PDA

View Full Version : FET induced glitch?


locwan
2006.08.30, 07:43 PM
I've just finished upgrading my blown stock FETs to IRF7317, 2x2 stack on the MR02 chassis. I'm running the Atomic Stock motor, with brand new neo magnets and a 9t pinion. Is it possible that upgrading the FETs can introduce glitches? Here's what I'm experiencing:

1) The range before the car glitches is decreased. (I'm running in the basement, and there are a few spots on the far side of the track that have been known to have interference) The car will noticeable lose steering input/angle in the middle of a turn, and throttle response becomes choppy whenever the car reaches the far side of the track. (This is especially severe if the throttle is punched in mid corner, instead of carrying a constant throttle input.

2) As the battery begin to run low, the car loses it's steering input/response, if I press the throttle, immediately followed by steering inputs. Loss of steering is less severe if I depress the throttle gradually, first bring the car up to speed, than turning. When using stock FETs, the car always runs until the there's not enough power to run the motor, but steering is always available.

Could it be that the upgraded FET's is allowing the motor to draw more current, faster, therefore introducing a voltage drop in the circuit before the battery could recover? If so, could a large capacitor help?

Please help!

rharris
2006.08.30, 08:08 PM
I'm having the same trouble and one more symptom: Sometimes the throttle sticks wide open!!!

From what I've been able to gather, this is what's happening:

When you stack the fets, you are allowing the motor to draw more power. The power that is now going to the motor is no longer going to other components of the board. Like the fets that control the steering servo. Your decrease in range, and steering loss at the end of battery life are all due to the fact that the motor is taking everything the battery has to offer.

Also, the problem gets more prominent as you increase winds on the motor. So, for example, you'd have more trouble with a PN Anima then with a PN Speedy 05. I've personally experienced this.

I recently installed a capacitor (around $10) to reduce the glitch. I haven't had time to test it yet.

Also, you can get a bit more range (and less glitch) with a better antenna. The trick is to solder a antenna wire into the board that is the Length of the original piano wire. The idea is to boost the signal by having a single wire soldered rite to the board in place of the piano wire setup that comes stock. You'll need to take some shrink wrap and put the whole mess around the stock piano wire so the antenna will still stick up in the air and not drag behind the car.

Good luck,

P.S. If anyone has any other advice, I'd love to hear it.

locwan
2006.08.31, 12:14 AM
Yes, your opinion matches with what I was suspecting; that the FETs are allowing the motor to draw more current from the battery. Can anyone else in the know second/confirm this?

Right after I experienced the glitch, I promptly made a custom antenna using a strand of wire inside a CAT5 cable, but this doesn't seem to help very much. Right now, I'm considering trying a capacitor too.

rharris, what capacitor did you use, and how/where did you install it? I've seen some installs that are directly connected to the motor tabs, while others have wires that connect to the battery terminals. If you have photos, that would be great. Please let me know how it works out once you've had a chance to test it.

rharris
2006.08.31, 01:32 PM
I'm going to test it out tonight. I'll snap some pics while I'm at it.

I soldered motor wires to the capacitor and mounted it on top of the chaise. It's wired rite into the battery leads. A very simple install.

One other thing I've been told, make sure you have fresh transmitter batteries.

imxlr8ed
2006.08.31, 01:53 PM
I would also check the feet of the fets to make sure they are making a full connection to the board, I had a similar issue a while ago with one of my triple stacks. It's tough to make sure that there is no cold connection on the front of the fets, but I just carefully touched the tip of the iron down in there.
Problem was solved.

rharris
2006.08.31, 04:13 PM
I would also check the feet of the fets to make sure they are making a full connection to the board, I had a similar issue a while ago with one of my triple stacks. It's tough to make sure that there is no cold connection on the front of the fets, but I just carefully touched the tip of the iron down in there.
Problem was solved.

Thanks. I believe this may be the issue with me. I've now tried everything else, and I'm still getting an intermitant glitch.

I'm going to try that next.

I've ordered a bunch of 4562 and I have a spare board to practice on. I'm also going to switch to a 2x2 stack.

locwan
2006.08.31, 09:38 PM
imxlr8ed, you mean that even with the FETs making poor contact, you were able to run you car, but the range is decreased and the car glitches?

Come to think of it, I should probably check the FETs again, since it is my first time doing a FET stack, and I've done it with a soldering iron with a pretty blunt tip..

I'll touch up the legs of the FETs again to make sure they're making good contact, and report back to see if that'll solve the problem.

Nightcargo
2006.09.01, 08:50 AM
All of my FET upgraded or turbo installed cars loose steering at the end of the run. This is due to the fact that the motor is drawing all of the power, as someone previously stated. This usually does happen when the batteries are almost completely drained so it has never been a factor for me. I have added a capacitor (http://www.rckenon.com/public_html/shop2/catalog/product_info.php?cPath=62_37&products_id=98)
to two of my cars and noted slight decrease in exhausted battery steering degradation.

One of my FET upgraded cars (4562 MOSFETs) goes to full power periodically when powering on, but never displays uncommanded throttle application during a run. I was told this is a standard problem with 4562's. Some also say it is a problem induced from a FET being overheated or electro-statically damaged during install.

As far as glitching, are you talking about the steering chatter associated with the steering servo recentering or true uncommanded steering/throttle inputs? The recenter problem can be addressed by the pot shim method or screw method. This holds the pot more secure reducing recentering issues. Due a search on steering glitching or chatter and you will find info on those.
Also check to the steering pot wires coming from the board. I had bad steerring issues after a FET upgrade and found I had only one straind of wire attached to the board. THey are easy to break when you manipulate the board during FET removal and resoldering.

Hope this helps!

locwan
2006.09.01, 10:13 AM
No, I don't think it is the steering pot. I've shimmed it long before the FET stack, and it hasn't been giving me trouble for a long time. I"ve also checked the wires to steering fets and pot; all are making good contact and connection is sound.

The "glitching" that I'm experiencing is a lost of steering input, and choppy throttle response when it gets to a weak reception area of the track.

I'm considering installing several 6v, 1000uf caps that I found on an old mother board. (2~3 of them in parallel), but I wonder if 2000~3000uf provides enough capacity to smooth out any current ripples, since PN offers caps with 4700uf.

imxlr8ed
2006.09.01, 01:39 PM
I'm not very scientific with my cars, it's just that the car ran fine with no glitches or loss of steering before I did the fet job... so when it started happening after, I figured that could be the only thing causing it. Therefore, I did like I said, made absolutely sure that all the feet were properly connected, did that and poof ! Car was fixed !

So, my not-so-scientific conclusion for that one was...

Must've been a weak connection causing all the wackiness !

I would check the pot also, antenna connection, crystals... go right down the line as usual.

SuperFly
2006.09.01, 07:33 PM
imxlr8ed, you mean that even with the FETs making poor contact, you were able to run you car, but the range is decreased and the car glitches?

Come to think of it, I should probably check the FETs again, since it is my first time doing a FET stack, and I've done it with a soldering iron with a pretty blunt tip..

I'll touch up the legs of the FETs again to make sure they're making good contact, and report back to see if that'll solve the problem.

A trick I just figured out for checking fet stacks is using a multimeter that has a circuit-checking setting. I put one of the probes on the top fet leg and search the board for the corresponding via (little hole that takes the signal to the other side of the board) to look for a complete signal path. I figure if the top fet is fully connected, the ones under it probably are too.

locwan
2006.09.02, 02:22 PM
A trick I just figured out for checking fet stacks is using a multimeter that has a circuit-checking setting. I put one of the probes on the top fet leg and search the board for the corresponding via (little hole that takes the signal to the other side of the board) to look for a complete signal path. I figure if the top fet is fully connected, the ones under it probably are too.

I like that idea. However, I do think that the fets must be connected in order for the car to run at all, so I think that is a good test to do to see if the legs of the fets are connected initially after the fet upgrade, but doesn't necessarily tell you if they're making good connection.

UPDATE:

I've touched up the legs of the fets again, and made sure that all connection is sound. (I'm pretty satisfied with how neat it is too :)) However, this doesn't seem to solve the problem for me. I've also installed 3 6v 1000uf caps in parallel, and have noticed a gain in on throttle pickup.. but this doesn't seem to solve the glitching problem either, and I actually find that the throttle is becoming too punchy for smooth acceleration with the caps on there. :(

Any other ideas? I'm starting to suspect that the glitching might not be totally due to a change in the car's setup, but of an external source. (New wireless or sound systems being setup in the houses near me perhaps?) But I'm not going to rule out an internal problem yet until I've exhausted all my options and checked everything single aspect that can affect the reception of the car..

rharris
2006.09.03, 12:53 AM
I put the capacitor from PN on my car to try and solve the glitch issue. I went to a Friends track on Thursday, and everything seemed fine.

I had thought this solved the problem. Then I ran at my house on Friday night with our club and the glitch is back.

The capacitor seems to be a bust.

For me it seems to be environmental, but only for this one car.

Other guys were having trouble on Friday. So on a hunch i took out my multi meter and checked all their batteries. Turns out everyone with glitch issues tested at less then 10v.

I run a Helios and was showing 12v. My fet stack car was glitching. Also my 3010 car was glitching (mr-02 converted from a awd), but my stock 02 ran fine.

What a PITA

In another thread, someone posted that you can overheat the fets. How do you test for this?

locwan
2006.09.03, 01:25 PM
Other guys were having trouble on Friday. So on a hunch i took out my multi meter and checked all their batteries. Turns out everyone with glitch issues tested at less then 10v.

Are you refering to the voltage of the transmitter?



In another thread, someone posted that you can overheat the fets. How do you test for this?

I've been having a discussion with LBRC on a similar issue in this tread. http://www.minizracer.com/forums/showthread.php?t=22740 Unfortunately, I don't know of any way to test if the fets were damaged during installation. But according to some new info that LBRC presented, a fet stacked car isn't pushing more current from the battery to the motor, but is simply allowing more of it to pass through with the lower resistence. However, I'm still under the impression that the rate of this current flow is increased, therefore changing the voltage levels in the circuits before the a worn-out set of batteries can recoup. (sort of like the lights dimming for a fraction of a second when you turn on the hair dryer in some older houses)

rharris, has the capacitor made no difference at all after you've installed it?

ph2t
2006.09.03, 09:04 PM
It's funny, what you're describing is more to do with iwavers. Not to sound like a bastard, but are you %100 sure you have a kyosho? The koysho steering circuit uses high quality parts, the steering motor is powerfull and the stock fets are good for the purpose.

You must be experiencing one hell of a current dump to loose the steering controll..... The motor si OK? try it on a single AAA cell and if the cell get hots quickly then the motor is binding somewhere...

Anways, here's my pointers.

1) Remote the steering pot and clean it in some contact cleaner spray. Let it dry and reinstall.

2) Replace the stock pot with a new replcement one.

3) Lengthen the arial to approx 15cm tall. Use heatshrink tube on the bast to protect it from shorting against the motor can and/or motor terminals.

4) Use a 6V 1000uF capacitor (min) to help smooth power fluctuations out and increase stability.

5) Clean your battery terminals!!!

6) Re-set the spring on the battery terminals. Over time THEY WILL go soft on pressure agianst the batts.

7) Run with different sets of batteries...Check the terminals on the batteries to ensure they don't have crap on them.

8) Check the two power screws that mount the PCB lugs to the battery power lugs. They are small screws located at the rear of the chassis each side of the stock spring damper. If these aren't tight, then you can have the problems you describe.

9) Check and/or re-do the solder points on the PCB -> steering motor, PCB -> steering pot, PCB to power switch, etc, etc.....

10) check the gear mesh on the motor pinion -> spur gear. If this is binding you're sure to have high current drain. Still, I doubt it's this one....


As you can see the list isn't short. Most of it is general maintenance though, and I'm sure I've missed some points that some of our more anal members would do, lol.....

ph2t.

locwan
2006.09.04, 04:04 PM
After all of the experimenting, trouble shooting and all that, I'm finally going to say that there is a 80~90% chance that the glitching is due to external interference afterall. Just a coincidence that the interference has become stronger just as I finished the fet upgrades. I based this conlusion on the fact that last night a few of us ran on the track, and the interference was affecting everyone's car, even those that did not experience this before. Of course, some had a little more severe glitching than others, (like my mr02).. :(

I say 80~90% and not 100% because there was one single car that wasn't affected.. my old and beat up mr-01, on which I've also upgraded the fets to IRF 7317 in the same time frame. This was the ONLY car that was not affected the whole night.. We have a mix of mr01 and mr02 running on the track, and this particular mr01 was different from the others because it was the only one running with a stock-motor, only upgrade was the fets and neo mags in the can. While all of the other cars had external escs or fet upgrades, one with just upgraded motor, or with both (like the mr02 I was running). The mr01 even had a stock antenna, no upgrades, and did not glitch a single bit the whole night. Go figure..

rharris
2006.09.04, 10:35 PM
Are you refering to the voltage of the transmitter?


Voltage of the transmitter battaries. There is definitly interference at my house. But strong transmitter battaries help to reduce it's effect.


rharris, has the capacitor made no difference at all after you've installed it?

I don't think the PN Capacitor made any difference at all.

GTHammer
2006.09.12, 10:39 PM
Just replaced my fets on my MR02 and have the same problem, today is the first day I have run the thing (other then in my kitchen which isn't that long) was feeling pretty proud of myself, now I guess it's back to the work bench to try and locate the loose leg.

GTHammer

madf1man
2006.09.13, 02:37 PM
Have you guys tried different crystals and antenna lengths on radios? Sorry for the basic ideas but sometimes those are the first things forgotten about! :)

locwan
2006.09.13, 06:25 PM
Sure have! Thanks for the reminder though.

New info:
I've put in an Atomic stock motor in the fet upgraded (only 2x1) mr01 today. The lost of steering input when the battery is getting low is there. That means that the fet IS allowing most of the remaining current through to the motor with the upgraded fet with less resistance, especially if it's a motor with less turns. (this doesn't happen with the Kyosho stock motor)

Which means, that with upgraded fets and a motor with less turns, if the battery isn't producing enough current, what remains will get drawn to the motor first, leaving the rest of the circuitry with less power.

GTHammer
2006.09.13, 09:48 PM
Hey Guys, GTHammer here, as soon as I arrived home tonight I pulled out the ole S Iron and went to work on the fet legs, added a bit more solder on each leg, a little heat and what do you know, problem fixed!!!! I now have full control once again. Great tips on the forum.

Hamm