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Current Transformer for Oscilloscopes

Potential-free (floating) measurement using current clamp techniques

Based on a Labs project | June 2017 | Find it here
Current Transformer for Oscilloscopes
Electric current can be measured in differing ways and processes. Frequently we make use of a shunt resistor, across which we can measure the voltage drop caused by current flow. Actually many oscilloscopes have a galvanic (electrical) connection between signal ground and the protective earth (ground) wire of the 230 VAC line supply and for this reason the shunt method is often inappropriate. Commercial current clamp devices certainly are able to measure current by potential-free means but they are not suitable for capturing rapid alterations in current over time. In addition many low-cost examples of these do not provide an output for connecting to an oscilloscope.
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Relevant websites
Extra info / Update
Oscilloscope, measurement, current, floating
Time required
3 hours approx.
Also required
Soldering instruments
Drill and drill bits
Lab power supply
€100 / £90 / $120 approx.
Component list
Component List
Default: SMD 0805, 1%
R1,R2 = 12kOhm
R3,R5,R10 = 10kOhm, 0.1%
R4,R6 = 20kOhm, 0.1%
R7 = 6.8kOhm
R8 = 7.5kOhm, 0.1%
R9 = 30kOhm, 0.1%
R11 = 0Ohm
R12 = 12Ohm, 0.25W, SMD 1206
R13 = 330Ohm
R14 = 1kOhm
P1 = 100Ohm, SMD, e.g. Bourns 3314G-2-101E
Default: SMD 0805, 50V, 10%
C1,C2,C4 = 1µF
C3,C5 = 470nF
C6 = 1µF 63V, PET, 5 or 7.5mm pitch
C7,C11 = 4.7µF, SMD 1206
C8 = 4.7µF, SMD 1210
C9,C10 = 100nF
C12 = 470µF, 20%, 5mm pitch, 13x21 mm
C13-C16 = 10nF 100V
L1 = 4.7µH, 20%, 1.2A, 0.12Ohm, e.g. Bourns SRN3015TA-4R7M
L2 = chip bead, 1kOhm@100 MHz, 200mA, 0.6Ohm DC, e.g. Laird HZ0603B102R-10
B1 = DF04S, bridge rectifier 400V 1.5A, SMD SDIP 4L
LED1 = red, SMD 0805
LED2 = green, SMD 0805
T1 = FZT751, PNP, 60V, 2W, 3A, SMD SOT-223
IC3 = LT1021DCS8-5#PBF, SOIC-8
MOD1 = LTS 25-NP, wired module, LEM
MOD2 = TMR 1-1211, SIP module, Traco Power
K1 = 2-way PCB screw terminal block, 0.2’’ pitch
K2,K3 = 6.35mm FASTON terminal, 3.3mm hole, e.g. TE Connectivity 42822-2
K2,K3 = 6.35mm FASTON female connector, blue, for 2.5 mm2 stranded wire
K4,K5,K6 = 3 pcs 2-pin pinheader, 0.1’’ pitch
K4,K5,K6 = 1 (only) two-way female pinheader, 1/10’’ pitch
S1,S2 = slide switch, horizontal PCB-mounting, SPDT, 100 mA, e.g. OS102011MA1QN1
BNC socket for single-hole mounting (see text)
DC power socket for single-hole mounting, 2.1 mm, insulated
Banana socket, red, 4mm, insulated, 30A, e.g. Cliff Electronic FCR14461
Banana socket, black, 4mm, insulated, 30A, e.g. Cliff Electronic FCR14460
Enclosure, Hammond 1455K1201, 78x123x43 mm
Stranded hookup wire, 5cm, 2.5mm2, red
Stranded hookup wire, 5cm, 2.5mm2, black
Stranded wire for K1 and BNC connector
4 pcs flat washer, steel
2 pcs standoff pillars, steel, 10mm, M3 thread
Plug-in power supply, 12VDC, ≥100mA
PCB # 150170-1 v1.1
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