DE-5000 question - Page 1
Forum section: electronics / testgear

author: AndyA
author_rank: Newbie
timestamp: November 05, 2018, 11:33:05 am
content: Hi,I ordered the DER EE DE-5000 and a capacitor discharge unit,https://www.amazon.co.uk/gp/product/B074PQF6GC/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1.Unfortunately the discharge unit discharges a capacitor fairly fast until close to 10Volts and then slows to a snails pace. Not sure if this is normal.Can the DE-5000 be damaged by a 10V charge from the capacitor?Thanks

author: indman
author_rank: Super Contributor
timestamp: November 05, 2018, 04:28:20 pm
content: DE-5000 have no protection of measuring ports! If on measuring contacts in the test mode without loading V=0.6-0.7V and a power supply of the chip ES51920 5V, then I think that in case of exceeding of this limit in 5V to measuring ports it can be already bad.

author: bugi
author_rank: Regular Contributor
timestamp: November 05, 2018, 04:32:39 pm
content: Considering that DE-5000's own supply voltage is 9V and many of the internal voltages are in the <3V category (especially ones at the measurement pins), and the device specifically says to discharge caps (without mentioning that below certain voltage is ok), I'd say 10V is calling for problems.Reading the description of that product, it seems to be aimed to bring the voltage down to just "safe range" for the human/worker, not for a total discharge. I guess it is simply a power resistor (100kohm/8W), with rest of the circuitry for the LEDs and such. RC time constant with 100k and, say, 1000µF=1mF gives order of magnitude 100 seconds per drop to 1/3rd voltage, so yeah, it'll be a coffee break.Easy DIY discharge for the rest of the way faster could be to simply manually add smaller (in ohms, may still need to be higher power type) resistor in parallel, after that device has done its job. But, I don't know if I would trust that device to even do that initial discharge safely. (If they can not even write proper English descriptions, how good can they be with electronics or high voltage safety? That is a simple device to make, so maybe they got at least the design right, but then the manufacturing quality... "4RD Gen Discharger" lol...)

author: Fred27
author_rank: Unknown
timestamp: November 05, 2018, 09:18:54 pm
content: If you don't discharge it fully, would you get a good measurement?

author: AndyA
author_rank: Newbie
timestamp: November 06, 2018, 02:32:13 pm
content: Thanks for the reply. I will send it back and solder a resister between 2 multimeter probes instead.What would you recommend the best value and wattage should be to discharge capacitors in switch mode power supplies in general?Thanks

author: cdev
author_rank: Super Contributor
timestamp: November 06, 2018, 03:00:41 pm
content: A dedicated tool to do this sounds like a really good use to put a cheap/free voltmeter to! You could even use more pressure to activate an additional switch after the initial resistor had done most of the work to completely short the cap at the end.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 06, 2018, 09:32:09 pm
content: An good 2k load at 20-25 watts should do a good job to bleed you capacitor 400v / 2000 ohm is 0.2 amps power would be 40 watts for an short instant ??

author: bugi
author_rank: Regular Contributor
timestamp: November 07, 2018, 08:32:03 am
content: Yes, 2k seems to be ok. As an example, my ad-hoc (not-so-safely assembled) discharger has 10x 200ohm 10W ceramics in series, and I used that to discharge PC PSU primary side main caps, working voltage somewhere above 300V. No idea what the voltage was at the time of starting discharge as the circuit has its own slow bleeder. It was not super fast, but I did not need to take coffee break, either. IIRC, the total price of those 10 resistors was cheaper than a single 2k higher wattage resistor. Also, the way I assembled it, I could move the "probe" connections to any point in that resistor chain, thus quickly turning the effective resistance to a lower value (down to a single 200ohm), for discharging lower voltages faster.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 07, 2018, 11:40:38 am
content: The ideal load would be somekind of intelligent voltmeter who will switch loads depending of the sensed voltages, the lower the voltages drops the lower the load would be ?You would need a cofee at all loll

author: bugi
author_rank: Regular Contributor
timestamp: November 07, 2018, 03:26:50 pm
content: Yes, indeed, the ideal would be sort of ramp-up from zero current to an adjustable constant current, and finally low resistance decay (and keeping the voltage down). Ramp up first to prevent sparking etc., and constant current would automatically reduce the load to match the remaining voltage.But, since these devices are typically not used that much (except in the safety purposes), the resulting devices are varying levels of compromises. I did start to design a multi-stage thingmagajic with LEDs and stuff for a general purpose solution, but, eh, that project is now in the end part of my queue.

author: AndyA
author_rank: Newbie
timestamp: November 07, 2018, 08:15:20 pm
content: So not one resistor value suits all capacitors to fully discharge.I was told this morning that a resister value of 470 ohms 5w was good for all capacitors in a smps but that doesn't seem to be the case.Do the resistors need to be wire wound or doesn't it matter much, so long as the voltage is higher than the rating of the capacitor value?

author: coromonadalix
author_rank: Super Contributor
timestamp: November 07, 2018, 11:33:38 pm
content: The problem is : even with an fixed 1 value of resistance, you will end with an timed discharge, you will have to wait until it is low enough to short it and drop all remaining voltage to do an safe measurement

author: AndyA
author_rank: Newbie
timestamp: November 08, 2018, 05:09:42 pm
content: Quote from: cdev on November 06, 2018, 03:00:41 pmGood idea. I have ordered these,https://www.amazon.co.uk/gp/product/B00COAX11O/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1To plug straight into my multimeter.That way I can put a resistor across the terminals of one and heat shrink it for safety. Then put a switched short across another just to discharge any remaining low voltage and plug them into one another. That way I can have the best of both worlds hopefully.What do you think?

author: cdev
author_rank: Super Contributor
timestamp: November 08, 2018, 06:39:37 pm
content: I would just have your resistor and a switch which pushing down a bit harder activated.If you're 100% sure its been shorted at the end you don't really need a voltmeter.Second thought, all that momentary current might toast a switch fairly fast, if you shorted it too quickly.Maybe in that case you might want to have some indicator.

author: AndyA
author_rank: Newbie
timestamp: November 08, 2018, 07:18:27 pm
content: Quote from: cdev on November 08, 2018, 06:39:37 pmCan you explain what you mean by pushing down a bit harder activated is please?

author: coromonadalix
author_rank: Super Contributor
timestamp: November 09, 2018, 12:46:41 am
content: maybe mean when you reach an certain discharge point in voltage you short it with an switch the hard way loll an pure shortYeah it will toast switch contacts really fast

author: AndyA
author_rank: Newbie
timestamp: November 09, 2018, 09:33:52 am
content: Quote from: coromonadalix on November 09, 2018, 12:46:41 amI see what you mean. No switch just a short when needed at the end of discharge to completely discharge the capacitor.Thanks

author: cdev
author_rank: Super Contributor
timestamp: November 09, 2018, 06:26:15 pm
content: Using a screwdriver assures that the charge is discharged but will cause severe damage to the screwdriver's finish and might even spot-weld it to the capacitor so tightly that its hard to pull off.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 09, 2018, 09:23:56 pm
content: tried the screwdriver thing, it made an hot metal in fusion residue who passed thru my jeans pants and burned my skin .......... never done that afterwards, do the resistor trick and be patient loll

author: commongrounder
author_rank: Frequent Contributor
timestamp: November 09, 2018, 10:07:10 pm
content: Discharging a capacitor, especially a large value electrolytic, through a dead short can damage the terminal-to-foil connections internally, due to extreme current flow beyond the ripple current rating. Much better to be patient and use a suitable resistor.