DE-5000 flashing indication - Page 1
Forum section: electronics / testgear

author: TimFox
author_rank: Super Contributor
timestamp: July 28, 2020, 09:48:23 pm
content: I had a strange indication today when measuring the magnetizing inductance (other windings open circuit) of an audio transformer at 100 Hz (minimum frequency of DER DE-5000 LCR meter).The winding had three taps, which measured 7.5 H, 13.85 H, and 24.7 H from the common pin. The strange phenomenon was that the middle value gave an indication that flashed on for a fraction of a second, then dark for a couple of seconds, then repeated. The values (L and ESR) were perfectly reasonable, midway between the other two values. The other values gave a steady indication (which took maybe 10 sec to stabilize to the last digit). Obviously, the middle inductance was not over-range, since it was between the other two values. When measuring magnetizing inductance, one must use a low frequency to avoid self-resonant effects with the transformer capacitances, as pointed out many years ago by Terman (Radio Engineers' Handbook, McGraw-Hill 1943, p 972).Can anyone enlighten me?

author: TimFox
author_rank: Super Contributor
timestamp: July 29, 2020, 01:10:23 pm
content: The DE-5000 is very popular amongst the members of this forum. Has nobody else seen this phenomenon?Since the full-scale at this inductance is 20 H, 13.85 seems like a strange value to get into an auto-range loop.

author: Mr Evil
author_rank: Regular Contributor
timestamp: July 29, 2020, 08:49:09 pm
content: I have a DE-5000 and an audio transformer, so I tried measuring the inductance to see if I could reproduce the flashing display. It's a centre-tapped transformer, so only two readings to make - one or both windings. I got about 4.5H and 18.2H @ 100Hz. Aside from taking a very long time for the higher value to settle down, it didn't do anything strange.

author: TimFox
author_rank: Super Contributor
timestamp: July 29, 2020, 10:06:15 pm
content: Thanks for the reply. It must be something strange about the value I was measuring: 13.85 H, ESR = 5 k, Q = 1.7 at 100 Hz.My other two values, where nothing strange happened, are similar to yours: 24.7 H, 7.37 k, Q = 2.1 and 7.5 H, 2.5 k, Q = 1.9 .Your two values, and all of mine except 24.7 H, should be on the same range. Your two values are in a 4:1 ratio, as expected for a center tap.

author: indman
author_rank: Super Contributor
timestamp: July 30, 2020, 05:46:55 am
content: TimFox, Can you show this flickering effect on video?

author: TimFox
author_rank: Super Contributor
timestamp: July 30, 2020, 01:38:54 pm
content: I'm not set up for video, but it is much slower than flicker. I'll re-do it later today and time the effect: there is a brief indication of both inductance and ESR, followed by a long pause with a blank screen, and then a repeat of the brief indication, etc. A few seconds period for the pattern.

author: TimFox
author_rank: Super Contributor
timestamp: July 30, 2020, 09:30:07 pm
content: After indman's post, I re-measured the same transformer two days after my original data. Of course, now it works fine, so I can't respond with the timing of the pauses.I re-calibrated the unit (as I had done two days before) and re-connected it to the transformer using the same fixture (original DER alligator clip fixture with the clips replaced with smaller Mueller copper alligator clips).The new reading, very close to the reading that flashed periodically two days ago, is 13.662 H and 4.65 k ESR.Two days earlier, I had tried re-doing the connections and cleaning the transformer pins, but always the same result. So this becomes a mystery.Before retiring, I had considered a new career in televangelism, since I could lay my hands on the CRT and it would be healed...