Relay switching 12v erratically

Much more usable diagram. :sunglasses:

joatmon13:
4) My understanding of a bridge rectifier is to switch ac to dc. So would it not in fact stop the system from reversing polarity to the actuator when required?

It is just a convenient way of attaching to each actuator connection, a pair of diodes which prevent the voltage either exceeding the supply voltage or going negative to the ground (12 V negative). Under normal operation, that clearly cannot happen, the diodes merely limit impulses out of the range of the supply voltage as the relay contacts change over. Two wires, two diodes each, a bridge has them all.

joatmon13:
5) Can anyone explain why the 12v output from the relays would switch to a negative value, less than required (-0.7 to -8v). Is this because of the long leads between the relays and the actuator? (2.5meters).

No. If it is wired correctly and you were measuring correctly, then other than switching transients - which you would need a scope to observe - no part of the circuit generates negative voltages.

Thank you both Southpark and Paul__B. This is a voyage of discovery for me and I really appreciate your perseverance.

Southpark, you ask have I considered a H-Bridge. As a newcomer to this, I just googled how to drive a linear actuator and the internet seemed to be consistent in recommending dual channel relays. I wasn't aware of the H Bridge solution, but if this is a more robust solution, then I will adopt this approach. What are the pros and cons of each approach? Are there any specific characteristics I need to correctly select a H-Bridge? Would this Amazon item work?

I only asked about fly back diodes in my OP, because I see this as a sticky post in this section and I thought maybe that was my problem. I have none in my project.

Paul__B - is a bridge rectifier as you suggest similar to a H-Bridge? When I search for a bridge rectifier, they all seem to be to convert ac to dc. Can you recommend what type I require, or do I need one at all, if I choose to use the H-Bridge approach?

To clarify further my earlier post # 4, when I say I get a negative feed from the relay, I should have stated more clearly that the voltage was reversed on the lines, as expected, it's just that the value was inconsistent - not + or -12Volts. I have a multimeter hooked to the relay outputs to the linear actuator and so it shows a negative value when the actuator is being closed.

Most welcome joat.

The bridge rectifier method includes diodes that achieves the desired voltage limiting function. So a properly controlled set of relays can be used to apply either a positive source polarity or a negative source polarity.

Off-the-shelf H-bridge modules generally have the necessary diode configurations too, and the switching is done using transistors instead of relays.

The bridge rectifier method could just be what you need. The diodes of the bridge rectifier will drop a few volts. So just take that into consideration too if it matters ------- as in --- if you want 12V or so across the linear actuator device, then could consider off-the-shelf H-bridge module ..... most likely costs more.

The 'flyback' diodes (they have other names too) are a name given to diodes that have the job of preventing a relatively large voltage from being generated (or developed) across the motor coil when somebody attempts to use a switch to abruptly disconnect the path (or line) of current (for the case where current is flowing through the motor coil).

The relatively large voltage generation an effect that the physics people found a long time ago. The developed large voltage during the 'switch-off' is known to have a polarity that goes 'against' the direction of the original flow of current. And this is why the fly-back diodes are arranged in the configurations that we see in circuit diagrams, which allows the original current in the coil to decay away to zero (during the switch-off period) and also at the same time prevents the large reverse-voltage from being developed. They are clever tricks (methods) for preventing large voltages, sparks etc ------ which could otherwise damage certain components in our circuits or wear out physical contacts of relays pre-maturely.

Thank you Southpark for your very clear explanation. Karma points added!

I have ordered some H-Bridge units and will try my luck with them. 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.

When I receive the H-Bridge units, I will update on the results.

Most welcome joat. When an inductor (coil) is involved with a current flowing though the inductor, and a physical switch is used to break the path (to stop the current from flowing) ----- the physics people found that the flow of current through the coil is maintained for a while even when the two sides of the switch (electrical contacts) are not touching each other during contact separation. Associated with the 'spark' ----- the two sides stay electrically connected for a bit - even when there is physical distance separating the contacts. The spark eventually distinguishes when the contacts become far enough apart (relative to the initial touching 'zero' distance). The spark can create residue on the contacts, and wearing out of the contacts -------- and it is the clever methods such as using flyback diodes in contactor-switch and coil combination circuits that helps to stop sparks from occurring.


Not sure what the intention of that diagram is, but it has nothing whatsoever to do with what I was explaining about using a bridge rectifier as a convenient form of voltage limiting for the relay switching.

It is in fact, nonsensical!

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

Paul__B:
Not sure what the intention of that diagram is, but it has nothing whatsoever to do with what I was explaining about using a bridge rectifier as a convenient form of voltage limiting for the relay switching.

oops..... ok ..... obviously misinterpreted the bridge rectifier scenario there. I assumed something different when looking at it ---- as in bridge rectifier diodes combined with a suitable set of relay switches (for flipping the polarity) and DC motor.

It is in fact, nonsensical!

I disagree with that.

Nice recommendations you made there Ron.

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.

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

Reversing_Relays.png

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

Reversing_Relays.png

yes, I like that site Paul, nice diagrams.

Ron_Blain:
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.

johnerrington:
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!

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

Reversing_Relays.png

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

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).

Reversing_Relays.png

@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.

johnerrington:
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.

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.

Southpark:
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! :roll_eyes:

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). :grinning:

johnerrington:
@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.