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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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how to calculate the base resistor
please can anyone show me how to calculate the base resistor for an NPN transistor used as a switch. the supply voltage is 12 volts, the load is a 12 volts relay with a resistance of 300 Ohms.
thank you. |
#2
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how to calculate the base resistor
" wrote:
please can anyone show me how to calculate the base resistor for an NPN transistor used as a switch. the supply voltage is 12 volts, the load is a 12 volts relay with a resistance of 300 Ohms. thank you. When the NPN transistor is saturated (fully on), the emitter-collector voltage will be about 0.2-0.3 volts. Using the other values you have, you can calculate Ic; the current flowing through the collector (and the relay). To saturate the transistor, the base current must be greater than Ic / beta where "beta" is the minimum current gain of the transistor. The last piece of information you need is that the base-emitter voltage is one "diode-drop"; about 0.7 volts. |
#3
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how to calculate the base resistor
On 10/8/2017 1:12 PM, natp wrote:
" wrote: please can anyone show me how to calculate the base resistor for an NPN transistor used as a switch. the supply voltage is 12 volts, the load is a 12 volts relay with a resistance of 300 Ohms. thank you. When the NPN transistor is saturated (fully on), the emitter-collector voltage will be about 0.2-0.3 volts. Using the other values you have, you can calculate Ic; the current flowing through the collector (and the relay). To saturate the transistor, the base current must be greater than Ic / beta where "beta" is the minimum current gain of the transistor. The last piece of information you need is that the base-emitter voltage is one "diode-drop"; about 0.7 volts. It's customary to overdrive the base. I normally use a factor of 10. It's also customary to put a reverse diode across the coil so that the turn-off transient doesn't overvoltage the collector. |
#4
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how to calculate the base resistor
On Sunday, 8 October 2017 19:53:50 UTC+1, wrote:
please can anyone show me how to calculate the base resistor for an NPN transistor used as a switch. the supply voltage is 12 volts, the load is a 12 volts relay with a resistance of 300 Ohms. thank you. 12v 300ohms is 40mA. Assume beta of at least 30, so we need 40/30mA into the base = 1.3mA V across base R is 12v - 0.7v = 11.3v R=V/I so R = 11.3/1.3m = 8.5k. Preferred values below that to give a bit more i are 8k2 or 6k8 or 4k7. NT |
#5
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how to calculate the base resistor
On Sunday, October 8, 2017 at 6:53:50 PM UTC, wrote:
please can anyone show me how to calculate the base resistor for an NPN transistor used as a switch. the supply voltage is 12 volts, the load is a 12 volts relay with a resistance of 300 Ohms. thank you. Thanks all your suggestion are well taken |
#6
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how to calculate the base resistor
On 09/10/17 07:33, mike wrote:
On 10/8/2017 1:12 PM, natp wrote: " wrote: please can anyone show me how to calculate the base resistor for an NPN transistor used as a switch. the supply voltage is 12 volts, the load is a 12 volts relay with a resistance of 300 Ohms. thank you. When the NPN transistor is saturated (fully on), the emitter-collector voltage will be about 0.2-0.3 volts. Using the other values you have, you can calculate Ic; the current flowing through the collector (and the relay). To saturate the transistor, the base current must be greater than Â*Â*Â*Â*Ic / beta where "beta" is the minimum current gain of the transistor. The last piece of information you need is that the base-emitter voltage is one "diode-drop"; about 0.7 volts. It's customary to overdrive the base.Â* I normally use a factor of 10. Beta is reduced near saturation. 10x is an attempt to compensate. |
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