Der EE DE-5000 Added Input Protection - Page 1
Forum section: electronics / testgear

author: MaxFrister
author_rank: Regular Contributor
timestamp: September 04, 2021, 05:24:24 pm
content: As implemented, the Der EE DE-5000 LCR meter has essentially no input protection.When I opened up my Der EE DE 5000 meter for arepairI found 16 unpopulated footprints on a version 8 board. ObviouslyMuntzhad been busy.Since mine had already been damaged, I thought I would try to see what they deleted and what might help protect it. Some of the missing components are decoupling caps. One is for battery reverse-polarity protection that seems to be redundant. However, some of them are on the input network and potentially would protect the inputs. There are 8 empty Sot-23 footprints marked Q5 through Q12.they map out as (only 1 input shown):One might expect an inputclampbuilt from opposing zener diodes. It turns out that transistors may be connected as didoes and can have lower reverse leakage currents when off [AOE 3rd p. 293]. The silk screen Q rather than D indicate that they meant to use transistors.This application is not only wiring the transistors as diodes but depending upon transistor zener/avalanche breakdown. I bought and installed some NPN transistors,BFT-25. They have the correct footprint, low Vebo, and extremely low leakage.The BFT25 cost ~$.25US each in quantities of 1k. That would have added a significant $2.00 to the BOM costs of the meter. I expect there are better choices.In addition to the missing transistors, the input network also shows three unpopulated capacitors, C17, C18, and C19. These are wired in parallel with input resistors R25, R26, and R27 respectively. It is not clear to me what useful purpose these would serve. If populated they would either have to be exceedingly small or compensated in software since their impedance would be frequency dependent.I measured a variety of resistors and capacitors before and after installing the BFT25s. The meter readings were essentially unchanged, within the limits of my experimental setup. So at least they have done no harm.

author: CDaniel
author_rank: Frequent Contributor
timestamp: September 04, 2021, 06:41:48 pm
content: Hard to say what level of protection you have with those zener transistors , of course something is better than nothing , but without a series resistor they could burn easily , hopefully will fail to short not open circuitSuch circuits are hard to protect in a non distructive way .

author: Trader
author_rank: Frequent Contributor
timestamp: September 04, 2021, 06:44:39 pm
content: This is a very interesting improvement, I wonder why they have the spaces and didn't use them.

author: Shock
author_rank: Super Contributor
timestamp: September 04, 2021, 06:56:15 pm
content: This datasheet?The caps look present in the IET DE5000 model in Daves video, a few members here own same version. Seen any drawings of the DE5000 front end? What do you calculate the input protection to be now?

author: MaxFrister
author_rank: Regular Contributor
timestamp: September 06, 2021, 12:49:07 am
content: Quote from: Shock on September 04, 2021, 06:56:15 pmGood questions.I have not seen any schematics for the DE-5000 but have reverse engineered some of the front end. The excitation signal is roughly 0.5Vrms riding on 1.5VDC. Frequency is from DC to 100kHz (DC is only for R). Input impedance ranges from 100 to 1M ohms. The LCR processor is 2 ICs: Cyrustek ES51919 and ES51920. I've only been able to locate a minimal sales sheet for these.Meter protection circuits seems to be a large field. Dave presented a nice review of some of the issues:https://www.youtube.com/watch?v=zUhnGp5vh60As an LCR meter some of the issues are different than the multimeter that he reviewed. In particular any L, C, or R of the input network will need to be compensated either in hardware or software to maintain accuracy.As for how much protection the zener connected transistors would provide, my guess is not much. I would not expect the meter to survive a 20mF cap charged to 500V but it would be nice if it was more robust than it is now.I started trying to analytically calculate the expected protection and found too many unknowns. So I decide to smoke some transistors instead.Here is the V-I curve of a BFT25 transistor wired as a zener diode:I only have 2 extra BFT25s so I decided to smoke 2n2222s instead. These are much more robust transistors but they sill let me get a feel for how they work as diodes (and how to capture it on a scope).Here is the correspond V-I curve for a 2n2222:In the next picture, the 2n2222 was connected to a 1uF cap charged to 20V. The load is 1-ohm in order to approximate the lead/connector/trace impedance.2n2222 connected to a 10uF cap charged to 20V:and finally a 100uF cap charged to 20V:With that final test, the transistor sometimes survived, sometimes survived with diminished beta, and sometimes failed short.I'll do some more testing using the BFT25s soon.

author: David Hess
author_rank: Super Contributor
timestamp: September 06, 2021, 03:05:57 am
content: Quote from: MaxFrister on September 04, 2021, 05:24:24 pmMy guess is that the capacitors are there to swamp the effects of the junction capacitance of the protection transistors.Quote from: Trader on September 04, 2021, 06:44:39 pmThey might not have bothered if the extra protection was irrelevant. If an overload shorts the protection circuits, then it still counts as a failure.