EEVblog Brymen DMM displays Incorrect AC Voltage? - Page 1
Forum section: electronics / testgear

author: speedlimit20
author_rank: Newbie
timestamp: May 24, 2017, 09:59:11 am
content: I'll see if I can explain my conundrum ...More modern model railroad / railway control systems are run by a system called Digital Command Control (DCC) in which there is a constant 12 to 15 volts square wave AC applied to the rails at all times. Superimposed over that is a parcel generated by the control station which is picked up by the addressed locomotive and decoded which then responds to the control command. Eg. speed up by one notch, turn the headlight on, etc.When I use my Fluke 15 DMM to measure an approximate AC voltage with the probes connected to the two rails, I get the expected voltage, say, 13.3v AC. (The actual constant voltage output by the control station varies by brand and their internal settings).However, and here's the issue, when I measure the same voltage using my EEVblog Brymen DMM, the voltage shows up as 2.9v AC ... obviously incorrect.When I measure the control station's input voltage (24v DC) I get the correct voltage with both DMMs.Why does the Brymen give a false reading?Is there a solution to this?

author: EEVblog
author_rank: Unknown
timestamp: May 24, 2017, 10:18:52 am
content: DCC operates at a frequency above the operational range of the BM235.Not all multimeters have the same frequency response specs.

author: speedlimit20
author_rank: Newbie
timestamp: May 24, 2017, 12:41:41 pm
content: OK ... that's no good then.So what is the Brymen measuring to give a display of approximately 2.9v AC?Can that value be interpreted as anything useful for me?

author: speedlimit20
author_rank: Newbie
timestamp: May 24, 2017, 12:55:02 pm
content: Forget that ... I've researched a bit more now that you've made me aware of the frequency differences between DCC output and the DMM's ability to read it.I'd need to put the DMM across a bridge rectifier to get closer to the voltage truth, and allow for the losses from 2x diodes of the rectifier.Thanks for the initial heads-up about frequency differences. The old Fluke must run at a high enough frequency, or it's also totally wrong. ?!?

author: TK
author_rank: Super Contributor
timestamp: May 24, 2017, 12:56:48 pm
content: Sorry if it is a dummy question, but why do you need to measure the DMM's AC voltage? I think an oscilloscope will give you more detailed information if you need to decode DMM

author: speedlimit20
author_rank: Newbie
timestamp: May 24, 2017, 01:12:22 pm
content: I'm trying to verify the DCC output voltage at the track. It's so I know that my DCC bus wiring is behaving properly and I haven't made any silly errors wiring up each length of rail to the bus wires.I'm not trying to measure the DMM's AC voltage ... I was originally asking why the Brymen DMM was failing to show the "correct" AC voltage across the tracks, particularly as the old Fluke could do it.I love my new EEVblog Brymen and I wanted it to work.I'm a junior at electronics and I don't have an oscilloscope to access. I'm using a car tail light globe inserted into the track feed as a short indicator. An analogue solution to a digital problem if something goes wrong. That should give you a clear understanding of where my expertise in electronics stands.

author: EEVblog
author_rank: Unknown
timestamp: May 24, 2017, 01:29:40 pm
content: Quote from: speedlimit20 on May 24, 2017, 12:55:02 pmIt could also be totally wrong. You jut don't know the frequency is higher than the rated AC bandwidth of the meter.Then you have the issue of True RMS and Crest Factor to contend with.AC measurement with a meter has a lot of issues to understand.

author: rstofer
author_rank: Super Contributor
timestamp: May 24, 2017, 02:22:40 pm
content: According to this article, it is more likely that both meters are wrong:http://www.dccwiki.com/DCC_PowerI would just use a small indicator lamp (locomotive headlamp?) as a voltage tester. I don't think I would use an LED for this but I can't nail down a good reason other than polarity sensitivity.

author: Wytnucls
author_rank: Super Contributor
timestamp: May 24, 2017, 04:09:29 pm
content: The Brymen is not very good at displaying true RMS voltages. The bandwidth is poor (400Hz) and so is the max crest factor (2 at full range).For just about anything else, it is a fine meter.If you mean the Fluke 15B, that is even worse, as it isn't even true RMS, but an averaging meter.

author: TK
author_rank: Super Contributor
timestamp: May 24, 2017, 07:55:00 pm
content: Quote from: speedlimit20 on May 24, 2017, 01:12:22 pmMaybe you can use a diode rectifier bridge with an electrolytic capacitor on the output and measure DC? At least you will get a consistent measurement. I tried it using my DSOX1102G with a square 5V signal (-2.5V - 2.5V) and I get 2V DC using 1N4148 diodes and 220uF capacitor. RMS voltage of the square signal is 2.47V (I don't remember if it is TRUE RMS or average...)

author: MacMeter
author_rank: Frequent Contributor
timestamp: May 24, 2017, 08:11:11 pm
content: Quote from: Wytnucls on May 24, 2017, 04:09:29 pmNow you tell me!

author: speedlimit20
author_rank: Newbie
timestamp: May 24, 2017, 10:17:54 pm
content: Quote from: rstofer on May 24, 2017, 02:22:40 pmThanks for the info.That reference material and confirmation from Dave's earlier post that most DMMs are hopeless at high frequency AC voltage measurement puts my mind at rest about the EEVblog Brymen ... I still love it.About 95% of DCC equipped model train layouts use LEDs exclusively so there's no issue there. It's just a case of needing to know the approximate AC voltage on the track to :1. confirm that the rail feeder wiring is OK as construction proceeds (each and every rail section gets its own feed from the DCC bus)2. know what the AC voltage is in order to calculate resistor values for LEDs. It obviously doesn't need to be precise, just in the correct ballpark.Thanks for the assistance, it's much appreciated.

author: P90
author_rank: Frequent Contributor
timestamp: May 25, 2017, 01:39:31 am
content: many meters won't display a proper reading when there is no zero crossing such as on a pwm output signal, but you can use a capacitor in series to remove the dc offset. of course the point is moot if the bandwidth of the signal exceeds that of the meter...

author: iainwhite
author_rank: Unknown
timestamp: May 25, 2017, 05:32:20 pm
content: Quote from: rstofer on May 24, 2017, 02:22:40 pmSome of the commercial Led track testers use two leds in parallel and in opposite directions, so one led will light up on a DC system and both will on a DCC (AC) systemseehttp://www.train-tech.com/index.php/accessories-extra/track-tester- this one uses red & green leds in a single housing.