Clipper Circuit - Page 1
Forum section: electronics / projects

author: SG-1
author_rank: Regular Contributor
timestamp: May 09, 2014, 02:20:09 am
content: I am beginning a project to build a prototype "black box" that has the following specifications:1. Must limit the primary side feed back of a transformer to 10-15 Volts if it is inadvertently connected backwards during a polarity test.2. Must allow polarity testing of the transformer.Background: We must perform polarity tests on Potential Transformers & Control Power Transformers to verify correct connections to bus bars. The tranformers have medium voltage primaries & low voltage secondaries, such as 12000V to 120V ( 100:1 ratio). Can go up to 300:1 or as low as 25:1.The test is performed using the inductive kick method. A low level DC pulse 3V - 12V is injected across the primary terminals & a milli-ampere meter with a zero center scale is connected to the transformer secondary terminals. When a positive signal is injected into the H1 terminal the meter needle indicator will move upscale if connections are correct when the circuit is made. The meters + terminal is connected to X1 & common to X2. ( Upscale kick on make & a downscale kick on break of the DC circuit. )For the most part the DC source current is limited to between 200mA & 2A. The usual current limiter is an "old timey" door bell, also used for continuity checks, hence the phrase " ring it out ".Management has become concerned that the meter & voltage source could be exchanged causing electric shock. The concern is real. The feed back will 'light you up'.I am thinking of using a pair of Zeners ( actually several pairs since this is a three phase assembly ) to short out any voltage , say over 10 Volts. I have a maxmum of 24VAC that cannot be exceeded. I am also considering using varistors in parallel with the zeners. The zeners would be connected across the primary terminals of the transformers during polarity tests.Sorry so long, but I would appreciate any feedback you may have to offer.Steve

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2014, 02:39:00 am
content: Yeah, that'll do. Nothing special, the current is never more than the current supplied to the circuit.Tim

author: David Hess
author_rank: Super Contributor
timestamp: May 09, 2014, 05:57:34 pm
content: I do not understand what the problem is. If the transformer is tested as a current transformer with high impedance drive and low impedance measurement, the voltages can never be high unless something is not connected and you can limit the compliance voltage on the drive side trivially.If I wanted to be really safe, I would synchronously demodulate the output so then the drive signal can be millivolts or smaller so even if the high voltage side of the transformer was disconnected, the output voltage would be low.

author: SG-1
author_rank: Regular Contributor
timestamp: May 09, 2014, 06:25:25 pm
content: Thank you T3sl4co1l for your reply.David, lets suppose the operator is having a bad day & somehow manages to mix up the test leads. Then the pulsing DC is applied ( from a bell type continuity tester ) The meter is not connected yet, error 2. The HV terminals are open, when the meter is connected one lead will touch first.The transformer has failed the polarity test. The operator is using the bell to verify the secondary wiring on the transformer. The HV terminals are open.I do see your point that the meter will short the primary windings & the voltage should collapse. Now I have an experiment to do. In my 30+ years I do not know a single person that has been shocked while performing a polarity test. That does not stop the safety department form being nervous.