Electronics > Projects, Designs, and Technical Stuff
How to pick a transistor to switch a relay
TomS_:
Hi everyone.
Im working on a little project at the moment, a relay timer, which will connect and disconnect a battery from a solar charger during certain hours of the day. The reason for doing this is because the charger draws from the battery to power itself and provide load output for lights etc.
But I dont want the charger drawing from the battery - I just want it to be a charger. I suppose I could use a purpose built charger for this, but the ones I already have I got for free, and I felt like doing a little project anyway, so what the hey. :)
I now need to extend this a little bit and add a second relay, because according to the manual I should connect battery before solar so the charger detects and uses the appropriate charging voltage, and at the moment solar is hard wired and the battery is switched in later, so the second relay will allow me to sequence those connections correctly.
But my question. Im using a BC547 as a low side switch for the relay at present, simply because I have a stock of them. "It works fine" (for now?) but I wonder if there is a "better transistor" to use for this kind of application, and how do you go about finding/choosing it? i.e. what sort of parameters are you looking at to determine the suitability of a transistor for switching a relay.
Ive included a back EMF diode (something like a 1N4004 or thereabouts, also because I have a stock of them) next to the relay itself.
Thanks!
moffy:
You need to be concerned about the maximum current the transistor has to switch, its voltage rating, and power dissipation (related to saturation voltage). If the BC547 works OK, then the 2N2222A should work better as it's designed for switching. Please provide more info about the relay coil resistance and the voltage to the coil. Kind of working in the dark.
TerminalJack505:
Ben Krasnow over on the Applied Science YouTube channel did a video that was pretty much about what you are asking about...
https://youtu.be/8DMZSxS-xVc
james_s:
Read the datasheet for the relay and for any transistors you are considering. Look at the current drawn by the relay coil, the voltage you are using, the current available from whatever source is controlling the transistor, the saturation current and gain and go from there. Low voltage switching is a pretty rudimentary use of a transistor and any number of parts will perform suitably. Which one is "best" probably boils down to what you have on hand or can easily get at a reasonable price with specs that meet your requirements.
DBecker:
--- Quote from: moffy on February 06, 2020, 12:19:32 am ---You need to be concerned about the maximum current the transistor has to switch, its voltage rating, and power dissipation (related to saturation voltage).
--- End quote ---
I would worry much more about the inductive "kick" of the relay than the simple parameters. There are parts designed specifically for switching relay coils because of the challenges.
That said, this doesn't seem like an application that requires a power-hungry relay. What voltage and current does the panel generate?
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