state? Perhaps your display switch algorithm is too sensitive.

In addition to what Perry suggested I would Isolate the T/C from the iron plate. You mentioned you had some heat resistant tape. I think a layer of that would be beneficial. Don't worry about any thermal drop across the tape, unless you have a Huge thermocouple the difference will be less that the error in the T/C reading.

You also need some filtering on the switch input to the nano and the Pot input to the Nano. I would think a 2k to 5k in series with a 0.1 µF cap to ground near the nano would do.

You showed an SSR operating the heater. Can we assume the SSR has zero crossing capability? This would mean the heater is not controlled by PWM but controlled by the on / off function of the SSR and control loop.

That's good to know, as I thought it may be drawing too much current, still yet to test. Ans yes, the pot makes the display change, which then shows the changing/selected set point. You've got me with the code part, I'll have a look but also post the original code (as I'm at work). I've only changed the digital pins, as the og post uses a D1 mini, rather than the nano.

I partly taped the thermo but possibly part of the sheathing is touching, as I taped the frayed end. The tip of the thermo is definitely touching, so I'll put a bit of tape between. I t was reading a degree or two above ambient, so that'll be fine.

I've got a 10k resistor on the signal end and vcc of the pot but no capacitor. Do you mean place the cap close to the voltage input to the Nano? or the pot and ground.

Yes the heater is controlled via an on/off function. So you're correct, there is no crossing capability there.

Thanks for all the help thus far

I did read that thread, very informative, although I didn't get the answer which I was looking for; AC and DC common ground.

Does current need to flow between the DC and the AC ground? Is there a common circuit between the 2 sides? In this case there isn't, so for functionality they do not need to be connected. Safety is a different question.

PerryBebbington: Does current need to flow between the DC and the AC ground? Is there a common circuit between the 2 sides? In this case there isn't, so for functionality they do not need to be connected. Safety is a different question.

Nope, no current flow. As stated, it’s fine to use separate power sources but at the same time, I’d like to get to the bottom of this.

Safety, would you like to voice your concerns?

exiledyorkie: Safety, would you like to voice your concerns?

I thought I already had done in reply #4:

You have mains involved, with the possibility of the mains finding its way into the low voltage circuitry under fault conditions. I would connect mains earth to the 0V for safety reasons.

If that's not clear ask another question.

PerryBebbington: I thought I already had done in reply #4: If that's not clear ask another question.

Ah fair point, I've slept since I last read that comment. I wouldn't have thought 230Vac would be able to get across. Yes, leakage but a full 230V, I'd have thought it'd burn the psu circuitry before that would occur but I'm speculating there, I'd have to lift the lid and inspec. Or I'm just completely wrong there.

You're probably right about the earth then and I will install the link. All I mean is, I already connected the -V to the mains earth and the problem persisted. I'm just wondering if that is common practice?

So I'm thinking either I need some filtering to clean up the DC or the 5V cable is picking up from the 230V cable, as I'm running them close together.

I wouldn't have thought 230Vac would be able to get across. Yes, leakage but a full 230V, I'd have thought it'd burn the psu circuitry before that would occur but I'm speculating there, I'd have to lift the lid and inspect. Or I'm just completely wrong there.

You're wrong to assume it can't get across. A 'fault' can be anything, possibly, probably, something you've not thought of. The best assumption is to assume that maybe it can get across somehow, even if you can't imagine how.

You're probably right about the earth then and I will install the link. All I mean is, I already connected the -V to the mains earth and the problem persisted.

This is nothing to do with solving your problem, this is about safety. Other people have given you good advice about solving your problem.

PerryBebbington: You're wrong to assume it can't get across. A 'fault' can be anything, possibly, probably, something you've not thought of. The best assumption is to assume that maybe it can get across somehow, even if you can't imagine how.

This is nothing to do with solving your problem, this is about safety. Other people have given you good advice about solving your problem.

Okay fair enough.

I've just replied to their comments as they've all given great suggestions, which make sense to me also. I'm not asking more questions, just reiterated it with regards to your comment.