I'm aiming to find a transistor that will be used to switch a relay, and can handle the following specifications. I think I've found one and would appreciate having the calculations checked and checking if I have missed anything important:
A device is an Arduino and is connected to base of transistor. Device voltage: 5V Device source current: <= 5mA The collector pin of the transistor is connected to a relay. Collector voltage (Vce): = 5V Collector current (Ic): = 90mA Transistor power dissipation: >= ( 5 * 90 ) >= 450mW DC current gain F.O.S. = 5 DC current gain >= ( Collector current / Device source current ) * DC current gain F.O.S. DC current gain >= ( 90 / 5 ) * 5 DC current gain >= 90
I've found the BC337 transistor by ON Semiconductors and using this datasheet.
Collector - Emitter Voltage (Vceo) is 45 Vdc, which covers the 5V required.
Collector current - continuous (Ic) is 800 mA, which is well above the 90mA the circuit requires.
Total Device Dissipation @ Ta = 25c (Pd) is 625 mW, which is above the 450mW required.
DC current gain is given for Ic = 100mA and Vce = 1V, which the Ic is quite close to the collector current (90mA), so this should be okay. The Vce given is 1V, although the circuit runs at 5V, but from all that is understood, the Vce is a bit on the meaningless side once the transistor is saturated.
The resistor is needed to restrict the transistor sourcing too much current from the device, which is limited to 5mA.
Base − Emitter on voltage (Vbe(on)) for the transistor has a maximum of 1.2V, which will be used for a worst case scenario. So, the resistor connected between the device and the base pin should drop:
Resistor voltage drop = Device voltage - Base − Emitter on voltage Resistor voltage drop = 5 - 1.2 Resistor voltage drop = 3.8
This means the resistor must have a resistance value of:
Resistance = Voltage / Current Resistance value = Resistor voltage drop / Device source current (A) Resistance value = 3.8 / 0.005 Resistance value = 3.8 / 0.005 Resistance value = 760 Ohms
- Is this all correct, or have I made some fantastic error or overlooked a large problem?
- Vbe(on) is quoted for Ic=300mA and Vce=1V. I think I understand that Vce isn't too much of a problem once the transistor is saturated, but the collector current is 300mA, while I'm passing only 90mA - is this something to be concerned about, and why should or shouldn't I be concerned over this?