Bypass thermistor safety circuit - Page 1
Forum section: electronics / beginners

author: raybies
author_rank: Contributor
timestamp: June 23, 2018, 07:48:30 am
content: Hi.I purchased a 58v cordless leaf blower without the 2x more expensive battery and charger to build a small wind tunnel.I've taken it apart; it has a nondescript DC brushed motor (supposed to have a brushless motor), a potentiomter and what appears to be 2 safety circuits.The battery compartment has 3 terminals (+, T and -) I'm calling the center one T bcos I assume this is for a battery thermistor.I'm supplying power w/ 14s LiPo; 52-58v.The motor switches on for 5 secs and then cuts off, and can keep doing this if I kill power every attempt.I had a spare thermistor and measured its resistance @ 100KΩ, and grounded(-) to T w/ 100KΩ; this made no difference.I have tried 100Ω, 220Ω, 500Ω, 680Ω, 1KΩ, 2KΩ, 5KΩ, 9KΩ, 20KΩ, 467KΩ, all with 0 difference.Now I'm here in need of help before I break itInfo:Blower Manual:http://www.datastreamserver.com/manuals/BLWCLSBMRATB5-manual.pdfT goes to the PCB > D10PhotosThanks

author: tsman
author_rank: Frequent Contributor
timestamp: June 24, 2018, 01:31:15 am
content: The "T" terminal probably is connecting to a big current shunt resistor in the battery pack. The controller expects a specific voltage range on that pin and will stop with a fault condition if it doesn't see it. The overtemp cutout would be handle by the BMS board inside the battery pack and not brought out to a terminal.

author: raybies
author_rank: Contributor
timestamp: June 24, 2018, 05:21:58 am
content: @tsman thanks for responding; are you saying that instead of resistance the T pin from the battery is carrying voltage? I've taken apart a few battery packs and they've always had a thermistor if anything, what would be the point of current shunt resistor in the battery pack? The voltage can already be measured.I agree the battery pack will have the BMS, but the blower and charger should also have a means to kill power if over heat.I have a variable PSU, do you think I should apply V+ to the T connector, and V- to ground somewhere on the PCB?I still believe it's expecting resistance, but a narrow range of say 100Ω but I just don't know how to determine it, so bypass will be easier.

author: SeanB
author_rank: Super Contributor
timestamp: June 24, 2018, 05:58:45 am
content: Probably has a 4k7 thermistor between that pin and ground, so just placing a 4k7 resistor between that pin and ground will fool it into thinking a battery that is cool is attached.

author: raybies
author_rank: Contributor
timestamp: June 24, 2018, 08:36:06 am
content: @SeanB... nope still cuts off after 5 seconds, so no change.Here are some more photos

author: Neilm
author_rank: Super Contributor
timestamp: June 24, 2018, 05:26:36 pm
content: If there is just a thermistor in the pack then I am surprised it matters - generally that would only be used for charging. Having said that, I can see use cases where they might monitor the temperature during run time but the units I have designed with Li-ion packs don't. All sorts of extra issues with monitoring that.There are battery management systems that will do a lot more - and the 3rd pin is not a resistor but a wire used for communication of pack parameters. If that is the case then you could be in trouble as that would be hard to easily fool.You have not been too clear on the application - I am assuming that you are using part of the control circuit to power the motor. In which case, there may be another signal missing, for instance a feedback from the motor to show it is turning. To prevent over heating, the circuits would stop the motor if they were not detecting it turning.

author: raybies
author_rank: Contributor
timestamp: June 25, 2018, 03:50:10 am
content: All I'm doing is replacing the battery pack. I have no idea what's in the original pack, bcos I don't have an original pack.My assumption is that the pack provides 58v and that's what I am supplying, but I have tried 48-58.8v.Everything works fine for 5 seconds.

author: DC1MC
author_rank: Super Contributor
timestamp: June 25, 2018, 04:53:32 am
content: Since the times of Nokia 3310 battery packs don't have a simple thermistor there, but a whole lot of electronics. Especially powerful batteries that can become deadly incendiary grenades very quickly.Have a look on this videos, around 1:30 you can see the electronics in the battery pack and it's a lot.IMO, your best way is to see if the battery management chip is separated from the motor control and try to bypass or remove it it, or if it's a CC brushed motor, to build your own control board. For a wind tunnel, you need a better control of the airflow anyway.Cheers,DC1MC

author: Zero999
author_rank: Super Contributor
timestamp: June 25, 2018, 08:58:11 am
content: How about bypassing the motor controller altogether and powering the motor directly?It's fairly easy to make you own PWM controller, using cheap, commonly available parts, if you want speed control.

author: raybies
author_rank: Contributor
timestamp: June 26, 2018, 03:30:21 am
content: Creating my own controller is the last resort.I figured bypassing would be easy... like 1 wire over a controller or at worst a cap somewhere.I still believe it should be an ez resistor of a specificbut I'm a n00b. The first chip in the chain looks like an amp (https://www.st.com/resource/en/datasheet/lm158.pdf), so tsman could on to something with voltage on the T pin. I'm sure somebody who knew what they were doing could fix this in 5mins... it's not like chipping a console, there's very little to the circuit.

author: firehopper
author_rank: Frequent Contributor
timestamp: June 26, 2018, 07:37:37 am
content: it could be a one wire type interface to read information from the battery pack. a lot of the newer battery packs have a lot of ic's in there that communicate back and forth. so it could actually be digital communication.

author: Zero999
author_rank: Super Contributor
timestamp: June 26, 2018, 07:44:01 am
content: Quote from: raybies on June 26, 2018, 03:30:21 amYou could try a potentiometer and alter the value, until it works, but from what others have said, I doubt it's that simple.QuoteYes, that's a very common part. You could have a go at reverse engineering it, which would be very educational, even though it might be more difficult, than designing your own.

author: raybies
author_rank: Contributor
timestamp: June 26, 2018, 11:49:58 am
content: Hi.Sorry I've made a mistake with tracing the yellow wire (T). A friend said I was wrong bcos DC wouldn't flow through the C24 cap, so I put acetone to try and remove whatever (epoxy) they spray on the board and trace properly.New:- T goes to D10.- Motor is getting 20v.- Large cap is 80v, but has 36v when battery pack removed. Small caps are 25v.Battery pack had 55v when testing.Hi res images: I've place a yellow line where T goes.https://goo.gl/BNXv4whttps://goo.gl/vgQowy

author: ledtester
author_rank: Super Contributor
timestamp: June 26, 2018, 06:42:20 pm
content: Quote from: raybies on June 26, 2018, 11:49:58 amThese links aren't working for me.

author: raybies
author_rank: Contributor
timestamp: June 27, 2018, 12:44:24 am
content: Sorry. they're not working for me either and I tested them with an un/authenticated browser... sorry to the 5 people who clicked the link.Updated links:https://goo.gl/BNXv4whttps://goo.gl/vgQowy

author: Zero999
author_rank: Super Contributor
timestamp: June 27, 2018, 08:41:34 am
content: Quote from: raybies on June 27, 2018, 12:44:24 amIt would be better if you uploaded the images, as attachments, on this site. If that site goes down, then this thread will no longer makes any sense.

author: raybies
author_rank: Contributor
timestamp: June 28, 2018, 01:08:23 am
content: 2MB limit; images are 6-8MB and optimized from 13MB jpeg.Also I updated the first post with the better photos, so I think the context will be preserved and the images are all hosted by google. I know goo.gl is being turned down but all goo.gl links are supposed to be remain active.

author: ranchero
author_rank: Contributor
timestamp: June 24, 2021, 08:01:45 am
content: Bringing this topic back to life, hopefully this information will help to someone.Some 60V tools are discontinued and can be purchased for pennies when they don't have battery and/or charger with them.For example, Craftsman 60V (not newer V60 line) tools, Lawnmaster 60V tools. It took me a while to figure out that they use same form factor battery (Lawnmaster 60LB5025-S similar to Craftsman 60LB2021-S similar to Craftsman 151.98833).I just purchased brand new Lawnmaster trimmer CLGT6014A for about $40 US, no battery, no charger. I was hoping to convert it to Dewalt 60V battery. Opened trimmer - figured out that there are 3 wires to the battery - plus, minus and mystery yellow wire which marked as TEM on the circuit board.Tried to run tool on the bench on 30V, just with 2 wires (power) - runs 2 sec, then dies. Then I did some research, found this thread. Someone suggested 4.7 kOm to the ground - no luck. Tried to short "temp" wire to 60V, to ground, tried 1kOm, 10kOm, 100kOm to the ground - tool either does not start at all or runs for 2-3 sec.Then I did some more research and found that temp sensor could have 10 kOm resistance. I had newer Craftsman V60 battery in my shop. It has completely different design, but I've checked it anyway and found that one of the contacts indeed has 10 kOm to the ground. I've tried 10 kOm already but with wrong voltage (30V instead of 60V). So I created approx 58V from 2 power supplies and tried one more time with 10kOm to the ground. Success! Tool runs continuously!So if you will see similar setup - 3 wires (actually battery has 4 contacts but 2 of them are ground), 60V, you can try 10 kOm as "temp sensor".Hopefully this post will save few discontinued tools from dumpster. Dewalt adapters are very cheap on Aliexpress, and most people have some of Dewalt tools and batteries in the shop. Don't like Dewalt? You can adapt any battery with 60V output. Just be careful do not overheat battery, or install proper temp sensor instead of 10kOm resistor.

author: Bigwavedave2u
author_rank: Contributor
timestamp: April 13, 2022, 01:58:49 am
content: Could you help me understand exactly what you did? Yep. I'm a total F. N. G.I have a 3tab kobalt. How do I bypass thermistor?One wire from Positive, one wire fromT tabAny ideas??Thanks

author: Bigwavedave2u
author_rank: Contributor
timestamp: May 08, 2022, 07:11:07 pm
content: There are a couple methods to address this issue. First, in newer tools (say 2020 forward) the third tab is both a thermistor and a comms* tab. In older cordless tools you are dealing only with a third tab connected to a thermistor.This can be fooled or bypassed generally by determining resistance between the third tab and the negative tab, then soldering a resistor of that amount between the third tab and the negative tab. This is usually done inside the tool case. The downside to this is you have eliminated your low charge protection and you could very well be drilling,sanding, or what have you with power tool hand grenade so let the solderer beware. In either new or old three tab tool This can be fooled with an adapter from an old battery which could in turn be attached to a homegrown mount. And just as you would harvest out the internal Samsung (or whatever brand) individual cells by snipping away the solder joints when attaching an 120v ac - 24v dc unit (or whatever volt your cordless tool is), you could use the post harvested kobalt battery top for your mount with a double spade +- conventional battery of your choice within reason. I would only recommend (at your own risk) using like volt with like volt. Or at a minimum a slightly larger volt battery with a lower volt tool such as a 24v kobalt with a 20v DeWalt. In point of fact this entire post is only reference material for my own use and anyone else who uses this information does so at their own risk.*This is a comms tab but not in the way you would think.Because you are dealing with a brushless motor you have three leads where the - tab, the + tab, and the T tab are operating in a forward sequence where upon rotation of motor stator the signal for say the current - tab moves forward to become the + tab and then on the next rotation becomes the T tab and so on and so forth. This is consistent with all three wires exiting motor.As for bypassing the T tab, I believe the only way to safely accomplish this is in the aforementioned explanation. HOWEVER, finding the proper thermistor resistance (generally anywhere between 4.7 and 10 ohms soldered between T and - tool tab should work. IMPORTANT: You'd damn well better make certain your power source is independently protected at all levels as you are disabling your low charge protection with this method and for me the thought "ABANDON HOPE ALL YE WHO ENTER" comes to mind.There is a third option and that is that there is a company out of China which manufacturers just about all the proprietary Li-ion brand battery management systems for pennies on the dollar. In most cases you could mount one of these on the battery to cordless tool of your choice and thereby have all your Li-ion safeguards in place. Should you desire this companies name just message me and I will get it to you.