| Electronics > Beginners |
| Current-limit circuits for 78XX voltage regulators |
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| khatus:
Hello guys i Want to find the value of RSC and R1 in the following circuit In this circuits, Q1 is the high-current pass transistor, and its emitter-to-base resistor has been chosen to turn it on at a given load current. In the circuit, Q2 senses the load current via the drop across the resistor RSC cutting off drive when the drop exceeds a diode drop(i.e 0.7V). Now i want to calculate the value of RSC and R1 But How can i solve this problem??? For linear voltage regulator |
| RES:
check this application note: http://www.ti.com/product/LM340/technicaldocuments (direct link) http://www.ti.com/lit/gpn/lm340 on page 16 you see your example.(figure 25, top right) two formulas: RSC = 0.8 / ISC R1 = βVBE(Q1) / IREG Max (β +1) – IO Max |
| iMo:
It will not work as a current limit. When the Rsc creates 0.7V the Q2 starts to conduct and the excessive current will flow through 7805. You have to put a resistor R2 into collector of the Q2. Set R1 such you get enough current for 7805 (10mA). For example 50ohm. The R2 for example 50-100ohm. Edit: I would put R3=100ohm into the Q2 base. With Rsc 1ohm it will limit to 1A, with Rsc=4ohm to 250mA, and so on.. |
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