DC Resistance after potting

[jason lollar]

Let it sit overnight and it should read lower than when you wound it unless your air temp is higher the next day. Your coil wire gets heated up if you run it through a felt tensioner so it will read lower the next day if allowed to sit. Potting can really raise the ohms dramatically but they should go back down if you let it sit at least overnight.

[MistaT]

Thanks Jason for your reply! I'm a bit confused. do you mean I should let the pickup sit overnight, and then pot it?

BTW, my winder is homemade (old sewing machine..) and my fingers are the tensioner. (at least for now..and definetly not the way to do it..)

Also, as an experiment, I placed my 7.1k pickup (after potting.. 6.5k before potting) into the freezer overnight, and it dropped to 6.1 k, which I would expect. then I let it warm up to room temp. for several hours, and it came back to 7.1k.

Finally as you suggested, my room temp. is the same from day to day.

Again thank you very much for your input! I am learning a lot!

-- Mike

[chevalij]

Thanks Jason for your reply! I'm a bit confused. do you mean I should let the pickup sit overnight, and then pot it?

-- Mike

I'm pretty sure he means to pot it, let it sit over night, then measure it. A pickup will hold the heat for a while.

[David Schwab]

Don't forget that you can check for shorts to magnets with your VOM. Just connect one probe to the start lead, and then touch each magnet. If you get a reading, you have a short.

[Down Time]

Let it sit overnight and it should read lower than when you wound it unless your air temp is higher the next day. Your coil wire gets heated up if you run it through a felt tensioner so it will read lower the next day if allowed to sit. Potting can really raise the ohms dramatically but they should go back down if you let it sit at least overnight.

This is good advice, I always wait a day or two before potting. The ohm reading always seems to read higher right off the machine. I like to let the coil relax before potting.

[MistaT]

Thanks David, yes I do check for shorts to the magnets. I have gotten past that one ok. It seems to me my issue isn't a short since the resistance goes up. If it were a short it seems like the resistance would go down.

[MistaT]

OK thanks. Yes that is exactly how I do it. I wind the pickup, then pot it, then let it sit overnight.

BTW, everyone... thak you so much for your willlingness to help out the new guy!!

[MistaT]

OK Down Time, that is pretty much the only thing I haven't tried yet. So later on tonite I will build a pickup, and let it sit overnight, then pot it the following day. I will take readings, and post my resluts here..

Again, thanks everyone for your input. It sure seems lik a great bunch of people on this forum!

-- Mike

[jason lollar]

you dont have to let it sit overnight before you pot it. even if you are tensioning with your fingers its creating heat- heat makes the resistance go up and it has to sit overnight before it comes back down. Measure a pickup get a reading and then hold it in your hand for a minute or two and measure it again- make sure you get good contact with your skin.
Measure a coil and then blow on it 4 or 5 times and measure it again- serious its that sensitive.
I dont know why it would read higher unless you had an intermittant short so when you maesured again it wasnt shorting out BUT I have seen people that cant get a consitant reading with an ohmeter. thier body chemistry changes.
I have seen this several times. One guy always got a higher reading than everyone else but most people it happens to get a fluxuation.
So maybe you are one of those guys that have to use test clips

[Down Time]

you dont have to let it sit overnight before you pot it. even if you are tensioning with your fingers its creating heat- heat makes the resistance go up and it has to sit overnight before it comes back down. Measure a pickup get a reading and then hold it in your hand for a minute or two and measure it again- make sure you get good contact with your skin.
Measure a coil and then blow on it 4 or 5 times and measure it again- serious its that sensitive.
I dont know why it would read higher unless you had an intermittant short so when you maesured again it wasnt shorting out BUT I have seen people that cant get a consitant reading with an ohmeter. thier body chemistry changes.
I have seen this several times. One guy always got a higher reading than everyone else but most people it happens to get a fluxuation.
So maybe you are one of those guys that have to use test clips

Yes, you may not have to let it sit overnight before potting. I always use test clips and have seen the reading come back down to normal in about two hours. I just like to let it sit a day or two. I'm not in a hurry.

[David Schwab]

So maybe you are one of those guys that have to use test clips

I always use test clips. I can't get useful reading without them.

[Joe Gwinn]

I dont know why it would read higher unless you had an intermittent short so when you measured again it wasnt shorting out BUT I have seen people that cant get a consistent reading with an ohmmeter. their body chemistry changes.
I have seen this several times. One guy always got a higher reading than everyone else but most people it happens to get a fluxuation.
So maybe you are one of those guys that have to use test clips

Contact resistance varies greatly with pressure and with the nature of the surfaces making the contact. To get accurate readings (versus simply steady readings) it's pretty well essential to use spring clips or soldered connections, unless everything is plated with gold (no surface oxide layer).

Fine wire is particularly hard to get a good grip on, so soldering it to a terminal or a piece of heavier wire helps a lot.

[MistaT]

OK I have completed my little experiment. Here are my resistance readings with my target resistance of 6.5k:

Right off the winder: 6.61k
The next day before potting: 6.45k
After potting and still hot: 7.54k
An hour later and cooled down: 6.53k

I have only wound about 6 or so pickups, and this is the first time my final resistance has come anywhere close (close enough for me..) to the wind off of my winder. Usually they have been off my anywhere between 1k and .6k

The only thing that changed here is that I used a different spool of 42 Ga. wire. Could it be that my old spool of wire had some sort of quality issue?

Also to reply to others suggestions, I tried measuring the resistance with clips, and just holding the wire against my fingers. For me, there is no difference. Also, after I finish winding, my next step is to solder the pigtails on. From then on I measure via the pigtails.

Again as always, I appreciate the input. You guys are great!

-- Mike

[Bill M]

The only thing that changed here is that I used a different spool of 42 Ga. wire. Could it be that my old spool of wire had some sort of quality issue?

That could be it. Here's something I'd like to throw out here. Your tension may have been too tight for the first few hundred wraps or so, and when you wax potted the pickup the magnets expanded from the heat and streched your wire, especially at the sharp radius around the 2 E mags. That could explain the higher DC resistance.

[Peter Naglitsch]

Right off the winder: 6.61k
The next day before potting: 6.45k
After potting and still hot: 7.54k
An hour later and cooled down: 6.53k

From 6.45 to 6.53 after potting and cooling sound normal to me.