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I am planning to use miniature BJT in a circuit , but I haven't used them before, The specs of NXP BC847BMB TRANS, NPN, 45V, 0.1A, SOT883B shows 100mA as collector current, I am worried if it can switch a G6K-2G-Y-DC5 relay.

The relay mentions 21.1mA as col current. I would be using a 3.8V value for VCC. Please help in understanding how to do the calculations and determine if this will work or not.

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3 Answers 3

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The link to the relay is broken, but it's for you to look up anyway.

You already know the transistor can handle 100 mA, so the question is whether the relay will require more than that. You say the relay only requires about 21 mA, which is obviously less than 100 mA by a good margin, so I don't see what you are actually asking.

You say you'd be powering the relay from 3.8 V. The transistor can handle 45 V, so that is clearly within its capabilities. That is again so obvious it's hard to see what you are actually asking.

You have to make sure that your relay can really operate from 3.6 V (the transistor will eat up about 200 mV when on) coil voltage, and that it really does draw less than 100 mA when it does. This again should be obvious, coming directly from the relay datasheet.

Two things that are less obvious to watch out for are: 1 - make sure to put a reverse polarity diode across the relay coil. 2 - Make sure the transistor can dissipate the power. That should be a no-brainer if all the current thru it comes when it is saturated. Since that can't be more than 100 mA and saturation is probably (your job to check) 200 mV or less, it can't dissipate more than 20 mW in that case.

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The "DC5" at the end of the relay part number indicates that the relay is intended for operation on 5 volts. If you intend to operate it from 3.8 volts, you probably need the DC3 version for reliable operation.

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It is probably adequate to switch the 3V version with about 50% more current draw ( which I think you'll need)

However, unless you're counting pennies you may wish to use something like the PBSS2515MB, which is rated for 500mA. The lack of an SOA ( safe operating area ) graph makes me a bit cautious when switching an inductive load, and more so if you intend to use a higher voltage clamp than Vcc.

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