Hello all first post here. I recently bought my first parallel box mod and I need some help. It is a cheap China mod bought from a local small vape shop so I'm sure that's half the problem but all the reviews I found online seemed to be good so that's the only reason I got it. Anyways to the point. I don't know what's going on with it but the mosfet gets very hot even at .15 ohm builds. It got so hot last night one of the wires came off the mosfet and I had to re attach it. So what's going on here and how can I fix this issue because it has me worried. Thanks.
Help with mosfet
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How is that the issue? I am running dual 38 amp battery's and in a parallel mod that gives me at least 50 amps to work with so .15 is by no means pushing it at all. There's guys that run .08 all day with these and don't have issues.
Can you see a part-number on the MOSFET? You are running at over 100W, which may be pushing it beyond it's limits. If you are handy with a soldering iron you can probably replace it will one rated much higher. For just a few bucks you can probably buy one with much lower on-resistance which will stay a lot cooler. Plus the wires running up to it - are they rated for 30+ Amps? Can you tell the gauge of wire used?
Thanks W00fy, the wires are 16AWG. I plan on upgrading all the wires to solid copper at some point and I'm sure that will help but here is a picture of the chip in the mod. Ignore the broken wire it popped off when I pulled the chip out. Also I am pretty decent with a soldering iron so that wouldn't be an issue to change it out.
I didn't realize the picture would upload so small the best I can see the numbers are
STPS30H100CT
G4 0SE VW
CHN 019
ST
I didn't realize the picture would upload so small the best I can see the numbers are
STPS30H100CT
G4 0SE VW
CHN 019
ST
Last edited:
Hmmm.... That device is not a MOSFET. It's used to combine two batteries into a single output, and is only rated at maximum 15A per cell. So that might be the problem - you are right at that limit. However, I have not seen a mod that doesn't use a MOSFET to switch the high current to the coil - do you have a photo of the whole thing inside so I can take a look? Is that the only device in there?
That's strange... Its a little cramped in there but maybe you can see what you need to see. Well besides the little LED light its the only thing inside it except the switch and all that.
Yeah, kinda hard to see! A couple of questions - the MOSFET (actually a Rectifier) must be screwed tightly to the case (with some thermal grease under it). Was it attached tightly? If it is floating it will get very hot! Also, the two outside legs should go to each battery, and the middle leg go to the switch. Is that how it's wired up?
Sorry about that but it wont allow be to post a link of the full size image. But the chip is attached to the case but not by thermal grease its stuck on by a piece of double sided tape and that's how it came to me out of the box. And it is wired correctly.
The case is ABS plastic
The case is ABS plastic
Okay..... I am assuming a few things here, but it should be accurate enough!
So, assuming you vape at 0.15Ω, thats about 120W. The Rectifier will draw total about 29A, which is 14.5A per leg. The Rectifier itself will dissipate some power and heat, but that should be no more than 27W. For that device, 27W will heat it up to about 50°C. That is pretty hot, especially if the Rectifier is not attached to a metal case or heat-sink. If you vape a lot, that heat will not be able to get out and the rectifier will keep getting hotter and hotter. For the solder on the wires to melt, you normally need something around 180°C. The rectifier itself is only rated to a maximum of 175°C, so you are exceeding that limit. That is bad, and will make the rectifier fail after some time - or possibly it could just break without warning.
So, you could replace it with a bigger device. This is the same thing, but has twice the current capability: STPS60H100CT STMicroelectronics | Mouser
It also has a lower thermal resistance, which means it will not get as hot (by 5-10°C), which is good.
But, the real problem is that you don't have a metal case. So please keep an eye on it, especially if you go even lower in resistance on your build. If you want, you could buy something like THIS, which is a small heatsink - bolt the rectifier to it. But without airflow it will still get hot... but it is better than nothing! If you want to drill some holes in the case that would help as well.
Hope this helps!
So, assuming you vape at 0.15Ω, thats about 120W. The Rectifier will draw total about 29A, which is 14.5A per leg. The Rectifier itself will dissipate some power and heat, but that should be no more than 27W. For that device, 27W will heat it up to about 50°C. That is pretty hot, especially if the Rectifier is not attached to a metal case or heat-sink. If you vape a lot, that heat will not be able to get out and the rectifier will keep getting hotter and hotter. For the solder on the wires to melt, you normally need something around 180°C. The rectifier itself is only rated to a maximum of 175°C, so you are exceeding that limit. That is bad, and will make the rectifier fail after some time - or possibly it could just break without warning.
So, you could replace it with a bigger device. This is the same thing, but has twice the current capability: STPS60H100CT STMicroelectronics | Mouser
It also has a lower thermal resistance, which means it will not get as hot (by 5-10°C), which is good.
But, the real problem is that you don't have a metal case. So please keep an eye on it, especially if you go even lower in resistance on your build. If you want, you could buy something like THIS, which is a small heatsink - bolt the rectifier to it. But without airflow it will still get hot... but it is better than nothing! If you want to drill some holes in the case that would help as well.
Hope this helps!
My eyes almost crossed reading that hahaha obviously you know your electronics and I'm glad you are the one that found my post! This helps a lot and I will defently be ordering these parts and probably doing some soft of vent hole to help also. Thanks again for all your help!
its a rectifier rated for 15A output.... ergo .15 ohms is the issue and most likely a cold solder joint on the chip
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