![]() ![]() In any case, my answer above is still valid. I might have skipped this question entirely if I had seen it post-edit for the first time. That's easier to answer than having to first dispel myths. Your original question was better, because it simply asked how to do something. ![]() I see you have substantially changed your question while I was writing this answer. The diode gives the inductive kickback current a safe place to go while the current dies down on its own due to the resistance of the coil. ![]() Without the diode, that would require abusing the transistor. When anything tries to shut off the current through it abruptly, the inductor will make whatever voltage it takes to keep the current flowing in the short term. The relay coil has a significant inductive component. 13.5 mA is well below that, so no problem.ĭ1 is not optional, even though it looks like it doesn't do anything. This means it can support up to 20 mA collector current. To guarantee the transistor stays solidly in saturation, let's say we only ask it for a gain of 20. This means there will be 1 mA of base current when the left end of R1 is held at 5 V. You didn't say how much coil current the relay draws at 24 V, so I picked an example part I had in my system (Zettler AZ8-1CH-5DSE).įigure the B-E drop of Q1 is about 700 mV. The real problem is driving a 24 V relay from a 5 V digital signal. So, for \$\mathrm\$.Converting to a "24 V signal" is missing the point. Since the BJT should operate in saturation mode here, a base current of one-twentieth to one-tenth of the load current is sufficient (this is a practical tip for small-signal BJTs). I selected BC547 as the load switching transistor because it has relatively high h FE (I personally tend to use 2N390x for relatively faster switching applications). Simulate this circuit – Schematic created using CircuitLab You can think of it as something like h FE of BJTs.Īs for using it to switch a load, here's a circuit you can use: The ones with relatively higher CTR (Current transfer ratio) would be better, so HCPL817 (any CTR rank starting from B) can be a good option.ĬTR for an optocoupler is simply a ratio of collector current to LED current. Nevertheless, I'll recommend you to use any 817 series optocouplers as they are too common and very easy to find anywhere in the world. Recommending specific parts is rated as off-topic here. How would I redesign my circuit to include an optocoupler and which optocoupler whould be necessary, considering I'm not sending data but only an on-off signal that doesn't need to switch back and forth too frequently? But it can be used as a pre-amplifier (or pre-driver) for a switching transistor. You cannot directly switch a relatively high-current load with an optocoupler. The available output current with an optocoupler is limited to small values. From the break datasheet I gather that it takes 0.175A of current which seems to be too much. There is so much information on optocouplers and here in chapter 3 I read that the available output current with an optocoupler is limited to small values. I have also noticed that connecting an oscilloscope including its GND will remove the 1.9V.įrom my research it leads me to believe that it might be a ground loop problem.Īs such I would like to electrically isolate the Arduino and the 24V circuit using an optocoupler to switch the 24V on and off with a 5V digital signal from the Arduino. (during this test, the 24V positive cable was disconnected) If I disconnect the GND cable, the 1.9V are gone. I have found an issue with the circuit that I have soldered where when connecting the GND from the 24V DC source to the GND of the Arduino, I get a voltage of ~1.9V on my digital pins on the Arduino. This is the datasheet and the brake is found on page 84 MCNB 2GR.Īt the moment I am using a 2N3904 transistor to switch the 24V which is generated from a Mean Well RS-100-24 DC power supply. The the important part is the right half, BRAKE1 is the spring applied brake with a resistance of 137 Ohm. ![]() The current circuit looks like this (please excuse if I'm using wrong symbols, I'm learning by myself): I am currently redesigning a circuit to switch a spring applied brake using an Arduino Micro. ![]()
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