Timeline for Help me calculate which transistor I need
Current License: CC BY-SA 4.0
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| May 31, 2018 at 17:16 | comment | added | Sean | The datasheet says the coil resistance is 4.75k+/-0.25k. Let's call it 4.5k. That means its load is 12V / 4.5k = 2.7 mA. If we take the BC109 transistor, the current gain listed on the datasheet is around 200 when the base is driven with 2mA (i.e. up to 400mA), which is more than enough for the solenoid. That means that you need a resistor that limits the base current to 2 mA. As you will use 3.3V from the Pi, and need 0.7V at the base, the resistor must drop 3.3-0.7V while providing 2mA, so it should be 2.6/0.002 = 1.3k. In reality, anything from 0.5-5k would probably work in this case. | |
| May 31, 2018 at 16:48 | comment | added | Detail | It's a solenoid valve for controlling water flow:coolcomponents.co.uk/products/solenoid-valve-12v-3-4 | |
| May 31, 2018 at 16:16 | history | edited | Sean | CC BY-SA 4.0 |
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| May 31, 2018 at 16:14 | comment | added | Sean | @Detail: not quite. The 12V at the solenoid is fixed by the 12V external supply you give it. While the voltage you apply between the base and emitter must be more than 0.7V to fully switch the transistor on, the current is also important. The 3.3V from the Pi is fine, but you must set the current flowing into the base to (a) enough to allow the solenoid to operate (remember the transistor's current gain is typically 100ish, but can be lower or higher - check the datasheet), and (b) not so much that the current flowing into the base exceed's the transistor's rating. What solenoid are you using? | |
| May 31, 2018 at 16:07 | comment | added | Detail | Ok, so if I want 12v at the Solenoid, I need to provide 0.12v to the base? Is this in addition to the 0.7v above ground.. making a total of 0.82v? So I need to add a resistor in between the 3.3v pin and the transistor base, which reduces 3.3v down to 0.82v. Is that right? | |
| May 31, 2018 at 15:57 | history | answered | Sean | CC BY-SA 4.0 |