Relay switching 12v erratically

[johnerrington]

Meanwhile, I have noticed that the relays' erratic switching can be coaxed into operation with a gentle flick (an old trick!!).  Seems to suggest that the contacts in the relays have somehow worn out, even though they have only been driving the actuator for less than 1 month.

I have not seen any decoupling on your diagrams.
The "gentle flick" provides a litle extra energy that allows an under-powered coil to actuate a relay. So I'd look for possible issues there, and definitely check for adequate decoupling, especially at the relay board.

[Paul__B]

OK, so we are clear on how the relays and the bridge are wired:

Diagram with the assistance of www.circuit-diagram.org.  

[johnerrington]

yes, I like that site Paul, nice diagrams.

[joatmon13]

When the 12 VDC to a motor starts becoming intermittent you may want to look long and hard at all of your connections. A loose or faulty connection will cause the symptoms you describe, especially under a higher current draw. How much current does your actuator draw under load? It's just a little peculiar it works fine sometimes and other times your 12 VDC drops down to 7.0 volts. If a breadboard is involved in this with wires poked into it I would wait for it to give the low voltage symptom and start wiggling and jiggling your connections. Nothing to lose trying that.

Ron

Hi Ron - Thank you for your input.  I have rechecked all 12v connections and all seem good.  My circuit is a soldered circuit, so no loose pins to jiggle.  The actuator draw about 0.7amps.  I'm fairly sure that the relay is bust on one side, as the circuit runs fine when opening, it's just on the close circuit, that the relay outputs insufficient volts to move the actuator.  Further attempts to flick the relay have proved unsuccessful, so I assume I will just have to wait until the new H-Bridge modules arrive.  Here's a picture of the project box.

I have fed the 12v supply from the battery and to the actuator, via 3.5mm plugs and used Molex connectors for the control inputs (PIR's and door reed sensor).  All connections seem solid and the control circuit is running fine.

[joatmon13]

I have not seen any decoupling on your diagrams.
The "gentle flick" provides a litle extra energy that allows an under-powered coil to actuate a relay. So I'd look for possible issues there, and definitely check for adequate decoupling, especially at the relay board.

Hi John, Thank you for your input and observation. 
1) When you say a little extra energy, is that on the control side or the 12v side of the relay?  I'm 99% sure the issue is at the 12v circuit, but your comment makes me question this.
2) As a greenhorn to this, I have not implemented decoupling (at least not knowingly!).  Can you please advise where decoupling should be implemented and perhaps point to some articles where I can educate myself?  Thanks!

[Southpark]

OK, so we are clear on how the relays and the bridge are wired:

Diagram with the assistance of www.circuit-diagram.org.  

At the moment, it just looks like the above configuration allows the relatively high reverse-polarity voltage to be developed across the motor coil (assuming a motor coil) upon motor switch-off ......... and sparking too.

My assumption is just based on a coil. So it all depends on what the actuator circuit is.

I will also add one diagram to just show what I meant from my own earlier diagram ----- regarding relay switching (and also noting that some voltage will be dropped across my diodes).

[johnerrington]

 @joatmon13, could you please post links to the specs of the devices you are using?

On Arduino relay boards I have found online the relays are labelled 5V so the activation energy comes from the 5V side. I'd check the 5V dc supply and make sure it can supply enough current to activate both relays at the same time.  And decouple it at the relay board with a big electrolytic cap.


Please dont get confused by all that stuff about bridge rectifiers and protection diodes; I'd suggest all you really need is a suppressor cap across the motor terminals.

[Southpark]

Please don't get confused by all that stuff about bridge rectifiers and protection diodes; I'd suggest all you really need is a suppressor cap across the motor terminals.

John ...... will a suppressor cap (with no flyback diodes being used) prevent the relatively large reverse-voltage upon switching off? And also prevent the sparking/arcing of contactors upon switching off? I'm thinking that arcing/sparking could possibly create issues with the contactors ---- residue build-up, wearing etc ...... after some time. But not sure how much time though.

[johnerrington]

The cap absorbs the energy so reducing the rise time of the spike and its max voltage.  Enough for the relay contacts to be open far enough to prevent arcing.  A bit like the cap & points on an old car ignition system.


An H bridge may need better protection.

[Paul__B]

John ...... will a suppressor cap (with no flyback diodes being used) prevent the relatively large reverse-voltage upon switching off? And also prevent the sparking/arcing of contactors upon switching off?

Actually, motors - permanent magnet or shunt would - do not have significant "reverse-voltage upon switching off".

This is confused with inductors - solenoids and such.  When running, a motor has a back-EMF; it acts as a generator.  The back-EMF is in the same direction as the applied voltage; it opposes that applied voltage and is the major limit to the current drawn.

There is no back-EMF at start-up or if the motor is stalled, so the current draw is limited only by the winding resistance.  OTOH if the motor is unloaded and "running free" the back-EMF is almost the same as the applied voltage and the current draw is minimal.  When you switch off the supply, the voltage across the motor is the same back-EMF which continues in proportion to how fast the motor is spinning,

So a theoretically perfect motor will have no inductive "kickback"; it is the "stray" inductances in real motors which are responsible for such and this is (proportionally) less so with larger motors with more commutator segments.  So you may need the diodes when using semiconductor "H-bridges", but not such a problem for relays.  The large switching arcs on contactors are purely due to the currents (and voltages) involved.

Sorry, but that diagram is still rubbish!  The rectifier bridge will cause the motor to run in one direction whatever the reversing switch does!  

The two SPDT relay control approach is extremely elegant as it uses only single pole relays and one per direction.  It does however cause full dynamic braking.  If you wish to avoid this, you can add diodes to the NC of each relay (and then implement dynamic braking by actuating both relays together).  

[joatmon13]

@joatmon13, could you please post links to the specs of the devices you are using?

John - here is a link on Amazon to the actuator which I am using.
Specifications:
Material: Aluminum alloy
Load capacity: 750N/160LBS
Input voltage: 12V DC
Speed: 10mm/s
Stroke length:100mm/4 inch
Working frequency: 20%
Rated power: 20W, maximum 30W.
Ambient temperature: -20°C to +75°C.
Standard protection class: IP54.

[joatmon13]

On Arduino relay boards I have found online the relays are labelled 5V so the activation energy comes from the 5V side. I'd check the 5V dc supply and make sure it can supply enough current to activate both relays at the same time.  

John - I am using a buck convertor LM2596S  to supply voltage 5v to the relay board.  I would have thought it sufficient.  Here is a link to the item.
"And decouple it at the relay board with a big electrolytic cap." - is this on the 12v or 5v side of the relay?  Can you suggest a suitable cap?

[johnerrington]

That actuator should not need protection diodes as it is loaded, so the load is absorbing energy.
I'd go for about 1000uF 12V on the 5V side at the relay board.

[joatmon13]

I'm struggling to keep up with the debate on how best to wire up my project, but I do appreciate the input and advice.  In simple terms, do I understand that EMF from the motor is depending on inertia continuing to turn the motor, after it's powered off?  In the case of an actuator which uses an acme screw geared drive, I can't see that onward inertia would occur.  

With regard to John's suggestion regarding using a cap, I don't understand why this would be on the 5v control side.  Is this how it should be positioned?   Should I use the same cap for the H-Bridge?  I cant seem to easily find 12v versions, so I have drawn in a 1000uF10v version.

[Southpark]

Joat ------ there is a heap of circuits and information out there - involving DC motors - particularly brushed DC motors, with driver circuits all involving features such as flyback diodes. Just check out this one of many links on the internet ---- (click here) ..... that could help point out the benefit of flyback diodes for this sort of DC motor driving application involving switching.

I think that in the given link above, who-ever wrote it got it mostly right, but some of the theory appears erroneous. Most of it is good though.

One important thing/lesson (I think ... maybe) that the information teaches us ------ is that if a current is flowing through an inductor (coil), then there is a known physics behaviour ---- where the current through an inductor will never change 'instantaneously' - that's a known physics behaviour of inductors.

If a mechanical switch is used to remove the current from the coil, then a related known physics (electronics circuit theory) effect will occur, which involves development of a relatively large voltage across the inductor, and also the development of a relatively large (but opposite polarity) voltage across the 'assumed large resistive air gap' between the switch contactor points.

When relay contacts are separating (to try break the coil current), the amount of current that flows through/across the largely resistive air gap will momentarily be maintained (ie. having the same value as the initial coil current ------ or at least momentarily has the same value of current). So for some regular value of current, combined with large resistance (air-gap) ....... means relatively large voltage across the air-gap.

Regardless of the large voltages developed ....... arcing/sparking will occur for the inductor circuits (shown in those web links) if flyback diodes aren't used ...... and arcing/sparking could possibly be linked with future performance issues (as in ---- arc/spark enough times, then enough carbon residue/pitting/burning or whatever could lead to reliability issues) ----- for the contactors/relay after some time. But not sure how long a time.

So ------- just in case ------ the various DC motor circuits out there typically do involve flyback diodes.

Sure ------ there's always the option to not use flyback diodes. We always have choices.

I reckon that ------- as long as we know the benefits of having them ..... or what they're supposed to do ..... and what could happen if they're not used ..... then that is good.