Transformer and switched resistive load - component voltage margins - Page 1
Forum section: electronics / beginners

author: e100
author_rank: Frequent Contributor
timestamp: August 28, 2015, 11:55:58 am
content: If you have a typical iron core mains step-down isolating transformer with a resistive load connected to the secondary, do you get a voltage spike when the load is disconnected, or is the magnetic energy diverted back though the core to the primary creating a voltage spike on the mains side, or do you get a combination of voltage spikes on both sides, or nothing at all?Likewise if the primary is suddenly disconnected does the remaining magnetic energy generate a voltage spike on the secondary?I haven't seen any info about this on the web and I don't have any mains (240v) rated test equipment for measuring voltage spikes.My particular application uses a 240v to 9v @ 10amps step down transformer.Mike

author: Simon
author_rank: Unknown
timestamp: August 28, 2015, 12:01:02 pm
content: Your best bet is to get an oscilloscope. Most will cope with mains voltage with the probe selector on the times 10 setting. Obviously make sure the probes can take the voltage. What you need to do is use a dual channel oscilloscope in maths mode and carry out the subtraction of the two channels. In this way you will have an isolated differential input to the oscilloscope. You do not connect the ground clips to anything. Beware if you connect grounding clips to any of the phases there is the chance of an almighty bang. Of course this is mains so be careful at all times. I have used this method to examine the power line when I have had to check for disturbances on it. You should be fairly familiar with your oscilloscope before you start carrying out this sort of measurement.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 28, 2015, 12:52:45 pm
content: There will a small spike in both cases. An ideal transformer would not have one. Typically a real transformer is modeled as a ideal one with addional series inductance and resistance at the primary and secondary side, and a one paralle to the ideal transformer to model magnetizing current. The spike comes from this additional stray inductance and the magnetizing current. How large the effect is depends on the type of transfomer. Ring type transformers have less stray inductoance and magentizing current. Also larger transformers are usually better - the magentizing current is not going up as fast as the rated power.You could measure most of the effect on the low voltage side. So you even get the small spike from diskonnecting the main side.

author: e100
author_rank: Frequent Contributor
timestamp: August 28, 2015, 01:58:41 pm
content: Quote from: Kleinstein on August 28, 2015, 12:52:45 pmThanks, you gave me some search terms to look for which lead me to the "AC Power Systems Handbook" which has some pictures of waveforms.https://books.google.com.au/books?id=a988UyrJttYC&pg=PA157&lpg=PA157&dq=eqipment+caused+transient+disturbances&source=bl&ots=kRRzhmXAcT&sig=TgNtXxJMZuCb0SRExvDIHlo_YOw&hl=en&sa=X&ved=0CCUQ6AEwAGoVChMIu56F-fbLxwIVwiumCh1X9wSp#v=onepage&q=eqipment%20caused%20transient%20disturbances&f=false