Tapping a relay - Page 1
Forum section: electronics / beginners

author: StyxF2
author_rank: Contributor
timestamp: December 22, 2023, 12:53:32 pm
content: I have a heater, it doesn't have a fan to push the air through, but I want it to have one. I only want to fan to spin when the heater element is active, and then turn off a minute or so after the heating element turns off.I opened the heater up and found the heating element is controlled by a relay, energised by an MCU output and transistor switch. I want to tap into this circuit somewhere here to get a signal to my own MCU which will control the fan. The MCU/fan part I'm fine with but wanting to know the best way to tap into this. (See attached circuit image). I don't want to interfere with what's currently there, so thinking a comparator for some high impedance. And would it be best to take the input from the MCU (Left side of R1), or from the Q1 base (Right side of R2). Or is there a better way altogether to do this?

author: SuzyC
author_rank: Frequent Contributor
timestamp: December 22, 2023, 01:18:47 pm
content: Take your fan on signal from the left side of R1 from the MCU. You will need to then program your mcu to keep the fan on for a minute or more after the heater turns off.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 22, 2023, 01:57:14 pm
content: Definitely you must read the output from the MCU side, left of the R1. The voltage on the Q1 base (right side of R1) will never go higher than 0.6V, because the max voltage drop on the BE junction of the transistor can not be more than 0.6 - 0.7V, and 0.7V will be too little for a logic 1 on your MCU.You don't need any high impedance comparator, the logic level from the CTRL point can be read directly by a digital input of your MCU. In practice, I would read the logic level through a series resistor, so yet another 2k from CTRL to a DI on your MCU of choice.Preferably, power your MCU at the same voltage with the heater's MCU. If not, you'll have to make sure they work at the same voltage, or else you'll need a voltage lever adapter between 3.3V and 5V (or whatever voltage might be using that MCU vs yours).Before modifications, make sure your heater has a bi-metal switch, or a thermal fuse, to protect against fire if the heating element get stuck in the ON state. That is often mandatory to have in many countries, but check your heater just to be sure.The idea is, if something (anything, a loosed wire, a software bug, etc) goes wrong, and the heater remains always on, youmusthave some electro-mechanical protection to cut the power. Not MCU protection, but a thermal fuse (not to be confused with normal fuses - a thermal fuse melts and cut the circuit by temperature only, no matter the current). Sometimes the thermal protection is played by a so called re-trigerable thermal fuse, a mechanical switch auctioned by a bi-metal: when too hot, the bi-metal band flips, and turns off the current. Some models return by themselves to the contact position when they cool down, other must be reset manually. Some other thermal fuses have inside a temperature sensitive alloy, and once it melts, the fuse has to be replaced.

author: StyxF2
author_rank: Contributor
timestamp: December 22, 2023, 02:54:01 pm
content: Thanks SuzyC and Roeorge. Makes good sense, and will make it all easier without needing a comparator. The heater does have 2 thermal switches which cut-off the mains directly, its a xiaomi retail heater. The circuit diagram was what i drew just by eyeballing the PCB... Will report back how it goes...

author: Siwastaja
author_rank: Super Contributor
timestamp: December 22, 2023, 05:40:44 pm
content: Before R1, in other words, "CTRL". You can use a series resistor an order of magnitude larger than R1 (e.g. at least 22k) so that if your microcontroller circuit is unpowered or accidentally configured as output, then it won't affect the level enough to get the transistor in wrong state.Transistor collector is another option. Again, series resistor is a good idea.