Test circuit for a withstand voltage tester

#The present invention relates to a withstand voltage tester, and more specifically to a test circuit for a withstand voltage tester, which has a safety means to keep the operator from the risk of suffering a sustained electric shock.

#When electric appliances are manufactured, they must be examined through a series of safety compliance tests before delivery. The withstand voltage test is a compulsory production test on electric appliances. The test voltage for such a withstand voltage test can be as high as 5,000V. Because an operator may have to test a large number of electric appliances within a short length of time, an accidental contact of the hand with the test circuit may happen, thereby causing an accidental electric shock.

#FIG. 1 shows a test circuit 1 for a withstand voltage tester according to the prior art. The test circuit 1 comprises a high voltage source 11. The high voltage source 11 is connected to a current detection device 15 through an electric line 13. The current detection device 15 is connected to the test sample (electric appliance to be tested) 17 through an electric line 13. The current detection device 15 is also connected to a withstand voltage tester (not shown), which is in turn connected to the high voltage source 11. The test circuit 1 is also connected to the grounding terminal 18 through the electric line 13. When the body 2 touches the test circuit 1 accidentally, electric current passing through the current detection device 15 is equal to the value of the electric current A1 passing through the test sample 17 plus the value of the electric current A2 passing through the human body 2. If A1=10 mA, A2=80 mA, and the preset trip current of the withstand voltage tester is 100 mA, the total current A=A1+A2=90 mA<100 mA. Therefore, the tester does not trip off, and the high voltage source 11 keeps outputting the high voltage A. Under this condition, the operator may subject to the risk of continuous electric shock by a high voltage.

#FIG. 1 is a circuit block diagram of a test circuit for a withstand voltage tester according to the prior art.

#FIG. 2 is a circuit block diagram of another structure of test circuit for a withstand voltage tester according to the prior art.

#FIG. 3 is a circuit block diagram of a test circuit for a withstand voltage tester according to the present invention.

#Referring to FIGS. 3, 4 and 5, the test circuit, referenced by 3, comprises a high voltage source 31. The high voltage source 31 has one end connected through an electric line 32 to a current detection device 33 (see FIG. 3) or alternatively, 33′ (see FIG. 4), and then the test sample, referenced by 35. The arrangement of the current detection device 33 or 33′ prevents the occurrence of an overload. The test circuit 3 further comprises a second current detection device 4 installed in the electric line 32. The second current detection device 4 comprises a resistor R2 connected in series to the test sample 35 through resistor R1, a comparator 41, the comparator 41 having a non-inverter terminal connected in series to the resistor R2 and the ground 47, an inverter terminal connected to a predetermined human body electric leakage current trip point AS, and an output terminal connected to the high voltage source 31, a latch 43 connected between the output terminal of the comparator 41 and the high voltage source 31, and a switch 45 connected between the latch 43 and the high voltage source 31.

#Normally, the test circuit 3 is not touched by the human body 2, and a voltage drop V2=0 is produced at the non-inverter terminal of the comparator 41.

#When the human body 2 touches the test circuit 3, the resistor R2 works as shunt means. When the high voltage current A2 passes through the ground 47 and the resistor R2 to the human body 2, a voltage drop V2=R2×A2 is produced at the non-inverter terminal of the comparator 41, and compared with the predetermined human body electric leakage current trip point AS.