Coffee grinder repair

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Denis Heidtmann

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Dec 19, 2016, 1:52:22 PM12/19/16
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I have been given a Jura-Capresso 560-04 with a damaged circuit board.

My first question is trying to understand the very simple circuit. The line power (115VAC) is connected in series through a mechanical timer switch and an MOV to a full-wave diode bridge.  The bridge drives a DC motor.  What I do not understand is the placement of the MOV in series.  (I am assuming that it is fact an MOV because the symbol on the silk screen of the board is the typical rectangle with skew line through it. )  The labels on the MOV are MZ23  20P1R5V  950AS.  It is 21.5 mm in diameter. 

The damage to the board is that the MOV became unglued from the board and hence the vibration broke the soldered connection.  The previous owner re-soldered the MOV a number of times but did not secure the thing to the board.  Now one of the leads is broken very short.  I might be able to reconnect it, but since I am not familiar with MOV's and how they fail, nor what this circuit is intended to do, I worry about safety.  This thing is UL certified, but I wonder how it got certified.

Looking for advice.

Thanks,

-Denis

Brandon

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Dec 19, 2016, 2:56:23 PM12/19/16
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That doesn't sound like a MOV based on placement...perhaps a NTC Thermistor to limit inrush on the rectifier?

Throw a multi-meter across it in resistance mode and you'll find out pretty quickly.

Cheers,

Brandon Mathis
KD7INF
brand...@gmail.com

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Denis Heidtmann

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Dec 19, 2016, 3:59:52 PM12/19/16
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On Mon, Dec 19, 2016 at 11:55 AM, Brandon <brand...@gmail.com> wrote:
That doesn't sound like a MOV based on placement...perhaps a NTC Thermistor to limit inrush on the rectifier?

Throw a multi-meter across it in resistance mode and you'll find out pretty quickly.

Cheers,

Brandon Mathis
KD7INF
brand...@gmail.com

On Mon, Dec 19, 2016 at 10:52 AM, Denis Heidtmann <denis.h...@gmail.com> wrote:
I have been given a Jura-Capresso 560-04 with a damaged circuit board.

My first question is trying to understand the very simple circuit. The line power (115VAC) is connected in series through a mechanical timer switch and an MOV to a full-wave diode bridge.  The bridge drives a DC motor.  What I do not understand is the placement of the MOV in series.  (I am assuming that it is fact an MOV because the symbol on the silk screen of the board is the typical rectangle with skew line through it. )  The labels on the MOV are MZ23  20P1R5V  950AS.  It is 21.5 mm in diameter. 

The damage to the board is that the MOV became unglued from the board and hence the vibration broke the soldered connection.  The previous owner re-soldered the MOV a number of times but did not secure the thing to the board.  Now one of the leads is broken very short.  I might be able to reconnect it, but since I am not familiar with MOV's and how they fail, nor what this circuit is intended to do, I worry about safety.  This thing is UL certified, but I wonder how it got certified.

Looking for advice.

Thanks,

-Denis

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Brandon

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Dec 19, 2016, 4:06:56 PM12/19/16
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Brandon Mathis
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Jerry Biehler

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Dec 19, 2016, 4:25:24 PM12/19/16
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Its a NTC, the resistive element has much more current flowing though it at cold than at operating temp so the ntc keeps the current down extending the life of the switch. 

Denis Heidtmann

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Dec 19, 2016, 5:42:08 PM12/19/16
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OK.  Now that I know what it is, how do I interpret the markings?  It measures about 2.0-2.1 ohms at 60F.  And when I glue the thing down to the board, what kind of glue do I use?  What is there with the impression of the device in it is clear and appears to be quite hard, perhaps an epoxy.

BTW, the grinder has a spec of 100W.

-Denis

Jerry Biehler

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Dec 19, 2016, 6:01:52 PM12/19/16
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Don't need to glue it. That's just because it is a relatively heavy component and it's glued to handle being bounced around during shipping. 

But if it is measuring 2 ohms now it is probably good. 

-Jerry
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Denis Heidtmann

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Dec 19, 2016, 6:20:40 PM12/19/16
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The vibration bouncing it around is what I believe to be the cause of the failure (broken lead and solder joint).  That thing hanging on by just its leads sounds to me to be a bad idea.  The shipping shock is likely much less than the grinding vibrations.

Nathan McCorkle

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Dec 20, 2016, 12:44:18 PM12/20/16
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While epoxy might work and be attractive in that sense... it doesn't
lead to easy servicing should this part need work again in the future.
Silicone seems more reasonable, but I know you don't want the kind
which cures via acetic acid (most silicones you get at Home Depot seem
to be this way), because the acid can interact with and exposed
wire/solder metals. I think the electrical term is 'potting' when you
form stuff around components for shock or insulative reasons.
Otherwise... what about zip-ties? They make them pretty small/thin so
you wouldn't need a terribly large hole in the board.

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Denis Heidtmann

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Dec 20, 2016, 12:58:38 PM12/20/16
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I agree with all you say, but I do not know where to get two-part silicone. I had thought about tying it down with dacron thread--much smaller holes than zip-ties.  Is there a retail supply of potting materials?


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Mark Medonis

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Dec 20, 2016, 1:10:53 PM12/20/16
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Tap Plastics has potting material, two part silicone.


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Erik Lane

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Dec 20, 2016, 1:57:18 PM12/20/16
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Why not give it a try with hot glue? I've seen that used well on plenty of circuit boards. I've used it myself, but never had a project that would get as much vibration as it sounds like you're talking about. Would it not be strong enough, do you think?


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Doug Ausmus

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Dec 20, 2016, 1:58:52 PM12/20/16
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There is the thermal issue to consider too.

Denis is correct that in a vibration environment, parts should not depend solely on a two-lead solder joint for mechanical strength, especially those standing off the board with a mechanical mass- they'll break every time. It is fun to watch circuit boards in a vibration test bed with a strobelight: it's astounding hum much they wobble back and forth and a wonder they don't break more than they do.

However, an NTC is a thermally operated component so mounting it securely takes a bit of care so as to not interfere with the thermal aspects (including timing of heat up and cool down).

There is a company, Fujiopoly, that makes thermally transparent (mostly) potting compounds that are very good for this type of thing. They have a bunch of products for thermal issues and I'm sure there's a solution in their product lineup for your application.

Or you can cobble something up that accomplishes the same thing, with a bit of care to the thermal stuff... ceramic lead-standoffs with a sufficient diameter to act as mechanical linkages to the board for the component mass come to mind. Avoid low-temp plastics for obvious reasons.



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Doug Ausmus

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Dec 20, 2016, 2:01:25 PM12/20/16
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Erik's idea is interesting- We've used hot glue for electronic mechanic strength as well as potting and it works great. Believe it or not, you can actually solder THROUGH the hot glue successfully... it aint pretty but it can be done. However I would think about the heating aspect of the NTC in this case.

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Jason Barnett

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Dec 21, 2016, 3:40:13 AM12/21/16
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At my work, we use a white RTV made by Loctite, which is designed for electronics. This is one of the many things we use it for. It can handle the heat, remains flexible, with strong adhesion.
If the board is easy to remove, is clean and you can get it to me in Hillsboro, I could apply some for you. Contact me off list if you want to do this.

Jason B


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Denis Heidtmann

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Dec 21, 2016, 1:03:28 PM12/21/16
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Thanks for all the suggestions.  Impatience convinced me that epoxy would be OK.  The machine is completely assembled and appears to be working fine--no evidence of messed-up thermal characteristics causing problems.  We will know for sure when my son grinds his first batch.

Having been told of some sources for potting stuff could be useful for future projects.

-Denis


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Andrew Melicharek

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May 4, 2017, 7:06:29 PM5/4/17
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Hi,

     I have one of these units. Mine was stinking to high hell and gave up the ghost. It's just the Thermistor that died out on it. Of course I called the company and they wouldn't disclose any information on the circuit. I couldnt read the part number very well since it was all charred. I could only make out MZ23. I was googling and MZ23 came up as a PTC. Does anyone know what the baseline resistance value of this thermistor is? I need to solder a new one in. The epoxy is blocking a good read on the diagram. Does the slope of the line going through the resistance figure provide any clue as to whether it is PTC or NTC? It clearly has a negative slope but I'm not sure if there is a serif on the opposite of the other side that has one. Any help would be appreciated.

Denis Heidtmann

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May 4, 2017, 10:10:40 PM5/4/17
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I got mine to work, but I have to check my notes to see what I concluded about the part.  The  purpose of the component is to limit the inrush current to the motor.  Perhaps it lost its heatsinking causing it to overheat.  I will look to see what my notes have to say about it.  If I find anything helpful I will post it.

-Denis

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Andrew Melicharek

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May 9, 2017, 5:32:42 PM5/9/17
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Hi Denis and everyone else,

    I just wanted to post a small update. I purchased, based on the information contained herein the following part from Mouser

The B57364S259M since I coudn't put a multimeter across my own I took the resistance 2.0ohm @ 60F rating and found that on the log plot in the datasheet. I'll update you all on how the grinder responds audibly and performance wise.
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Andrew Melicharek

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May 21, 2017, 11:30:41 AM5/21/17
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FYI everyone these worked. The gauge of the posts was 18 instead of 26 so I had to drill out the pcb a little and scrap some of the silk screen off the traces but It's all back together and operational.

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