darkith
Posts: 523
Joined: 2/19/2007
From: Fredericton, NB, CANADA
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quote:
ORIGINAL: firenick
I've been watching this thread because I am about to order a Stug and a DBU/DBC from RC Command. This will be my first non Tamiya battle ready tank although not my first Heng Long tank. I have a PIII with Tamiya electronics in it. I'm not too familiar with the RX-13 or RX-14 boards, only what I've read about them here. Now here's a question that I've been wanting to ask. I've seen the layout of the FET's on the RX-18 board and was wondering if you could stack the FET's to provide less resistance and heat. I used to own some Kyosho Mini Z's and an Xmod and the mini racer guys were stacking the FETs to run higher voltage and hotter motors through the stock boards. Atomicmods.com has some silly fast upgrades and cars which do 35 mph with 180 size motors. They acheive this by upgrading and stacking the FETs. By stacking the FETs you can double the current through them. Like I said, I'm not too familiar with these but thought maybe someone more familiar with the Heng Long electronics could check this option out. Here's an example. http://www.atomicmods.com/Products/XMOD-Evo-Board-Pro-Ready-Stacked-FETs__10614.aspx
Here is a tutorial on how to do this modification. http://www.atomicmods.com/Categories/Tutorial-1-28-XMODS-Generation-1-Stacked-FET-Installation.aspx
I hope these 2 links work.
Nick
Possibly. There are three main issues with motor driver transistors in general:
1. Voltage drop (how much voltage you lose through the transistors
2. Max current handling (the amount of power they can handle before they go pop)
3. Max power handling (basically how much heat they can handle before they cook themselves)
Voltage drop depends on the transistor type. Bipolar transistors drop around 0.7v. A Darlington pair like the RX13 drops about 1.4v. A FET will drop virtually nothing, so replacing a bipolar or darlington pair with a FET will net you more voltage to the motor, less heat in the transistor (fewer blown boards) and therefore more torque. The RX14 and RX18 boards already use FETs, so no immediate gain there.
Max current handling is pretty basic. Add bigger/badder FETs until they can handle the peak and sustained current you expect the motors to be drawing, preferable including stall current (sustained) for tanks. Heng Longs RX-18 FETs look okay for peak current, not sure about sustained yet. This is one of the reasons why you'd replace FETs or stack multiples in parallel.
The heat handling may be the issue. The FETs that Heng Long use are reasonably low resistance, but lower resistance would help. More FETs in parallel (not necessarily stacked, but side by side as well) also help (as this lowers the resistance which creates less heat), but after a certain point, stacked FETs can't get rid of their heat (normally, the FETs conduct the heat down their legs to the PCB). The top FETs start experiencing diminishing returns, so I suspect the 6-FET-tall stacks of XMOD FETs are more for show then they are for effectiveness. [sidenote: too many stacked FETs increases the gate capacitance, so they can actually malfunction because the gate voltage takes too long to change]
So, if you're running a particularly heavy tank or hot motors, the stall current might be enough to bake the FETs, then stacking a 2nd set on top will reduce heat build up and accommodate higher peak and stall currents (two birds, one stone). If you're running a heavy tank with draggy axles in a hot climate with poor ventilation, the 2nd set might not buy you much if the problem is heat, a fan over the FETs might help more, or better internal ventilation.
On a sidenote, when it comes to stacking FETs, you want to use the same type of FET so that it switches on at the same gate voltage. And avoid stacking too many to avoid messing up the gate capacitance.
D.
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RCU Required Disclosure: Manufacturer of DBC battle circuit for Heng Long tanks.
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All Forums >> RC Cars, Buggies, Trucks, Tanks and more >> RC Tanks >> RE: HL Stug rebuild 
RE: HL Stug rebuild - 
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) - those chips look a whole load more toasty than I've seen in 10 years of viewing failed computer