EMRFD Message Archive 4513

Message Date From Subject
4513 2010-04-06 14:50:29 R Wall Contact bounce problem with AADE LC meter....
Hi all,

I've just assembled a AADE LC meter. But I had problems with the zero bush button switch. The instructions indicate that on power up, the Lx and Cx buttons should not be selected. This then puts the meter in a mode where you can then select one of the 5 operating modes by pressing the zero button. But I found that the zero button had contact bounce and that it didn't step on to the next mode but would skip a few modes. The instruction manual indicates, if you have problems with the switches to spurt some contact cleaner into a hole in the side of the switch. Before I did this I thought I would see if operating the switch would clean the contacts. This reduced contact bounce but did not solve the problem. After squirting WD40 into the switch there was no contact bounce, pressing the zero button it stepped between each mode without skipping any modes.

My Question:
These are new switches, I would have thought that if the contacts needed cleaning that the manufacture would have done that. If contact wiping action wasn't enough to clean the contacts then the problem may come back when the WD40 stops working. To me it looks like the switch may not be designed to operate at the low voltage logic levels that are used in the AADE LC meter. My experience has been that most contacts need a certain voltage across the contacts to cut through any oxidation on the contacts, especially if contact wiping doesn't clean the contacts.

There has been a large number of AADE LC meters built, why should I have this problem. I note that the manual on AADE's website shows the old type of switches that were used. The new manual and kit that I received has different smaller switches. I don't know how long AADE has been using the new switches for.

Your comments and experiences would be appreciated, is this a problem that will come back,

Roderick Wall, vk3yc.

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4514 2010-04-06 17:26:07 Paul Re: Contact bounce problem with AADE LC meter....

I ordered and built mine about 6 months ago.

My switches look exactly like those that are pictured in the manual that is online at aade.com, except mine have black butt
4515 2010-04-06 18:05:50 R Wall Re: Contact bounce problem with AADE LC meter....
Hi Paul,

Thanks for your response, I suspect that the old switches were OK. My kit has smaller switches and it's in the new manual to use contact cleaner if you have problems. You are lucky in getting the old switches I don't think you'll have any problems with them.

Roderick Wall, vk3yc.

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4516 2010-04-06 18:46:38 Kerry Re: Contact bounce problem with AADE LC meter....
G'day Roderick.

WD-40 is not a great contact cleaner; it dries to a film.

A "proper" contact cleaner such as the CRC one sold by Jaycar et al would be better.

I have an old AADE meter; it has done a bazillion measurements and never given trouble.

4517 2010-04-06 22:45:40 Vern VanZ Re: Contact bounce problem with AADE LC meter....
This is just my opinion, but...

...I suspect that the bounce has more to do with mechanical response to actuation of the spring metal switching component(s) (i.e. - dampened oscillating motion) than oxidation or anything else...perhaps a poor mechanical design(?) A liquid (like WD-40) inside the switch housing could act as a damper for any secondary [mechanical] motion. Sort of like the shock absorbers on your car...it resists motion.

Now, I do understand the difference between electrical 'bounce' and mechanical 'bounce'. But where [mechanical] switches are concerned, one can certainly have both. I'm only suggesting (given your account of events) that you probably experienced the more the later and less of the former kind...

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4518 2010-04-07 07:18:36 Tim Re: Contact bounce problem with AADE LC meter....
Contact bounce can be especially bad if insufficient wetting current is used. I think that extra voltage can help "punch through" contact oxidation/poor contact but that a better solution is a reasonable wetting current.

Most small tactile switches don't have an official rating for wetting current leaving a good amount of FUD when it comes to substituting parts in producti
4519 2010-04-07 14:27:37 R Wall Re: Contact bounce problem with AADE LC meter....
Hi Kerry Vern and Tim,

WD40 was all I had. I was using the term "contact bounce", I could have said, "during the contact sliding wiping action the contacts made contact then disconnected a number of times". This was what I was calling contact bounce. The contacts bounced whenever the zero button was pressed and again when it was released. Wetting current and operating voltage. Good point it may only need another resistor to increase the wetting current to a level to fix the problem. I also wondered if the problem could be fixed by increasing the software switch bounce delay, but it wouldn't fix the problem when the button is released. Problem at the moment is the switches are working (with WD40), I can add a wetting resistor but I can't test the fix. I was also thinking of installing the old switches (that were used before) but the board layout has been changed for the new switches.

Thanks for your feedback, I'll send this on to AADE as It may help them with their otherwise excellent product. I don't think asking customers to use contact cleaner is the correct solution to this problem.

Talking about switch contacts, I remember about 30 years ago, we used a normally closed (NC) momentary switch that was specified to work at low audio signal levels. The switch was used to put the machine into a test mode. The problem was that the switch relied on the switch being operated to clean the contacts. But whenever the silver contacts oxidised the machine went into a test mode. The problems when interfacing to mechanical devices.

Thanks for your feedback on an interesting subject,

Roderick Wall, vk3yc.

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4520 2010-04-07 21:35:30 Bill Wright Re: Contact bounce problem with AADE LC meter....
WD40 is not a contact cleaner.  WD is "water displacement" with a lubricant.  Lubricants tend to be dust collectors as well as an insulator.  Debounce on switches is accomplished by a capacitor across the switch contacts.

4521 2010-04-08 02:51:53 R Wall Re: Contact bounce problem with AADE LC meter....
Hi Bill,

Thanks for your comments, Yes I shouldn't have used it and I may have to unsolder the switches and clean them. For the moment it is working so I'll do something about it when and if I have problems.


Roderick, vk3yc.

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