EEVblog #330 - Medela Swing Teardown & Repair - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: August 07, 2012, 02:56:52 pm
content: Dave.

author: Monkeh
author_rank: Super Contributor
timestamp: August 07, 2012, 03:11:32 pm
content: Am I the only one thinking that it's not bad design, but rather an intentional decision to increase sales?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 07, 2012, 03:23:12 pm
content: I think it's just product safety - so reverse batteries or shorted tants don't set things on fire.It is certainly bad design if the battery compartment is designed in a way that allows reverse insertion.

author: Monkeh
author_rank: Super Contributor
timestamp: August 07, 2012, 03:30:22 pm
content: Quote from: mikeselectricstuff on August 07, 2012, 03:23:12 pmBut as Dave pointed out, it's easy enough to entirely protect the circuit from reversed batteries with a diode in series.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: August 07, 2012, 03:39:41 pm
content: This device is probably classified as medical equipment in various parts of the world.And I would not be surprised if medical standards prohibit the use of polyfuses or end-user replacable fuses.A series diode would work, but that wastes quite a bit of power at these currents.edit:Indeed this is a medical device; here's their FDA approvalhttp://www.accessdata.fda.gov/cdrh_docs/pdf5/K053052.pdfSo it should comply with IEC 60601-1, however there does not seem to be a restriction regarding fuses.

author: croyleje
author_rank: Regular Contributor
timestamp: August 07, 2012, 04:02:19 pm
content: hi Dave & Everyone,I think that sense line is probably some reading of how much vacuum is on the breast. It appears that as vacuum decreases that sense voltage increases to the point where the pump needs to turn on again. Well that would make sense to me.As for the battery setup I think its just a case of engineering over practicality.Jason

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: August 07, 2012, 04:10:54 pm
content: Nice video Dave. Logical diagnostics is an often overlooked skill set in electronics.I'd bet that black foam is actually a muffler, not a filter.And I think the previous poster is correct. This would be classified a medical device, and thus designed so that if something goes sideways, it should die and stay dead.I've seen other brands of these things in thrift stores, and am now wondering if you could hack it into a desoldering pump. 8-)

author: transmissiongate
author_rank: Newbie
timestamp: August 07, 2012, 04:21:40 pm
content: The "sense line" could actually be connected to a solenoid valve. Probably the polarity of the signal was inverted? The signal would make sense then. You typically need a short current pulse of a couple of milliseconds to open the valve and then a much lower current to hold the valve in an open position.

author: arekm
author_rank: Unknown
timestamp: August 07, 2012, 05:35:47 pm
content: The valve in this device is a bit weird. I mean if you remove that black "filter" then you will get very low sucking. Thus the material under the filter is a part of valve. Here it broke due to small hole in the under the filter material.

author: ftransform
author_rank: Frequent Contributor
timestamp: August 07, 2012, 06:22:44 pm
content: hahahahahahaha the fluke 87 of breast pumps

author: SeanB
author_rank: Super Contributor
timestamp: August 07, 2012, 06:28:06 pm
content: I am going to guess it mimics a baby sucking ( like a bovine milking machine does with a mechanical valve assembly) and the sensor is a resistive vacuum sensor, basically a diaphragm attached to a slider on a cermet substrate. The pulses are probably the micro reducing noise by only powering the pot when the motor is not powered so as to have a stable supply. The rising value is typical of a slider moving on the track, and represents the inlet suction.At least you saved a lot from Sagan's feed fund......

author: T4P
author_rank: Super Contributor
timestamp: August 07, 2012, 06:34:49 pm
content: Quote from: ftransform on August 07, 2012, 06:22:44 pm

author: firewalker
author_rank: Super Contributor
timestamp: August 07, 2012, 07:09:11 pm
content: Quote from: Short Circuit on August 07, 2012, 03:39:41 pmI think you are right. I have seen it to an old blood sugar tester. Maybe they want to make sure that you wont be able to use a potentially damaged instrument.Alexander.

author: Christopher
author_rank: Frequent Contributor
timestamp: August 07, 2012, 09:10:48 pm
content: I did laugh at the melf resistor (MILF resistor)..

author: creakndale
author_rank: Newbie
timestamp: August 07, 2012, 11:52:49 pm
content: If the rail voltage is directly charging up the big 100uF capacitor, that might explain the eventual fuse blow.I've ran across VXI/VME boards that would blow a fuse on power up after working perfectly for a year. In troubleshooting the problem, the inrush current pulse, charging the onboard filter capacitor, stressed the fuse on each power up. Eventually the fuse failed.creakndale

author: JoannaK
author_rank: Frequent Contributor
timestamp: August 08, 2012, 01:38:08 am
content: Wold be nice to know what kind of data they have on the I2C memory,... just in case it could be hacked to do something totally different.

author: SeanB
author_rank: Super Contributor
timestamp: August 08, 2012, 04:53:24 am
content: I would say the lead resistance and the battery contacts, along with the long traces on the board, limit the current to a low enough level to not worry about fuse aging. It was most likely user error, and a poor design that did not have physical reverse battery disconnection ability, though you do find designs that do this often cannot use certain battery makes as they simply do not exactly conform to the specified AA dimensioning.

author: SeanB
author_rank: Super Contributor
timestamp: August 08, 2012, 05:00:04 am
content: 24C01 so probably only a cycle count and the programming for the different modes ( number of cycles and time) along with perhaps a calibration offset for the vacuum sensor.

author: tom66
author_rank: Super Contributor
timestamp: August 08, 2012, 06:09:55 am
content: At 3:14 why is the Fluke reading a negative resistance?

author: T4P
author_rank: Super Contributor
timestamp: August 08, 2012, 06:22:54 am
content: Quote from: Christopher on August 07, 2012, 09:10:48 pmIronic that MELF resistors are old and abandoned already ...

author: EEVblog
author_rank: Unknown
timestamp: August 08, 2012, 06:26:41 am
content: Quote from: tom66 on August 08, 2012, 06:09:55 amA cap would have been charged from the previous resistance reading.Dave.

author: tom66
author_rank: Super Contributor
timestamp: August 08, 2012, 06:47:55 am
content: Quote from: EEVblog on August 08, 2012, 06:26:41 amI would have thought in ohms mode it would have some kind of brains and say "that's not right; I'll show 'OL'."

author: Short Circuit
author_rank: Frequent Contributor
timestamp: August 08, 2012, 11:12:45 am
content: Of course not, why wants a smartass meter!?Negative resistance does exist;http://en.wikipedia.org/wiki/Negative_resistance

author: T4P
author_rank: Super Contributor
timestamp: August 08, 2012, 01:50:38 pm
content: Quote from: Short Circuit on August 08, 2012, 11:12:45 amFlorescent lamps for example ...

author: tom66
author_rank: Super Contributor
timestamp: August 08, 2012, 01:53:04 pm
content: I don't see why you'd ever need to measure the negative resistance of a fluro. But I wonder if it would actually work, or the MCU in the Fluke is "dumb" and just sticking an ohms sign on the end of a voltage.

author: SeanB
author_rank: Super Contributor
timestamp: August 08, 2012, 03:23:11 pm
content: Just a voltage measurement, with a ohm sign. Not possible to read negative slope resistance without doing a sweep of the voltage or current to obtain the unknown and calculate the slope resistance

author: IanJ
author_rank: Unknown
timestamp: August 08, 2012, 08:20:17 pm
content: Dave,"The Ducks guts"......................hmmmm, much prefer "The Dogs Bollocks"........lets hear you using that one pls.Ian.

author: Monkeh
author_rank: Super Contributor
timestamp: August 08, 2012, 08:21:13 pm
content: Quote from: IanJ on August 08, 2012, 08:20:17 pmOh, but he might upset a poor delicate old lady somewhere!

author: fcb
author_rank: Super Contributor
timestamp: August 08, 2012, 10:05:57 pm
content: Pretty certain that the second set of wires (black) is to a vacuum release solenoid - otherwise the unit would create a constant vacuum on the breast, rather than a suckling action. They are PWM'ing the solenoid as well.I would also guess that there is no pressure sensor in the unit, or calibration required (they will have a vacuum test-jig as part of final test). The 24C01 may record cycle counts for service feedback, but it's main goal will probably be to remember the pressure settings since last time. Perhaps Dave can monitor the I2C packets with his MSO and see when the unit reads/writes to the memory.The unit can be powered from an external mains adaptor, so the non-reset nature of the fuse is probably wise, also - it may a zener diode to protect against over-voltage (although I doubt it, I would design it withstand 15v and have a processor 'over-voltage' detection routine).The black foam is probably a dust-filter for the vacuum release solenoid.

author: amyk
author_rank: Super Contributor
timestamp: August 09, 2012, 09:04:07 am
content: The mention of it being Swiss makes me think that MCU might be a custom part fromEM Microelectronic. Oddly enough there are plenty of Chinese sellers saying they can sell you that part... with the exact part number (344-1.3M3) and package listed.

author: EEVblog
author_rank: Unknown
timestamp: August 09, 2012, 09:19:37 am
content: Quote from: Monkeh on August 08, 2012, 08:21:13 pmI think every time I make a video, a puppy dies!Dave.

author: Pat Pending
author_rank: Regular Contributor
timestamp: August 09, 2012, 09:32:48 am
content: The design patent for this particular unit is US8075516, other Medela patents describe the vacuum waveform shaping.The vacuum signal is monitored for different user comfort programs.

author: SeanB
author_rank: Super Contributor
timestamp: August 09, 2012, 12:33:09 pm
content: Dave, only in Shenzhen...................... Then it is eaten.

author: fcb
author_rank: Super Contributor
timestamp: August 09, 2012, 02:02:04 pm
content: Quote from: Pat Pending on August 09, 2012, 09:32:48 amWhere is the vacuum/pressure sensor? I can't see one - unless they sense pump motor current (which is possible).

author: reagle
author_rank: Unknown
timestamp: August 09, 2012, 07:58:04 pm
content: This episode is in the today's Product Design And Development newsletter"Dave does a teardown, troubleshooting, and repair of a Medela Swing Breast Pump.View Video "http://pddnet.com/video-eevblog-medela-swing-teardown-and-repair-080812/