Timeline for Boosting current to trigger 3v relay
Current License: CC BY-SA 4.0
13 events
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| Nov 6, 2019 at 3:37 | comment | added | jonk | @TravisBallard \$R_{15}\$ helps to dissipate that stored coil energy more quickly. That's why it's optional. Lots of folks don't worry about it and I can't complain. But I added that resistor to roughly double the speed at which the energy would dissipate. If you don't want it, though, feel free to dump it. It's not required. \$D_{21}\$ doesn't have to be a 1N4148, though. If you have different diodes on hand, chances are they will work okay. Any remaining questions? | |
| Nov 6, 2019 at 3:34 | comment | added | jonk | @TravisBallard \$R_{15}\$ and \$D_{21}\$ are there to snub the stored energy in the relay coil when you direct that the relay should be turned off. While it is on, current is flowing through the coil and creates a magnetic field storing energy. When you cut the coil off from its source, the magnetic field must collapse. It's physics stuff. And in collapsing, that energy has to be dissipated. Those two parts provide an "easy path" for this dissipation and helps to make it more rapid, too. Without at least the diode, it's possible to damage the BJTs when they are hit. The resistor helps. | |
| Nov 6, 2019 at 2:32 | comment | added | TravisBallard | is it just so that it can charge the base to feed the base on Q1? | |
| Nov 6, 2019 at 2:28 | comment | added | TravisBallard | @jonk What is R15 and D21 for? and what does D21 hook to? one end goes to the 6.8 ohm resistor but the other end goes to the 3.3v output on the MCU? I know you said it's optional, but if it's better for the circuit to have it then I should probably add it. I ordered the 1N4148 anyway after I saw it on there. I just didn't know what it's purpose was so I was curious. (well, i think, i know the diode is to protect against reverse polarity, but i meant the whole thing. resistor, diode, and hooking to 3.3v) Thanks! | |
| Nov 5, 2019 at 5:52 | vote | accept | TravisBallard | ||
| Nov 4, 2019 at 20:30 | comment | added | Hearth | I don't see any reason to remove the resistor, I was just wondering what its purpose was. Shouldn't do anything positive or negative there. | |
| Nov 4, 2019 at 14:14 | comment | added | jonk | @Hearth Got to a PC, so added what I forgot to add. Thanks for the clue! | |
| Nov 4, 2019 at 14:14 | history | edited | jonk | CC BY-SA 4.0 |
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| Nov 4, 2019 at 13:49 | comment | added | jonk | @Heath I added it because the OP said they'd used one and because it didn't hurt. BJTs don't really need one for this circuit. The relay won't turn on from static charge like a FET might. But it was harmless. Should I remove it as confusing? I like the addition of the diode. I simply forgot, is all. I'm away from a PC right now, but can address it later when I get a chance. | |
| Nov 4, 2019 at 13:43 | comment | added | Hearth | What's the purpose of R14? Pulldown resistors like that I usually see used with FETs, not BJTs. Also, might be a good idea to put a flyback diode on that relay, though I would assume the BC547s can probably handle the spike. It's just good practice. | |
| Nov 4, 2019 at 13:30 | history | edited | jonk | CC BY-SA 4.0 |
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| Nov 4, 2019 at 6:42 | history | edited | jonk | CC BY-SA 4.0 |
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| Nov 4, 2019 at 6:12 | history | answered | jonk | CC BY-SA 4.0 |