This PSU has been abnormally advised for current-hungry ham radio transceivers. It delivers cautiously about 20Amps at 13.8V. For lower currents, a abstracted accepted attached output, able of 15ma up to a absolute of 20A has been added. Let us see what we accept got here. The ability agent should be able to bear at atomic 25A at 17.5 to 20V. The lower the voltage, the lower ability dissipation. The rectified accepted will be �ironed� by the C1, whose accommodation should not be beneath than 40.000uF, (a aureate aphorism of about 2000uF/A), but we acclaim up to 50.000uF. This accommodation can be congenital up by several abate capacitors in parallel. The abject of this architecture is a simple 12V regulator (7812). The achievement voltage can be brought to adapted amount (here 13.8V) by two alien resistors (R5 and R6) application this formula:
U= 12(1+R5/R6)
The low currents (here 15mA) will accumulate the 7812 in its approved function. As anon as the accepted rises over 15ma, the voltage bead on R4 will open the Q3, absolutely administration the aerial achievement current. This is a PNP transistor (Ic>25) and accepted addition agency of at atomic 20. The one that has been activated and accurate actuality is the 2N5683. The accepted attached attrition RL, for the best achievement of 20 Amps should be 0.03 Ohms, rated at
least 15W. You can use the attrition wire or about-face several resistors in parallel, accretion the resistance/power values. Ethics for added currents can be affected by the rule:
RL=0.7/Imax
The RL and Q2 (3A PNP such as BD330) anatomy a abbreviate ambit automatic fuse. As anon as the best accepted alcove 20Amps, the voltage bead over the resistor RL will accessible Q2, and appropriately absolute the B-E Accepted of Q3. Alongside to Q2 is Q1, which lights the LED 1 whenever the accepted attached ambit is active. When the �fuse� is active, the Q2 bridges the R3, so the abounding accepted would breeze through the IC1, and accident it. Therefore the R4 is inserted, as to absolute the IC1 accepted to 15mA. This makes it accessible to run the IC1 afterwards any cooling aid. The LED 2 will ablaze up every time the PSU is switched on.
There is an adjustable accepted limiter in alongside to the anchored output, appropriately accouterment adjustable accepted antecedent for abate currents.
This ambit is actual simple too. You will apprehension that there is no accepted analysis resistor. But it is absolutely there, in a anatomy of the Rds-on attrition of the N-channel FET, which absolutely handles the amount blow from the source. The action of the FET is apparent in the diagram 2. When the accepted Id is rising, the astriction Uds over the attrition Rds rises actual boring in the beginning, but actual fast afterwards the knick. This means, that afore the knick the FET behaves as a resistor but afterwards it, works as connected accepted source.
The D2, R3 and B-E affiliation of the Q4 will faculty the Uds voltage of the FET1. When the voltage rises enough, the Q4 will adjustment the FET1 aboideau to mass, and cut the accepted breeze through the FET 1 off. However, to accredit the FET1 to open, there is assertive aboideau voltage necessary, which in this case is brought up by the voltage affiliate consisting of R8, Z1, P1 and R9. So the best Aboideau voltage will be the one of the Z1, and the basal will be about 3V6. The Z1 voltage (Uz1) will appropriately actuate the max accepted abounding through the FET 1.
The diagram 2 will appearance that for 5 Amps the Uz1 should be 5V6, and for 20Amps about 9V6. The Capacitor C4 will actuate the velocity or the acknowledgment time of the limiter. 100 uF will accomplish the acknowledgment time to be about 100ms, and 1n will accomplish it 1us.
Within the advised limits, the P1 will absolute the accepted achievement in the ambit of 15mA to 20A. You can use both achievement simultaneously, but the absolute achievement accepted will be bound by the amount of the RL. This PSU can be congenital additionally for college outputs, as continued as the agent will handle the accepted requirements, and you accommodate acceptable cooling for the Q3.
source : www.electronics-lab.com
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