External FET upgrade

Phred168 · #1 09-06-2004, 06:57 PM

As the N channel FETs required for Neuro's turbo are on back order from digikey.com until 10/8, and the only other places i could find them you had to buy in bulk(200+), is there anywhere that sells an external FET upgrade for xmods? I'm having trouble finding them for less than $45.

Jshwaa · #2 09-12-2004, 11:41 AM

Quote:

is there anywhere that sells an external FET upgrade for xmods? I'm having trouble finding them for less than $45.

I believe Rob at Atomicmods.com sells them for about that price($45). I hear that ph2t sells an external for about $35 to $40.

The price of the boards are due to the cost of getting manufactured PCB's(printed circuit board). And if you want to have quality FET's(which is the point), then you're going to pay a hefty price for those as well, because parts retailers had to buy thousands of them to be able to give them away at THEIR price and make a profit. If you want to get cheaper, you'll have to buy your own PCB's and big lots of parts at a wholesale cost. And we haven't even talked about the labor aspect of it Nobody builds these things but us(external FET board retailers). We have to spend our time to build the thing, which ends up being squat if we want to give them away at an affordable price.

So how much are you willing to pay for cutting edge power? I think $45 is a steal. Mine almost cost that to manufacture. This is my xmod with the prototype version of my board.

Here's what it does to plasma dash motors. Eats them.

The left is a plasma dash motor after one run in the car you see above it. The motor on the right is a brand new plasma dash that's never been run in a car. Notice the melting of the bell-cap, the scorching of the commutator, and the brush is almost gone. I probably couldn't get another run out of the brushe if I wanted to. Also, after analysing the armature, I found that the current had melted the magnet wire in a spot, causing the armature to draw a ton of current and output little power. Time to rewind the motor. The point is that this is the difference between stock'like FET's(SOIC 8) and quality FET's. With run-of-the-mill FET's, it would have been the FET's that ended up charred and not the motor. How many people can say that they got a full run out of a plasma dash motor(27 turn, 11 tooth) with li-ions and 2x2 stacked SOIC-8 FET's? Not too many. The FET's would go poof. If you think that >$45 is too pricey, then you'll hate me I suppose. I have to charge $50 plus $25 for install. But you should never have to worry about FET's again. Put as many cells as you can cram in your xmod, and the hottest motor you can find. My H-bridge will love you for it.

hotrod29728 · #3 09-12-2004, 10:01 PM

Jshwaa I talk to you on AOL sometimes and I admire you for your talent as an Electronics Junky I was wondering what is the price for your boards? 75? Thats what I'm guessing you said... Not really sure...

Phred168 · #4 09-12-2004, 10:17 PM

Whats wrong with neuro's external setup? The FETs are sound. You can make it for >$20 with shipping. I agree the labor sucks, and I understand the charging so much for them, but it seems odd that no one manufactures them. But regardless, if I were to get one of your h-bridges, where would I go about ordering them from? I'd like more information, but I dunno if I really need a $50 h-bridge, as I don't intend, at least in the short term, to do anything quite so crazy.

Jshwaa · #5 09-12-2004, 10:35 PM

Quote:

Jshwaa I talk to you on AOL sometimes and I admire you for your talent as an Electronics Junky I was wondering what is the price for your boards? 75? Thats what I'm guessing you said... Not really sure...

The H-bridge itself will go for $50, but I'm assuming most will need an install, which I will do for another $25. I know it sounds a bit pricey, but with the boards I had manufactured, my cost went up there as well. I'm not making the PCB's like other H-bridge retailers are. I'm having them professionally done, to where all I have to do is solder parts on, when I receive them. I'm not into the etching and cutting of PCB boards. When I receive my boards this week, you will see.

hotrod29728 · #6 09-13-2004, 06:25 PM

Okay Jshwaa thanks for the update... I hope to see these things in action... Lemme know...

Jshwaa · #7 09-16-2004, 08:16 PM

Got the PCB's today. Production versions of my h-bridge are already selling. Got my first paypal customer 'bout 5 minutes before this post. First come, first serve.

Phred168 · #8 09-17-2004, 09:39 PM

I'd like some info about them, as much as you can give, anyway. I understand the need for a bit of secrecy, but anything you can say about them, please do

Jshwaa · #9 09-17-2004, 10:28 PM

Quote:

I'd like some info about them, as much as you can give, anyway. I understand the need for a bit of secrecy, but anything you can say about them, please do

No problem. The way any H-bridge works is to utilize a dual push-pull arrangement of power transistors, which acts as a power amplifier that drives the motor. With most H-bridges there is a small signal that's put through a stage(s) of amplification before reaching the gate/base of a power transistor, which turns power on/off straight from the source(cells). The signals are what's called PWM(pulse width modulation), and is a pulsating DC signal which varies in duty cycle, which nets a varied level of DC that's applied to the motor and allows for various speeds. The signals can also turn on certain FET's, while turning off others in a way that allows for dual polarity(forward, reverse). If this is all quite confusing, then check this site out for information about H-bridges in general.

XMODRC THREAD ON H-BRIDGES, MODERATED BY YOURS TRULY

Hopefully something in that thread or the websites linked will clear things up. H-bridges are like riding a bike though, once you learn.......

Anyway, I'm sure you're interested in what's different about MY H-bridge. Let me first say that I've analyzed the signaling of the stock FET's and have found that IC1(xmod's on-board CPU) only ouputs a PWM signal of 0V to 3.4V. That is not sufficient voltage to turn on/off an FET of a considerable power rating. Anyone using IRF-7389's would be dissappointed to know that they are not even turning their FET's on and off at their full potential, and that means more resistance and less power from cell to motor.

What I've done with my H-bridge design was to implement an input pre-amp stage. The pre-amp brings the 3.4V PWM up to whatever voltage you are running, which is what FET's like. If the FET has 15V from DRAIN to SOURCE(VDS), you had better have a 15V PWM signal applied to the GATE to SOURCE(VGS) to optimize the FET's current transfer and minimize resistance. Look at an FET's data sheet and you'll find a spec called RDS(on), and that will be associated with a given VGS voltage. RDS(on) is the resistance from DRAIN to SOURCE. The lower the RDS(on) the better, but you'll notice that for the best RDS(on), you'll need a VGS of about 10V(IRF-7389). 3.4V PWM won't cut it, but strap 10V worth of cells to my H-bridge and the pre-amp stage is going to make sure the FET's get that drive.

In other words, you can put the biggest baddest FET's you can find, on your xmod, and they won't do squat for you if not driven properly. My H-bridge drives them properly, which ends up to be jaw dropping power deliverance from cell to motor. Motors scream in agony and cells drain faster than you might be comfortable with, but all the power goes to the ground and your xmod takes off faster.

Phred168 · #10 09-18-2004, 01:14 PM

So basically, It works a bit like a preamp/amp on an audio system?

Jshwaa · #11 09-18-2004, 01:27 PM

Quote:

So basically, It works a bit like a preamp/amp on an audio system?

Sort of, yes. With any amplifier, you must have an appropriate input signal in order for the amplifier to be able to work with it and amplify it. The pre-amp to an audio system is another example of this, yes. Some car stereo receivers will boast of having a certain amount of pre-amp output wattage. Most of this falls by the way-side as people don't know what to make of it, and they just connect receivers to amplifiers without paying any mind to that. The result of coupling a stereo, with no pre-amp, to an amplifier that needs an apropriate level of pre-amping, would be distortion and noise. With a power amplifier, such as an H-bridge, it ends up to be loss of power.