2 VOMs = 2 different Diode Mode voltages... - Page 1
Forum section: electronics / projects

author: Holes Flow
author_rank: Contributor
timestamp: December 27, 2021, 12:50:42 am
content: [There are a variety of posts on DM (Diode Mode), and some even talk about voltages, but none address this issue directlyThere are even web sites I've seen where someone has written down all the DM voltages at key points of working electronics...]One meter's DM "output" is6vdc, and another's is3vdc...Story: I bought a friend with a phone repair store aSiglent Technologies SDM3045Xfor Christmas, as he only uses this minimalistic analog power supply and a 'DM' on it. At this time, I do not know the brand/model, but will update when I find it.Here is the rub: Siglents seem to put out 6vdc in DM, and his old/existing "meter" puts out 3vdc. I just measured the normal leads with another VOM for each meter. Why should this matter? Well, he works on cell phones, and a LOT of the parts in those will toast over 5vdc. He hasn't modified this existing meter.He needs a true VOM desperately, but when he tries the DM on the Siglent, it's way too 'hot'. Some might not know that the MBs often have 7 layers, and are almost all full of 0201s.So I'm thinking I need to ween him off this old piece of equipment, and somehow modify the rig to allow a much lower DM output. Perhaps aa tiny box with a switch to pass the leads through, so it could be turned off for non-DM uses...What do you think? Would a simple resistor bridge do the trick?P.S. I never use DM myself.

author: ledtester
author_rank: Super Contributor
timestamp: December 27, 2021, 01:33:36 am
content: QuoteThis is interesting... has he damaged a MB using the Siglent's diode test function?Most digital multimeters use a constant current source in diode test mode.On page 33 (physical page 46) of the Siglent DM3045 user manual it states that the diode test current is 1mA:https://siglentna.com/USA_website_2014/Documents/DataSheet/SDM3045x_UserManual_UM06034-E01A.pdfA higher test voltage enables the measurement of higher voltage LEDs and zener diodes. It's the low test current that prevents damage to the diode being tested.

author: David Hess
author_rank: Super Contributor
timestamp: December 27, 2021, 03:59:06 pm
content: Quote from: ledtester on December 27, 2021, 01:33:36 amEven at a low test current, reverse breakdown can damage PN junctions.Very old multimeters had a compliance of 10 volts but this was reduced to about 3 volts to prevent damaging the base-emitter junction of bipolar transistors, while still being high enough to test LEDs. Later after blue LEDs became common, this voltage was raised to about 5 volts so that they could be tested also.

author: tooki
author_rank: Super Contributor
timestamp: December 27, 2021, 04:38:07 pm
content: Quote from: Holes Flow on December 27, 2021, 12:50:42 amSo what? Those have nothing whatsoever to do with the question at hand.(Also, other than single-sided boards, board layers always come in even numbers: 2, 4, 6, ... 70+.)

author: Holes Flow
author_rank: Contributor
timestamp: December 28, 2021, 01:47:12 am
content: Found the meter in use...It is a15V 2A ZHAOXIN RF-1502T High quality Power supply for mobile phone repairDC Power Supply model RF-1501T. I found one on alibabahere.It has a test knob on the left, and the farthest left position of it is the diode/continuity mode:I certainly don't see many PS with a diode mode/continuity tester!But regardless of the model, the question is how to tone down the Diode Mode voltage.TIA!

author: Holes Flow
author_rank: Contributor
timestamp: December 28, 2021, 01:51:31 am
content: Sorry to offend, @tooki.To the question, and not how it is worded, what is the best way to vary the diode mode voltage on a DVM? A switch inline with probes? A pot/bridged, zeners, and ideas?Thank you for pointing out my attempt to be inclusive of all readers.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 28, 2021, 09:08:28 am
content: Quote from: Holes Flow on December 27, 2021, 12:50:42 amIMHO the best way to lower the voltage would be to use 2 low current cheap shunt reference in series, e.g. LM385-2.5, like thisin ---->|----|<---- outthat would reduce the voltage by 3.1-2V independently of the polarityif you need a different voltage drop you can use 2 LM385-adj

author: tooki
author_rank: Super Contributor
timestamp: December 28, 2021, 02:05:50 pm
content: Quote from: Holes Flow on December 28, 2021, 01:51:31 amI'm not offended. I was trying to figure out why you think the layer count or component size matters. Specifying those suggests a connection that doesn't exist, and that can confuse people who don't know that.