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enter image description here enter image description here

I can't get the relay to work in my example. Transistor gets really hot. When connecting&disconnection relay wire between diode and transistor and I can hear soft click from relay. But when changing digital pin output high/low then relay doesnt react.

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    \$\begingroup\$ The schematic shows the negative side of the 12V connected to the relay/collector side, and the positive side unconnected. Is your circuit set up this way? What transistor are you using? \$\endgroup\$
    – Oli Glaser
    Commented Sep 28, 2013 at 23:15
  • \$\begingroup\$ The 2n3904 is a 100mA npn transistor. Is 100mA enough to turn on your relay? What relay are you using? \$\endgroup\$
    – Passerby
    Commented Sep 28, 2013 at 23:27
  • \$\begingroup\$ I am using this relay from radioshack. Nominal current is 133mA so I guess you are right. It doesn't get enough current to trigger. \$\endgroup\$
    – jM2.me
    Commented Sep 28, 2013 at 23:29
  • \$\begingroup\$ @jM2.me 100mA to 133mA isn't too much. The coil is 90Ω, and needs only 60% of the typical 12v voltage to work (7.2v / 90Ω = 80mA). 133mA is also not excessive to the transistor. It's not ideal, and you might want a beefier transistor like a common 2n2222 for just piece of mind, but If anything it depends on which manufacturer's 2n3904 you are using, as it does come in 200mA max versions. With your base resistor, you are allowing up to (4.4mA * 30 hfe = 132mA). As you have noted, the wiring was the issue. \$\endgroup\$
    – Passerby
    Commented Sep 29, 2013 at 0:37

1 Answer 1

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If you are using a 2N3904, then you have it the wrong way round, notice the emitter is at the left (flat side facing):

enter image description here

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  • \$\begingroup\$ This was the answer. On package it didn't say top/bottom view therefore I assumed it was top view and ended up burning three transistors. \$\endgroup\$
    – jM2.me
    Commented Sep 28, 2013 at 23:49
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    \$\begingroup\$ @jM2.me - If a drawing doesn't explicitly say the perspective, check three times and look if there are other datasheets with a better drawing. I've been bit by unclear drawings once I had boards made. That's not fun to fix. \$\endgroup\$ Commented Sep 29, 2013 at 5:05
  • \$\begingroup\$ @OliGlaser-Can you please tell the correction is to flip the V2 power supply or use a PNP transistor? Thanks \$\endgroup\$
    – AKR
    Commented Sep 29, 2013 at 12:05

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