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Soft Start for Amplifiers

saves fuses and electronics

Soft Start for Amplifiers
No worries, it's happened to everyone: you switch on an electrical device and the mains fuse blows. In most cases that does not mean the equipment in question is faulty; the cause is usually much more prosaic: the switch-on current surge made the fuse melt prematurely. This inrush current (or surge current) occurs in traditional power transformers as well as in switch-mode power supplies, although it has different causes in either type of power supply.
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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor GREEN and GOLD members can exclusively download these files for free as part of their membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service.

Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

Download the BOM list
The BOM (Bill of Materials) is the technically exhaustive listing of parts and other hardware items used to produce the working and tested prototype of any Elektor Labs project. The BOM file contains deeper information than the Component List published for the same project in Elektor Magazine. If required the BOM gets updated directly by our lab engineers. As a reader, you can download the list here.
Want to learn more about our BOM list? Read the BOM list article for extra information.
Component list

Components List


R1,R3 = 33Ω carbon film, 5%, 0.25 W, 250V

R2,R4,R25,R26 = 680Ω carbon film, 5%, 0.25W, 250V

R5 = varistor, Epcos B72210S2271K101 (for 230VAC mains voltage) *

R5 = varistor, Epcos B72210S2151K101 (for 115VAC mains voltage) *

R6.R7 = NTC 16Ω, 2.1W, Epcos B57236S160M (for 230VAC mains voltage) *

R6 = NTC 16Ω, 2.1W, Epcos B57236S160M (for 115VAC mains voltage) *

R7 = wire bridge (for 115VAC mains voltage) *

R8,R14 = 25Ω, 1%, 15W, T)-126, Caddock 915

R9,R10,R15,R17 = 22kΩ carbon film, 5%, 025W, 250V

R11 = 470Ω carbon film, 5%, 0.25W, 250V

R12,R13,R18,R19 = 100kΩ carbon film, 5%, 0.25W, 250V

R16,R22 = 4.7kΩ carbon film, 5%, 0.25 W, 250V

R20,R21 = thermistor, ICL NTC, 10 Ω, -20% to +20%, radial, Ametherm SL15 10006

R27,R28 = 220Ω metal film, 1%, 0.6 W, 350V



C1,C3,C6 = 470µF, 50 V, 5mm pitch (13x21 mm)

C2,C4,C7 = 47µF,100V, 5mm pitch

C5,C9,C10,C11 = 100nF, 50V, X7R, 0.2’’ pitch

C8 = 1nF, 100V, X7R, 0.2’’ pitch

C12 = 4.7nF, X1/Y1, 500V, Y5U, 15mm pitch



D1–D4 = 1N4148

B1 = bridge rectifier 600V, 4A, GBU405

LED1,LED2 = green, 3mm

T1,T2 = BC337

IC1 = 7805 (+5V, 1A)

IC2 = 7818 (+18V, 1A)

IC3 = 7918 (–18 V, 1A)

IC4 = ATTiny44-20PU microcontroller, programmed, Elektor Store no. 170462-41 *.

IC5,IC6 = TLP222A-2 double optocoupler



K1,K9,K11 = 2-way PCB screw terminal block, 0.3’’ pitch, 630V

K2 = 8- way PCB screw terminal block, 0.2’’ pitch(300V)

K3 = 6-way 2-wire PCB terminal, Wago 806-106 *

K3 = matching 6-pin header, Wago 806-906 *

K3 = 6-way PCB screw terminal block, 0.2’’ pitch, 300V, e.g. Multicomp MC000039 *

K4,K5,K6,K8 = 3-pin pinheader, 0.1’’ pitch w. matching cable parts

K7 = 2-pin pinheader, 0.1’’ pitch w. matching cable part

Shrink connector 1-pole, minimum 14 pieces

TR1 = safety transformer, 1.5VA, primary 2 x 115V, secondary 2 x 6V, e.g. Block AVB1.5/2/6

RE1,RE2 = power relay, SPDT, 12VDC, 16A, through-hole (Omron G2R-1-E 12DC)

RE3,RE4 = power relay, DPDT, 24VDC, 8A, non-locking (Omron G2RL-24-CF-DC24)

GRV IR receiver (Seeed)

60°C thermostat, e.g. Thermorex TK24-T02-MG01-Ö60-S50

PCB no. 170462-1 (v2.21) from Elektor Store


* see text

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