DER DE-5000 - Has anyone modified to make it save for checking charged caps? - Page 1
Forum section: electronics / testgear

author: Acecool
author_rank: Regular Contributor
timestamp: July 14, 2020, 04:15:10 pm
content: I was thinking of modifying the case of the unit to add a discharge capacity location... I may still do this ( cut out a rectangle top right just under display - buy a 3d printer or just order a part to be printed which clips into the area... have a pcb pad to discharge on, but also a hole / slits for positive and negative [ reversible, already have breadboarded the circuit with leds, etc.. ], and then there is a zone to discharge as there is plenty of room inside the units case to add the components for this simple circuit and using a 3d printed plate, it could fit quite nicely in the unit and make it easy to handle discharging caps up to a certain size )...However, I had another idea... First: Why does it warn against using charged capacitors directly? and, Is there a way to modify it so that charged caps automatically get discharged before checking...Worst case scenario, this would mean bypassing the entire circuit, through another pcb and check to see if it is a cap, and if it has power. If it does, then discharge it. When discharged, switch circuit back to PCB.This should be possible as there is enough room; but it would add extra time to the entire system and there must be a simpler way...I'm more of a software guy - I can create anything you could ever dream up in code. I've been doing it forever. However, hardware is more of a new thing for me... For my computer science degree, we didn't really touch on it much other than basic circuits ( inputs go in, output comes out - this is the logic which is processed ) but not the actual hardware components, uses, etc...I do have some basic knowledge, but not near enough to come up with a solution without a lot of time; I am trying to learn as much as possible.Has anyone done this with their unit? Would you be open to sharing your design? Would it negatively affect the testing unit to add this feature ( likely the biggest issue - ie why I was thinking of a bypass circuit which could introduce its own issues with switching )...I just want to make sure I don't accidentally fry it in case I forget to discharge a cap, or if I don't discharge one enough, or if I thought I did but didn't... etc..Cheers.

author: bob91343
author_rank: Super Contributor
timestamp: July 14, 2020, 05:26:21 pm
content: I don't think it's practical. My reasons for saying that are multiple.First, discharging a capacitor doesn't necessarily remove all its charge. There is a property of capacitors called dielectric absorption that causes the charge to return after discharge. So multiple discharges are good.Second, the current that flows during discharge may be quite high, especially for electrolytic capacitors, often large enough to damage the capacitor and/or any traces on a circuit board.As for your measurement device, its circuitry is delicate and applying a voltage to it may damage it.Once again, the situation calls for understanding what's going on and not just guessing.

author: David Hess
author_rank: Super Contributor
timestamp: July 15, 2020, 04:32:02 pm
content: I looked at this problem last year and concluded that it was possible, and very feasible, to design an LCR physical interface which can handle charged capacitors, but retrofitting an existing LCR design would require details which are not made available. I would rather design an LCR meter from scratch than reverse engineer an existing flawed design.

author: tom66
author_rank: Super Contributor
timestamp: July 15, 2020, 05:19:51 pm
content: I think the easiest way would be to use a PTC which could dump the energy from the capacitor, combined with an isolation change over relay - before component is connected the device checks voltage using a high impedance circuit and if >5V then it leaves the relay off until the voltage drops below the limit. The PTC "Low-Z" device would behave similar to Fluke multimeters with their LowZ range, which would allow you to connect (up to) 500V capacitors without having to discharge them.Thing is, this type of circuit would add cost and potentially the relay would add inductance and capacitance components that would need to be nulled out, so that is a pain. And there may need to be some human interaction to anticipate when the relay needs to be in the "safe" position (how else do you tell the difference between an open circuit about to be connected to a charged cap, and a 40 megohm resistor?) so ultimately I'm not convinced it would be worthwhile.

author: CDaniel
author_rank: Frequent Contributor
timestamp: July 15, 2020, 07:03:53 pm
content: I doubt such a circuit could be added later with succes ... a good multimeter is designed to withstand a charged cap , a LCR bridge not , thats life

author: coromonadalix
author_rank: Super Contributor
timestamp: July 16, 2020, 05:34:09 pm
content: yep it is loll

author: TimFox
author_rank: Super Contributor
timestamp: July 16, 2020, 06:48:53 pm
content: Not designed for capacitor discharge, but I built two boxes to connect the four signal wires to two good binding posts for component measurements (different spacings on the two boxes). To facilitate the calibration, I added a shorting switch between the two posts, using multiple poles of 3PDT or 4PDT miniature toggle switches in parallel for a low ON resistance. I started with the DER box that came with two mediocre alligator clips and replaced the wires with appropriate length shielded twisted pairs. In use, the metal box connects to the "guard" banana jack on the DE 5000.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 16, 2020, 07:43:34 pm
content: Modifying the DER EE will "void it's warranty", but more important. How would you guarantee that your added circuit does not make tolerances of mearusements worse?If you just put a piece of metal with hotsnot, double sided tape or whatever to the side of the meter, then you can short the capacitor before measuring.If it's a big elco and it goes **BANG** then you'll know why discharging is important, and you've just saved your meterFor a controlled discharge, you can make something with two anti-parallel LED's and some current limiting.If one of the LED's light up, you have a charged capacitor, and have to wait until it goes out, and then (for big elco's) maybe short it hard for a few seconds.But when the voltage is below a few volts you're unlikely to damage the meter.