How bad is a 1Meg/Ohm "short" on DCC ?


D

DALDEI

Member
How bad is a 1 Meg/Ohm "short" on DCC rail ?
I'm laying track and switches and cant remove this short. I've tracked it down to 2 of my tortoises (across pins 2/3 or 6/7) ... this is a consistent occurance. I solder a connector to the tortoises and frequently get a very small short. Usually scraping the hell of them cleans it up so I think its likely residue from resin ...
but this time I cant get rid of it entirely.

I use a hand-made buzzer thing which lets me "hear" even tiny shorts but this one is strange. Hooking up my digital VOM I measure about 4.5 meg/ohm across the tracks *without* the torts. Plug in one or the other tort and it goes down to about 3.5 meg/ohm.
I verify about a 1meg/ohm resistance across pins 2/3 and 6/7

the interesting thing is my buzzer thing is silent at 4.5 meg/ohm but makes a tiny noise at 3.5 ... so I'm thinking its the threshold of the devices.

I also measured current. With a 9V battery I'm getting about 100 micro-amp ....

What I'm wondering is how bad is this ? I hate inperfection, but then I've also used 1k ohm resistors across programming tracks and it doesnt seem to hurt any. Plus block detection uses resistors in the k range so how bad can 1/1000 of that be ?

Suggestions welcome. How worried should I be ?

-David
 
I'm no wizz but are you sure it's not just showing resistance from inside the torise? the windings?
 
1 megohm is not a short. A short is zero ohms, or close to it.

What you have is an unexplained 1 megohm resistance across the track. It is unlikely to have any negative impact on things. It may in fact be normal, as has been suggested.
 
I looked into this more today and the problem (99%) of it is in the tortises themselves. I have soldered a connector onto the tortoise board. Taking apart the tortoise and removing the circuit board and testing it (on several tortises !) shows a "leak" across most or all of the traces in the 100k - 1meg range. This is not going through the coils but rather traces totally physically seperate, even when removed from the mechanism that bridges them. I tested against a brand-new-in-box tortoise with 0 leak.

I am thinking that maybe the flux I used with the solder has left a residue that is conductive. I've tried scraping and cleaning and it shows improvement which hints that may be in fact the problem, but I cant reliably get it under 1Meg/Ohm.

As noted this shouldnt be a concern, but I'm wondering if maybe my process is bad. What can I do to remove residue from a circuit board besides scraping it to hell, cleaning with Acetone and sanding it ? What else is there ? there are no *visible* connections < 1mm between any of the traces but it still shows a leakage.

Any suggestions ?
 
Whoa! Hold on a minute. Pins 2&3 are contacts for pole 4 and 6&7 for pole 5, i.e. they are single pole single throw switches built into the Tortoise for external use. They have no internal connections and operate when the Tortoise changes position. Unless you have something wired to them, they are not in the circuit. 1&8 are the motor coil terminals to operate the Tortoise.

1 Megohm measured anywhere is something you do not need consider for any problems you are having unless you are not getting power somewhere. With our voltage/currents, you are looking at an OPEN.

BTW: what are the symptoms or problems you are having??
 
YES !!! "Who hold on there" is exactly right.
Pins 2 & 3 should be totally electrically issolated.
Opening up the box (once you remove the board) you can see they dont touch.
But I lie not. I'm getting a current through them !

The problem I'm trying to solve ? Probably a problem that doesnt need solving, but I am perplexed why I am getting ANY connectivity, enough to make a pizeo buzzer buzz slightly, ...

I think its my soldering .. I soldered pins to the board ... they LOOK clean, and I have scraped them to death but maybe theres some magic pixie dust still throwing elecrons over the mm wide gaps.
 
I doubt if it is your soldering if it doesn't show any continuity between pins. Besides, 100micro amps?? You must have one heck of a good meter to accurately determine that, but I know of nothing in the hobby that would even flinch at that value. Maybe it's eddy currents :D.
 
Depending on the composition of the plastic, you can have some leakage. This is because they generally mix some very slightly conductive things in for color. This has especially been a problem lately with things from China.

There are other things that can cause some slight leakage currents, including the flux. This is especially true if you used the wrong flux, like an acid based flux or a paste flux. What type of flux did you use??

9 volts and 100 uA is 9/.0001 or 90k ohms. If you put 100 volts there it might act like 10 ohms. :) An ohmmeter typically runs at a few volts, not 9 volts. Sounds like the resistance is non-linear with voltage....which implies it is from some sort of contamination.

While I would not worry about 100k to 1 meg ohm, or even 10k ohms if it was from a known source, I'd probably want to learn what I did to contaminate the turnout. Did you char the plastic from heat?? Did you use a conductive non-electronics flux? Is something that was supposed to be insulated now melted out of place??

I would wash the thing off with denatured alcohol, let it dry, and see what happens. I wouldn't worry about the 90k to 1 meg resistance, but I sure would worry about why it appeared and why it is unstable with voltage. Makes me think you used an acid paste.
 
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