And you’re ready for your next project where you might have 2 or more relays. You could use another drive in the package to turn on the LED. You waste a lot if you just want one relay, but, it’s cheap (about $.70, and in stock at Mouser) and a good go-to part for driving relays. Note that the input voltage is 5V though. They are relay drivers with the snubber diode built in. There are a number of parts in this series. Don’t pinch pennies for a low volume design.Īnd finally take a look at a ULN2003. ![]() The current through the diode will be the same as the coil current, so if the coil current was 70 mA, I’d look for a diode that could conduct 150 mA or more. For this use case, you want to be sure the diode has a sufficient voltage rating when it’s reversed bias (taking into account what spikes might be on the +12). Note that the design may add some noise or spike to the +12 since the diode will be dumping the energy stored in the coil back out to the 12V line. If you don’t have a scope, since all fun hobbies are expensive you can get a nice 2 channel 100 MHz digital scope for reasonable price.ĭesigners have their favorite high-speed diode, and you can see one that works OK in elekgeek’s post. When you get things working, if you have an oscilloscope you can trigger on the SPK_RELAY control line and look at the A2 to see all this happen. In the design, the delay in having the snubber diode conduct means that the A2 connection to the coil could briefly rise far above 12V, which might be more than the reverse bias on the LED than it can take or might exceed the voltage rating of the transistor. So when you de-energize the relay, and the inductor/coil does what inductors do when you stop the flow of current, there is a big voltage spike that will occur before the diode starts to conduct. The 1N400X parts were designed for rectification of low frequency AC. If you can, try to make sure the part you pick for the transistor/FET and snubber diode have a part with “bigger” specifications available in the same size package. Direct coil drive or logic level input (Any pin can drive, and you’re not using your uP as a relay driver).Drop-out speed for contact life (probably not a concern with speakers).Spikes on the power supply line (might see a volt of spike).Summarizing: what are your worries about: Note that the resistors used in the FET design deal with this nicely, and a biased transistor (MMUN2214 type) does this too. I suspect that with a speaker load, the number of cycles and current make this a minor concern.ĭo you want to drive the coil directly or is the cost of a transistor/FET not a problem? With a transistor/FET your input is now any logic level from 3 to 5 volts, at low-to-no (transistor to FET) current in.įWIW, I really like the pre-biased transistors, if the input is open (and it just about always is for while until the uP wakes up and sets up the IO) you can be certain the relay will be off. Is the load on the contacts such that the slower drop time due to the diode will cause contact life concerns? (and never, never use a 1N400x rectification diode as as snubber !). I’ve always used 12V relays whenever I can, seems like the 5V coils run hotter, and the current eliminates using the smaller 100 mA transistors/FETs becuase it is to close to their limit, but, sometimes you just don’t have 12 V…Īre you worried about the spike on the power supply when the relay is turned off and the inductive kick dumps power back into the supply? There are probably thousands of alternatives here, but some more details could help. Perhaps, if you drive the transistor directly from a microcontroller output, you could consider a darlington transistor with high hfe.īut on the other hand, if the Arduino pin is 5V, the 1k base resistor will give a base current of several mA, so even a BD139 could be enough. I looked at a random 10A/230V relay with 12V coil voltage, and it has a current draw around 55mA nominally, so a transistor with at least 0,2A current capacity, 50V voltage rating and hfe >100 could be okay. This is mainly a factor when designing for long contact life of the relay. What voltage is your VCC and what relay (specific model or coil voltage/current/resistance) do you use?įor typical small relays a 1A rectifier diode 1N4007 or equivalent could be a suggestion.īe aware that the switchoff time of the relay is lengthened by the diode in order to compensate for this sometimes a transistor that can withstand higher voltages and a zener type diode is used, such that the reay coil voltage during the collapse of the magnetic field is higher than the 0,7V you get in the above scenario.
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