| Electronics > Beginners |
| Data sheet question. |
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| sureshot:
I have been looking through data sheets to glean some circuits, psu circuits to be more precise. I'm unsure of two resistors value and why there resistance is this high. Normally these circuits have these resistors in a few tens of ohms, but in this schematic they seem very high. The two resistors I'm referring to are the base rsistor for the 2N2905 at 5K and the emitter collector resistor of 500 ohms for the TIP73. I've seem maybe up to 100 ohms to maybe 150 ohms, but never this high as in this schematic. The image is from a genuine data sheet, would anyone know why there so high in value ? Thanks for reading, any help appreciated. |
| sureshot:
With the above posts question, its as this image below. I know the transistors are different, but there still being used for same purpose. The rsistors in the image below are a lot less resistance than in the image in the post above. And I've no idea why. |
| lapm:
Thats current drive. LM317 handels voltage and transistors handle current. 5K resistor is there to limit first transistors base current since its not power transistor and 500 ohm is there to ensure second transistor starts to conduct when enough current flows. Very handy if you need more current drive capasity then lm317 has by itself. |
| ArthurDent:
This schematic may be similar to what you are referring to. In this circuit the added pass transistor is a low gain power transistor so the base resistor is a low value because more current is needed to control the low gain pass transistor. In your circuit there are 2 stages used where the first is a high gain lower power transistor that will work properly with a higher value base resistor. This transistor drives the lower gain higher power pass transistor and the 500 ohm resistor supplies voltage to the first transistor. |
| David Hess:
Those resistor values are not critical in any way. The series base resistor protects the 2N2905 from excessive base-emitter current at the start of current limiting. The base-emitter shunt resistor on the TIP73 sinks leakage and removes base charge for better response. |
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