For a project, I plan on using a SSR to cycle on/off a load (an induction heater). I want to go with an SSR for quiet operation and the fact this will (in use) cycle on and off for hundreds of cycles per use, and want to ensure longevity.
Given that SSRs typically fail closed (so I've read), I want a backup to prevent runaway heating. My plan was to put a standard electro-mechanical relay in line with the SSR, and have the setup code close this relay to allow current to flow (probably put it downstream of the SSR current). Next, I need a way for the Arduino to sense if current is flowing through the SSR after the SSR was supposedly shut off (indicating SSR failure) and if it DOES sense current, release the electro-mechanical relay, opening the circuit, stopping the heater.
I'm not an expert by any stretch, so I need help determining the best way to set this up in terms of which hardware components would be best (ACS712 perhaps?), as well as if my proposed circuit is acceptable or if there are better methods. Also in reading about the ACS712 I read these are prone to false readings so you must make several readings and average them...is this true for getting a simple yes/no on current, or is a single reading good enough for simply determining IF current is flowing, regardless of the amount?
The current flowing through the SSR (on the load side) would be 24V DC at probably 15A, but it may be as low as 10A. I'd like a single solution that can handle either current.
Also, not sure how any proposed sensing device works, I suppose I can program a pin to listen for a change, so if there is any advice you can give me on what coding would look like to sense any current, that would be appreciated as well.
If DC, How about using the SSR for high-side drive, and measure the voltage applied on the load?
EDIT:
Regarding the you additionally edited contents...
Current can be measured even with some modules with ACS712.
Note:
That the model number differs depending on the measuring maximum current.
Please found a 20A or 30A model.
Its seems that what you want to do is detect when the DC SSR fails and not to measure current. If the induction heater operates with a relatively high frequency square wave signal, then it should be easy to detect failure mode (100% duty cycle or steady 24VDC). If you use a resistive voltage divider to bring the voltage down to about 2.2V square wave, then filtering would bring it to 1.1V for normal operation. When it fails ON (100% duty), the signal will increase to 2.2V.
Here, the failure detection response time is about 10ms ...
For 24V DC the ACS711/ACS712 etc are good choices (note they are not properly rated for mains, for that
a current transformer is the way to go).
Using a shunt resistor for high currents is problematic as you have to amplify the voltage across the shunt
if its sized not to waste lots of heat. The ACS7xx family of sensors are Hall-effect and there is no shunt
resistor, just an internal copper bar.
dlloyd:
Its seems that what you want to do is detect when the DC SSR fails and not to measure current. If the induction heater operates with a relatively high frequency square wave signal, then it should be easy to detect failure mode (100% duty cycle or steady 24VDC). If you use a resistive voltage divider to bring the voltage down to about 2.2V square wave, then filtering would bring it to 1.1V for normal operation. When it fails ON (100% duty), the signal will increase to 2.2V.
Here, the failure detection response time is about 10ms ...
Would it still be a square wave if I tapped into it before it goes tot he induction board/unit? I'd think only the output coil would have a square wave. A constant 24V would be fed from the power supply to the SSR, then the SSR would feed into whatever method I'm using to detect the 24V, then that would proceed to feed into the Electro-Mechanical Relay, and then that would feed power to the induction heater.
I tried playing with an online schematic builder but apparently has a steep learning curve. I might need to play around with it a bit more. Separately, any tips on which software is easiest to use?
Ah, so no Arduino involved. Yes, would at least need schematic and link to parts but will be difficult to get much help here unless someone else has built this or has experience with induction heaters.
If power for the Arduino has a common ground with the 24V power supply, and if it's a constant 24V where you want to test for shutoff, then I think you could just divide down the 24V with resistors to something a bit under 5V, and read that as a digital input on the Arduino. If the supplies are separate, you can use an optocoupler (like a 4N25) with the right input resistor and with the output collector connected to a digital input pin with INPUT_PULLUP enabled.
Can you not just switch the relay off when you turn the heater off ?
Also if you measure the heater voltage when the SSR is off , it should be zero - if not you have a fault !
So get your software to briefly turn the heater off via the SSR then measure the voltage , if all is well turn the heater on again !
Don’t forget to have an over temperature trip
Too ( which would also show up a fault )
