Mosfets?

[LV LOU]

Anybody know anything about Mosfets? I have two MRC 2800's.A few days ago,I turned one of them on,the locomotive started moving even though the throttle was off.I ripped it apart,checked it out,everything looked okay,I figured it was the Mosfet.I took one out of an RC car speed control,replaced it,and now it works fine.I was thinking while I have the both of them taken out of the panel,I might like to upgrade all 4 Mosfets.The one I put in doesn't quit put out as much as the old one,and I'd like a little more power to run some of my older stuff.I know what they do,but know nothing about grades,ETC,is anything from say,Radio Shack worth my time? Anything online I could buy that would be a higher quality than what's in there? Also,anybody besides MRC and Troller make momentum powerpacks? Are there plans online or a kit available to build a good momentum controller?.I'm gonna be posting pics in a few days of a walk around I built from an MRC pack with momentum/brakes...THANX!!!

[mmagliaro]

In general, my experience with Radio Shack's parts is very good.  They are way more expensive than what you will pay at any on-line electronics or surplus house like AllElectronics, JameCo, Hosfelt, etc.  But the parts themselves seem to be
good quality stuff.

If you don't replace the MOSFET with one that has similar gain characteristics,  I'm not surprised that you see it putting
out less than the old one.   It would be better to try to read the number off the old one and buy an equivalent
replacement if you really want the throttle to work the same as it used to.

In fact, the only negative experience I had with a Radio Shack part in my life was back in the 70s when I built an SCR
throttle using "whatever SCR I could get" by buying one that RS carried, instead of the correct part.  The one I bought
had completely different trigger characteristics, so the throttle worked like junk.   I then bought the correct part
from an electronics store and it worked perfectly.  But that wasn't RS's fault.  I was using the wrong part.

[LV LOU]

Hmmm...This is the right part #..Big bucks,80 cents..LOL!! I'll hafta check the specs,and see if there's something better..At this price,maybe I could use two..
 
http://www.futurlec.com/Transistors/TIP101pr.shtml

[LV LOU]

Well,solved that problem..Just bought two huge,like new Troller Autopulse 5's on Ebay,LOL!! Rather than try to make the MRC's better,I got everything I want,bigger power,brake,and momentum..I'm still gonna replace the mosfets in the MRC's,but I'll keep one for other RR I'm building,and dump the other..Never really like these,they didn't seem to really have any real power.I have several sets of 3 units that will kick the breaker after a few laps..Don't have any idea how you could use them for HO..

[mmagliaro]

But Lou, the TIP101 is not a MOSFET.  It is just a Darlington power transistor (a Darlington pair of conventional
bipolar transistors).

So now even though you worked around your problem,  I'm curious. 
What did you put in its place, temporarily, which worked, but didn't put out the same power
as the original part?  With regard to your original problem, I suppose the original power transistor could get leaky
and start turning on when it shouldn't.    This smells (ha ha) like a heat problem.   Pehaps that
TIP101 wasn't tightly screwed to the case or heatsink?

[LV LOU]

 Max,I'm just going by what I found on the case,there's another # under TIP101?  Maybe they just have that # on it to be goofy? I know you could never find the right ones for brakes on RC speed controls.I replaced it with a piece from a Tekin RC speed control I was using for parts.Works perfectly,but the full on voltage is one or two volts less than the MRC piece.It will probably handle 10 amps,though.
   Yep,first thing I checked was the screw holding it to the heatsink...Hmm..What's the saying? Lightbulb? I just realized the ground on the heatsink was through the body of the pot,which was only grounded through the heatsink with cheesy tabs that held the pot to the sink through the sink & the plastic case.If those tabs came loose after all these years of use,THAT might have caused the problem.Just happens that the first thing I did was take the pot off and clean it,and I reinstalled it,but I didn't tighten the tabs before I tried it..I tightened the poo out of those tabs when I put it back together....Maybe I'll put that Mosfet back in,and see if it's okay..I was kinda puzzled,usually when those things go,you see a crack or something,this one looks perfect..

[peteski]

Replacing electronic components with something you think might work will eventually get you in trouble.  There are lots of transistors which externally look the same (all use the same case).  But inside you can have a bipolar transistor, darlington pair of bipolar transistors, P-channel or N-channel FET (or MOSFET). All of these types of transistors also come in many flavors (voltage, current and gain ratings can very greatly between the various types).  Some cases have the metal heat sink tab isolated electrically, others have it connected to one of the leads.

[up1950s]

MOSFET and static are to be avoided , or so I have read , it leaves craters in the metal oxide surface which damages them .

[peteski]

MOSFET and static are to be avoided , or so I have read , it leaves craters in the metal oxide surface which damages them .

Many nowadays have built-in protection diodes, but you're right, static electricity and MOSFETs (or any subminiature active electronic components) do not play well together.  It is best to avoid zapping them.