Hi
Need a little help – this is new to me so assume simple explanations are necessary.
Propagator control, I've been trying to track down a fault for some time, had thought it was a power supply issue but today realised the true culprit, 'the relay'.
The Propagator control works OK when no load is on the relay but stops working when the relay switchers a 240V load.
Searching has pointed me at back EMF and possibly using a separate 5V to work the relay.
I've googled this and have a fair idea what generates back emf – all new to me.
Apparently a diode across the 5V to the relay might fix this – it is across the 5V yes ?
What size and type of diode would I need and where does it go.
What is a 'fly back diode' – is this just a name for what the diode is doing
What is a snubber, do I need and how would I use one.
I can if wanted upload a photo, a circuit diagram I don’t have but as I said it is working when no load is applied.
I'll work through whatever is suggested and hopefully fix this
When the relay turns off, it generates a voltage opposite that which turned it on. It can be much larger than the turn on voltage. The diode is reverse connected across the relay coil. When the turn on voltage is applied, it is reverse biased so has no effect. When the relay turns off, the voltage forward biases the diode and the voltage is shorted by the diode.
Any common diode like a 1N4004 or similar will do.
I'm switching on a 240v 35W Reptile Heating Pet - which is being used in a plant propagator
If the heat mat doesn't provide enough heat I'll replace it with 240v Heat cable that will be about 75w
Any inductive load needs provision for a path for current at switch-off, otherwise very high voltages
are generated (inductors force current to keep flowing using the stored magnetic energy - if you
provide an easy path for the current nothing gets fried)
Thanks for the input people, I will be following and acting on those links - I'm going to need to.
I'm already using a optically-isolated relay - thanks for the suggestion
Unfortunately the diode didn't fix the problem
So
look into a separate 5v supply just for the relay
try a ssr
distance the relay from the arduino
and i thought this was going to be simple and fun ho ho
The Propagator control works OK when no load is on the relay but stops working when the relay switchers a 240V load.
So your problem has nothing to do with back emf from the relay coil, your relay looks as though its fitted with all the right stuff, including isolation.
Your symptoms point to possibly 240Vac wiring layout, keep all your 240Vac wiring well away from the arduino and low voltage wiring.
Can you post a picture of your project so we can see your layout.
An SSR would help, but wiring layout is very important.
Hi,
Why are the green,red,yellow wires running under the relay assembly.
If its the wires to temperature sensor, or any 5V wiring get it away from the 240V wiring, how long is that tri-coloured sensor wire, does it run parallel to the 240V wiring?
It should have its own entry hole to the box on the other side of the assembly.
What type is the temperature sensor?
If it is an analog type, have you got its input bypassed?
What is happening is the mag field produced by the heater current is possibly inducing spikes in the wiring.
If the PCB with the transformer is the 240V to 5V supply, then keep it well away from the switched 240V.
Move the relay assembly away from the other circuitry, turn it so its 240V wiring is as short as possible inside the box.
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
The fact that you are using prototype wiring, long leads, would not help either,
A blank solder shield plugged on top of the UNO with your RTC and connections would be better with soldered and routed wiring, in your final product.
The green, red, yellow wires are from the temperature sensor probe, connecting them underneath seemed an ok idea at the time, externally yes it does mostly run parallel with the 240v cable. The temperature probe is a DS18B20 the wire is one meter long, its digital.
The relevance of the three wires running under the relay has been realised.
The transformer supply’s 9V to the Arduino
I'm quite new to Electronics, Arduino etc and though modules, sensors etc make assembly fun and relatively easy there's obviously peripheral knowledge that has to be learned. Your feedback regards running higher voltages is one, no doubt there will be more to come.
Being 'new' to all this I’d wondered what the 'shields' were all about, why/how to use. I knew using wires just pushed in wasn’t a good idea, I’ll now be looking at shields.
Yes I’ll do all that you suggest, though the distance between high and low volt components still wont be that great.
Will putting in a physical barrier like a metal plate do anything to shield the components ?
A circuit diagram, is using ExpressSCH ok and from that a .jpg ok, - will still take some time though.
Again thanks for the input, I will remake the thing using your suggestions
Rebuilt the thing today moving the high voltage away from the Arduino and the low voltage components, exchanged the mechanical relay for a SSR and moved the temperature sensor cable to exit at the opposite end of the enclosure.
It now works without problem a big thanks for your help
And a thanks to the other people that had input - apology for the duff info i gave
It now works without problem a big thanks for your help
Congratulations on your Persistence (The main Engineering requirement!) ..
Often taking the thing totally apart and putting it back together carefully and with considerations of physical layout and grounding makes all the difference.
2 Days ago I had a nice nRF24L01 Radio example working. Then I 'Neatened" things up and headed down to the 'darkroom' to take photos. See the RoboREDs and radios down a little on THIS: page.
Look closely to see the Dumb Thing Terry Did This Time (tm). And after that nothing worked! Tried "everything" (which of course was NOT)...
So I built up a totally new 'Arduino', connections, nRF24L01 radio with base module (for good 3.3V power and bypass capacitors), and it WORKED! Visually compared it to the 'bad' ones. AWWWW!!! THAT was dumb. When I neatened things up I put the POWER cable on 3.3V not 5V! 3.3V into a 3.3V regulator does not work well. Grrr...
So now it's all working and I'm back to working on good how-to examples of using nRF24L01...
Sometimes when you really hit the frustration point, it's time to Just Start Over.
Just some general thoughts and experience on em noise. Back when I worked for a living we started having noise problems as electronics came in for control systems. Eventually we went to snubbering every relay or other noise source in sight and the problems were radically reduced. The snubber was usually a .1 mfd 600v capacitor in series with a 100 ohm 10w resistor connected across the coil or other inductive device. The flywheel diode works in many cases but is not always the best way. It introduces a time delay in dropout of the relay which may be irrelevant in many cases but every once in a while can cause a very difficult to find problem. It also cannot be used on an ac relay.
One basic rule is that it is always better to stop the noise at its source rather than to let it go then try to protect
sensitive devices from it.
We also separated the wiring into several noise levels so we could keep sensitive wiring away from noisy wiring. Sensitive was op-amp and hard wired logic stuff and noisy was relays and other electromagnetic stuff.
