Question about polarity (solved) - Issue with dremel not running (solved) - Page 1
Forum section: electronics / repair

author: vgamesx2
author_rank: Regular Contributor
timestamp: July 28, 2015, 10:42:43 pm
content: Hello, I recently bought a Dremel 8100 off ebay for a mere $14 what a deal right? well it doesn't include a battery and I planned on making my own Li-ion pack for it, however as you might have guessed by the title, how do I determine the polarity for the contacts without actually powering it up? I don't think it would kill it but I figure better safe than sorry and I should probably ask on here first.Does anyone have this or a similar tool who could test their battery for me?On a side note, if you know where I could find a used battery pack or want to sell me one for the tool, (currently none on ebay/amazon) that'd also make this a lot easier and look nicer.Thanks for the help.This pretty much answers my question Thanks everyone.http://www.batteries4pro.com/4249-pos_thickbox/batterie-808-dremel-72v-multi-usage-8100.jpg

author: ivaylo
author_rank: Frequent Contributor
timestamp: July 29, 2015, 07:08:27 am
content: Depends which way you want it to turn. Just kidding... Not sure what the 8100 is but most others I've seen have universal motors so it's safe to reverse polarity.

author: Psi
author_rank: Super Contributor
timestamp: July 29, 2015, 07:37:13 am
content: It will have electronics for the speed control, so it will definitely have a correct polarity.

author: tec5c
author_rank: Frequent Contributor
timestamp: July 29, 2015, 08:40:18 am
content: I have a Dremel 8100 but I am away until the weekend. If you don't mind waiting until then I am more than happy to confirm this for you.

author: Jorpy
author_rank: Contributor
timestamp: July 29, 2015, 09:03:45 am
content: Do a teardown and look for clues. There might be color coding, protection diodes or even silkscreen markings on the speed controller pcb, etc..

author: McBryce
author_rank: Super Contributor
timestamp: July 29, 2015, 09:19:11 am
content: Look for the largest electrolytic capacitor on the PCB (if it has one) and check which battery terminal is connected to the negative side.McBryce.

author: jdraughn
author_rank: Regular Contributor
timestamp: July 29, 2015, 12:25:42 pm
content: Knowing that the main switching mosfet is going to be NPN, and seeing as how most mosfets usually have the same pin out, you could check which battery input has zero resistance (pretty much) between the third leg of the mosfet. The center pin (number 2) probably goes to one of the motor wires. The other motor wire probably goes to the other battery input, which would be positive.

author: IanB
author_rank: Super Contributor
timestamp: July 29, 2015, 12:49:59 pm
content: Quote from: jdraughn on July 29, 2015, 12:25:42 pmHuh?

author: jdraughn
author_rank: Regular Contributor
timestamp: July 29, 2015, 12:57:06 pm
content: Quote from: IanB on July 29, 2015, 12:49:59 pmIs that wrong?

author: IanB
author_rank: Super Contributor
timestamp: July 29, 2015, 01:14:51 pm
content: Quote from: jdraughn on July 29, 2015, 12:57:06 pmThe NPN designation applies to bipolar transistors (from their construction, having a layer of P-type material sandwiched between layers of N-type material).

author: jdraughn
author_rank: Regular Contributor
timestamp: July 29, 2015, 01:16:08 pm
content: Quote from: IanB on July 29, 2015, 01:14:51 pmAhh yeah, but I think he knew what I meant

author: fcb
author_rank: Super Contributor
timestamp: July 29, 2015, 01:48:46 pm
content: I just BING'd "dremel 8100 battery", on several of the pix you can make out a + and - on the battery casing.http://static.rapidonline.com/catalogueimages/Product/S85-0504P01WL.jpg

author: vgamesx2
author_rank: Regular Contributor
timestamp: July 30, 2015, 12:02:17 am
content: Quote from: Psi on July 29, 2015, 07:37:13 amThat's what I figured thanks.Quote from: tec5c on July 29, 2015, 08:40:18 amYes if you don't mind that would be great, Thank you. (actually I think I got it, found a pic below)Quote from: Jorpy on July 29, 2015, 09:03:45 amQuote from: McBryce on July 29, 2015, 09:19:11 amQuote from: jdraughn on July 29, 2015, 12:25:42 pmI would certainly do that if I could however, I probably should've mentioned I actually don't have it yet.I bought it just a few days ago, so I'll get it as soon as maybe friday or saturday if I'm lucky.. So I'm just looking for suggestions at the moment, Thanks guys.Quote from: fcb on July 29, 2015, 01:48:46 pmYa know, I Googled it beforehand and most of the pictures I found were the same or unrelated Lol, at least Bing is useful for something I should remember that next time I have trouble.Thanks man, I found a clearer image herehttp://www.batteries4pro.com/4249-pos_thickbox/batterie-808-dremel-72v-multi-usage-8100.jpgso I think can go off that...

author: vgamesx2
author_rank: Regular Contributor
timestamp: July 30, 2015, 08:12:00 pm
content: Hey I was just wondering if anyone had a suggestion on what connectors I should use? I could just solder wires right onto the contacts but I would like to eventually buy an old battery and use its housing instead of having exposed batteries sit in it so I'd rather not mess with them, I was thinking something along the lines of a couple blade connectors to sit on the contacts attached to a deans plug, so that way I could just pull the battery right off and charge it with my balance charger.Whatcha think? Know of anything better I could use? if you need a picture to get an idea of what it looks like then here and the inside is about the size of two 18650s.(also just a mention that the left contact is + and right is -)

author: McBryce
author_rank: Super Contributor
timestamp: July 30, 2015, 08:23:15 pm
content: Drill two small holes somewhere suitable beside the contacts and solder the wires to the other side of the contacts.McBryce.

author: vgamesx2
author_rank: Regular Contributor
timestamp: August 02, 2015, 03:03:32 am
content: Hey its me again, I got my dremel, my batteries and I'm ready to get started however.. I found a new issue while trying to test it, it won't run if I simply connect power to the contacts, the motor is perfectly fine tested it by directly running it off the battery, but the circuit is blocking it from running.... I believe it's because it has a temp sensor and disconnects power if it doesn't sense anything, am I correct? How can I bypass/trick this into working?Here's a picture (too big for an attachment)

author: SeanB
author_rank: Super Contributor
timestamp: August 02, 2015, 07:17:37 am
content: Take that third wire ( white) and wire it via a 10k resistor to the battery negative. If that does not work then to positive. That should work, and if not try a 4k7 resistor. Basically you want to fool the controller into thinking a battery that is not too warm or too cold attached, and, not having the original battery to measure the value of the thermistor and to which terminal it is connected as well, 10k and 4k7 are pretty common values as nominal temperature for small NTC thermistor beads, and it has to be one rail or the other.

author: vgamesx2
author_rank: Regular Contributor
timestamp: August 02, 2015, 06:39:28 pm
content: Quote from: SeanB on August 02, 2015, 07:17:37 amAlright, thanks man I'll see what I can find lying around, not sure if I have any. (by that I mean I'm going to steal them from stuff I don't need)

author: vgamesx2
author_rank: Regular Contributor
timestamp: August 03, 2015, 10:57:02 pm
content: Hey thanks again SeanB, I just found an old FM microphone kit I got a long time ago, which had a 1.5k, 4.7k, 6.2k and 47k resistor.It worked great with the 4.7k resistor, as you suggested, I'm happy with that but.. it does however make a whining noise when I slow it down to around half speed or less, I can live with it I suppose but what's causing it? Is it the resistor? I was playing around seeing if the other ones would produce different results and I noticed it still made the noise, just sooner with the other ones or does it possibly just need some lubrication?

author: McBryce
author_rank: Super Contributor
timestamp: August 04, 2015, 07:48:44 am
content: Those variable speed motors do tend to whine at slower speeds (at least my Bosch Oxi has always done it).McBryce.

author: vgamesx2
author_rank: Regular Contributor
timestamp: August 04, 2015, 05:43:52 pm
content: Fair enough, but I watched some videos/reviews of the dremel and they didn't make the same noise from what I could tell, then again I did have it apart so its entirely possible that it wouldn't happen with a case holding it together or would at least have different acoustics or heck its used afteral.. still trying to find a 10k resistor though to just try it out and see if it makes any difference.Edit: instead of bumping this up again, I'll just leave this edit if anyone wants to know... I did the same test as before but this time I held down the motor and it sounds perfect, so it was just vibrations in the open case causing that noise, all is good and if I feel like I may even upload a short video showing off how well it works when I'm done.