Voltage drop with load - bridge rectifier, PTC or zener problem? - Page 1
Forum section: electronics / repair

author: okw
author_rank: Frequent Contributor
timestamp: November 24, 2021, 04:48:03 pm
content: I have a heated floor thermostat (smart with TFT display) that brown outs immediately when the relays kicks in. At first I thought one of the relays was broken and consumed too much, so I removed both and checked the coil resistance. They both read 1.65k (PB134024 datasheet says 1.6k and 24V coil voltage). I then hooked each up to a power supply and set current limit to 15mA (24V / 1.6k ohms). Both relays consumed 13mA (turning up the current limiter to 20-30 mA did not make the relays consume more). Then measured resistance between all combination of the 4 pins. Nothing wrong. So I assumed they were both OK.With the relays out, the unit stays on. I reseated relay 1 and it stays on. Removed relay 1 and reseated relay 2, it stays on. But with both in place, the 22V rail drops drastically, until the unit restarts (Atmega MCU). That releases the relays, voltage rises to 22V and the unit tries to power again.I followed the money:Line 250V in -> C883 120'C PTC thermistor -> AV R75 MKP 47J 400 (47pF capacitor) -> 37G bridge rectifier -> 22V 1W zener -> 120uF 25V capacitor.I hoped the cap would be bad, and replaced it with a 100uF 50V (just for test, didn't have 120uF - and designing with a semi-odd value, I figured they must have had a reason for it). Didn't help. Paralleled two 100uF and the unit stays on for longer (2-3 seconds after relay kicks in), but eventually dies. Either something is consuming too much further down in the circuit, or something supplying is half-dead.What is most likely the problem? The declining voltage, would that be a PTC thermistor behaviour (either the PTC is faulty, or something draws too much)? The zener could also breakdown at less load than spec'ed (if faulty), but would this cause a more rapid voltage decline? The bridge rectifier either works or not, right?Any help is appreciated.

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 24, 2021, 05:44:04 pm
content: Could you draw a schematic of this power supply?Edit: Pictures are often helpful too.

author: fzabkar
author_rank: Super Contributor
timestamp: November 24, 2021, 05:48:09 pm
content: That appears to be a transformer-less supply which relies on the impedance of the series capacitor to reduce the voltage. If so, then 47pF would be far too small. :-?https://www.homemade-circuits.com/cheap-yet-useful-transformerless-power/

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 24, 2021, 05:56:47 pm
content: That's why I asked for schematic, sounds almost like the usual capacitive dropper with shunt regulation but 47pf can't possibly be the dropper cap.

author: james_s
author_rank: Super Contributor
timestamp: November 24, 2021, 06:06:56 pm
content: It's fairly common for the film capacitors used as droppers to deteriorate over time, Big Clive did a video about this. My friend recently replaced several of them in smart sockets, they exhibited the same symptom, they would fail when trying to pull in the relay.

author: okw
author_rank: Frequent Contributor
timestamp: November 24, 2021, 07:36:58 pm
content: I don't have the schematics, then it would be fairly easy. I'm trying to eye-trace but it's hard under components etc.The design (w/ values, e.g. 47pF), is made from a very reputable company (ELKO).So you think the 47pF might have deteriorated and could be the culprit? PCB is stamped May 12, and since it's floor heating, it's probably been energized non stop for 8-9 years.Is the value important? I have a few 10nF (X2 310V). At least for test.

author: james_s
author_rank: Super Contributor
timestamp: November 24, 2021, 09:21:44 pm
content: There's no way the dropper is 47pf. Either you're mis-reading the part, or it isn't the dropper.

author: okw
author_rank: Frequent Contributor
timestamp: November 24, 2021, 10:04:29 pm
content: Maybe misreading. "AV .47 J 400". I figured it was 47J (47pF) 400V. Maybe is it .47 (470nF) and the J stands for "Type J - Dipped Tantalum Capacitors"?Attached some pictures.

author: fzabkar
author_rank: Super Contributor
timestamp: November 24, 2021, 10:12:37 pm
content: Definitely 0.47uF.

author: okw
author_rank: Frequent Contributor
timestamp: November 24, 2021, 10:20:10 pm
content: I have MPEM-470NR15/400 SR PASSIVES (Capacitor: polyester; 470nF; 400VDC; Pitch: 15mm; ±10%; -40÷85°C):https://www.tme.eu/no/en/details/mpem-470nr15_400/tht-polyester-capacitors/sr-passives/Would this be a good permanent replacement? If not, maybe just for test?

author: Gyro
author_rank: Super Contributor
timestamp: November 24, 2021, 10:28:35 pm
content: The original is a metalised Polypropylene. The 400V DC rating of your capacitor is too close for standard 230-240V AC mains (340V peak + spikes), the next standard voltage is 630V DC irrc. Although not strictly necessary because it is not an across-the-line application, buying a 470nF MKP X1 or X2 rated cap is the normal way ensuring adequate rating.

author: okw
author_rank: Frequent Contributor
timestamp: November 24, 2021, 10:33:29 pm
content: Thanks. Then I'll do that.But for testing now, just to see the unit functions properly, it wouldn't hurt to run it a few minutes?

author: Gyro
author_rank: Super Contributor
timestamp: November 24, 2021, 10:38:11 pm
content: Well, spikes can happen at the worst moments but for a quick test, using a non-inductive, or no load (just to make sure the relay works), maybe... but at your risk. The datasheet doesn't specify a test voltage (X2 is 2500V test).

author: okw
author_rank: Frequent Contributor
timestamp: November 24, 2021, 11:10:02 pm
content: SuccessThank you guys.I'll go ahead and order a the proper cap.Just for the discussion, is X1 and X2 (and Y1 / Y2) caps from Aliexpress generally to be avoided? Or are the big stores serious players? For example, I've done some business with "shenzhenYida Store" and "Fantasy Electronics CO., Ltd". They are huge, and have every thinkable part.

author: okw
author_rank: Frequent Contributor
timestamp: November 25, 2021, 10:12:10 am
content: I think this is the original (AV R75 series), but Kemet:https://eu.mouser.com/ProductDetail/KEMET/R75MI3470GY30J?qs=pfd5qewlna67o5JcBurPvw%3D%3DIt's 400VDC/220VAC. Mine is 400VDC (doesn't mention AC in the datasheet).In the datasheet of my cap, it says it's rated to withstand 800VDC for 10 sec:Rated Voltage(UR): 400VDC.Withstand Voltage: 2UR(10S)In the datasheet of the original, it says it's rated to withstand 640VDC for 2 sec:Test Voltage 1.6 x VR VDC for 2 seconds (between terminations) at +25°C ±5°CNeither are X2.Wouldn't mine be almost an exactly match, except the dielectric (MKP vs MKT)?

author: mariush
author_rank: Super Contributor
timestamp: November 25, 2021, 11:10:17 am
content: You could also tweak the capacitance to give you 26-30v output and then use a linear regulator like 7824 to output exactly 24v and replace that 22v zener diode :The regulators are cheap and they can handle up to around 37..38v input voltage ... ex.https://www.tme.eu/gb/details/l7824abv/unregulated-voltage-regulators/stmicroelectronics/https://www.tme.eu/gb/details/l78s24cv/unregulated-voltage-regulators/stmicroelectronics/or 0.5A max dpak versions :https://www.tme.eu/gb/details/l78m24abdt-tr/unregulated-voltage-regulators/stmicroelectronics/and the relays don't consume that much power at 15-30mA each, so the regulator wouldn't heat much even with the higher input voltage.May want to add a diode from output to input (see examples in datasheet) as extra protection for the regulator.

author: Gyro
author_rank: Super Contributor
timestamp: November 25, 2021, 11:26:25 am
content: Quote from: okw on November 25, 2021, 10:12:10 amYes, I'd avoid Ali stuff, you want decent life.In terms of the voltage rating, the thing to remember is that, when exposed to overvoltage breakdown, metallised film capacitors self-heal by evaporating an area of metallisation around the puncture in the dielectric film. All well and good, but this results in a reduction in electrode area, and so, capacitance over time and repeated healings. This is what has happened to your existing capacitor, causing the failure in the first place.The easiest way of life limiting such products is to cheap-out on the capacitor voltage rating. Eventual failure is inevitable, but the manufacturer can do things to nudge it along. If you want longevity and to avoid having to repeat the process, I would go for the highest voltage rated part that will physically fit in the available space, and from a good manufacturer - Epcos springs to mind. Personally I'd go for a 630V rated part with decent datasheet test data.As I mentioned previously, X rated caps are often used in this application, but are not actuallyrequiredas it isn't a 'directly across the line' application. However, they do tend to be cheaper due to volume. An X1 rated capacitor has higher breakdown test voltage - 4kV for X1 vs 2.5kV for X2, so will probably self-heal less frequently in practice. Decent pulse rated capacitors tend to have thicker metallisation than X types though.

author: okw
author_rank: Frequent Contributor
timestamp: November 25, 2021, 11:50:11 am
content: Quote from: mariush on November 25, 2021, 11:10:17 amThanks, a good suggestion. However, I'd like to keep as original as possible. Also, space is limited

author: okw
author_rank: Frequent Contributor
timestamp: November 25, 2021, 11:52:43 am
content: Quote from: Gyro on November 25, 2021, 11:26:25 amThanks. I'll do that.Out of curiosity; since both are metallised film capacitors, why is MKP preferred over MKT in in this application (capacitive dropper)?

author: Gyro
author_rank: Super Contributor
timestamp: November 25, 2021, 12:02:32 pm
content: MKP has a lower dissipation factor than MKT. In theory, that might make the capacitor run a bit cooler. I think Polypropylene (MKP) is probably a 'tougher' film than Polyester (MKT) but I've never compared things like melting point. Pulse rated capacitors are almost inevitably MKP.