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Old 2015.02.25, 12:17 PM   #16
LED
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Whoa, take it easy. Adjustable power supplies aren't rare by any means, and that is the reason for the adjustability, so you set a max current. No matter what, your elecs will only draw up to that setting. Nothing to worry about. The dumb part would be to not heed the disclaimer, and especially to attempt a FET surgery and test by way of roulette.

So, there are no electronics geeks that are into RC cars? I beg to differ, but I'm not here to argue or trash other people's input either, so...

Put a lighter to your car all you want, but doing so would suggest a lack of sanity on your own part or taking what was posted completely wrong on purpose. If you have a better way of testing FETs in the mini-z application I'm all ears, sir.
Yes I do, put them in and see if the motor runs :-)
And I also agree with imxlr8ed. You provided some very useful info over the last months, and I also follow each one of your threads. That is why I am really really surprised to read such a comment from you. Because obviously you are one of those electronics geeks (no offense intended, I wish I was one to). But come, you must admit that using a shortage to test something is really far fetched.
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Old 2015.02.25, 03:45 PM   #17
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Yes I do, put them in and see if the motor runs :-)
And I also agree with imxlr8ed. You provided some very useful info over the last months, and I also follow each one of your threads. That is why I am really really surprised to read such a comment from you. Because obviously you are one of those electronics geeks (no offense intended, I wish I was one to). But come, you must admit that using a shortage to test something is really far fetched.
I understand your hang-up with this, really. Shorting your motor leads is the last thing you'd want to do with cells, but under a controlled current source, it is just setting up a condition that your FETs are in all the time.

Let's say that you just did a FET job and you accidentally shorted a couple pins together. Throwing your cells in afterwards could be causing a worse condition than shorting your motor wires under a controlled current source.

If it would make you feel better, put a motor at the output, but you should still control the current so as to limit it to a level that won't damage anything in the event of a soldering error. And FYI, a motor only poses an ohm or so resistance to the FETs anyway, so your FETs see a 'near' shorted condition most of the time. 1 or 2 ohms is not much resistance.

I apologize if this is too much information, but this internet thing is pretty big. I'll try not to fill it up. But there's nothing wrong with being skeptical either. I get it.
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Old 2015.02.25, 05:08 PM   #18
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Instead of a dead short on the output I'd suggest a low value resistor - such as 10 ohms. It could also be used to ensure the current limit on the supply is set correctly before starting the test and is just "good practice" in general. The resistor should be capable of at least 5 watts.

btw, since we're talking about testing FET's I'd not recommend the original procedure mentioned earlier in this thread. The gate of a FET should never be left floating - IE nothing connected to it. When you remove the positive voltage and apply a negative voltage or vice versa you are leaving the gate pin floating. Just touching the gate pin with your finger can cause it to partially turn on which can very easily lead to an instant catastrophic failure. Yes, I am not going into the details of why this occurs but can if needed.
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Old 2015.02.25, 06:03 PM   #19
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Instead of a dead short on the output I'd suggest a low value resistor - such as 10 ohms. It could also be used to ensure the current limit on the supply is set correctly before starting the test and is just "good practice" in general. The resistor should be capable of at least 5 watts.

btw, since we're talking about testing FET's I'd not recommend the original procedure mentioned earlier in this thread. The gate of a FET should never be left floating - IE nothing connected to it. When you remove the positive voltage and apply a negative voltage or vice versa you are leaving the gate pin floating. Just touching the gate pin with your finger can cause it to partially turn on which can very easily lead to an instant catastrophic failure. Yes, I am not going into the details of why this occurs but can if needed.
Sure, you could put a 10 ohm resistor at the motor output, but then during test when you throttle, you'll be drawing 4.8/10 = 0.480A or 480mA at the motor end at most. This would be a good cursory test to begin with, but your motor may want a few amps from time to time, so it would be prudent to test in increments to a max value of what you believe your FETs SHOULD handle, which is a data sheet spec you should reference before testing. I'm thinking that a pulse in the forward and reverse direction could be handled by the FETs up to at least 2A. Time is crucial though. You can't keep high amperage on them constantly. Your motor doesn't either, because as your motor accelerates to top speed it draws less and less current, until the only current it draws is that which is required to overcome drag.

You can't up the voltage either, because the voltage is across the elecs, and there is a limit to what the components on the board can handle, so you must lower resistance to allow the elecs to draw up to, but not exceeding, the 'controlled' current setting, see how the elecs respond, and go from there. It is how the industry does it, however they go straight to max current spec and allow the defect to fail, and then recycle it through a repair process. My procedure is trying to avoid the shotgun approach and walk it through in increments so as to safely determine whether or not an error has occurred before going to full run-mode.

I remember back to when I first came to the boards and posted that I was changing my FETs out of the box. There was a reaction as if I was going to destroy my mini-z before I had a chance to drive it. I don't recall anyone deferring me to a safe procedure for changing FETs, and no, throwing your cells in after a FET job isn't safe, and as TheSteve indicated, neither are any of the previous methods introduced.

Here's a link to a good power supply which you can use universally to power whatever for the rest of your life. There's probably at least a metric ton of information to post on how you could use one of these in your mini-z hobby, and maybe, not to mention, a greater understanding of all things electrical.

POWER SUPPLY

Drop in the bucket compared to what some of you have probably spent on mini-z's, and can enhance your hobby going further.

I'm sure some of you have messed around with a multimeter....

MULTIMETER

Another universally useful device that has a multitude of purposes in aiding you with anything you put batteries in. Buy one. Learn how to use it and what the values are telling you. You may find yourself thinking of edges you can apply to your race scheme. You can bet some of the professional RC racers do.
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Old 2015.02.25, 09:25 PM   #20
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[QUOTE=Jshwaa;456259]
I remember back to when I first came to the boards and posted that I was changing my FETs out of the box. There was a reaction as if I was going to destroy my mini-z before I had a chance to drive it. I don't recall anyone deferring me to a safe procedure for changing FETs, and no, throwing your cells in after a FET job isn't safe, and as TheSteve indicated, neither are any of the previous methods introduced. [QUOTE]

Well now... ain't gotta be all attitude about it. There are a bunch of us on here who followed newbies over the years only to watch them get fet crazy and then disappear because they fried their car messing with it. Maybe if you would've put it in your sig that you are an electronics nut, I wouldn't have warned you about all that fryin stuff.

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Old 2015.03.01, 05:02 PM   #21
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Ok, I suppose a 'better' test circuit would be to do this...



However, circuit-wise, it is no different than the previously mentioned circuit, as the 'real' difference is where the current meter is placed, and the current meter is a short as far as the circuit is concerned.

mleemor60 was kind enough to donate a set of elecs with apparent range issues. I hooked them up to my supply and meter and found that the stock elecs are pretty stout in terms of current flow per heat dissipated at the FET's. A constant 2A didn't phase them...

So here is the setup...

The current meter is where a motor would normally be, essentially shorting the motor output through the meter....



And then the power supply is connected to where the batteries normally connect.





At first I started with the lowest current setting my supply can be set to, which is about 0.25A. I do this by turning the 'Course' and 'Fine' adjustments of my supply all the way down. I can then press the 'CC Limit' button (middle of supply), and a dummy short is internally placed across the output, and the current output is read on the supply. I then know that a short circuit condition will only put out that much current.



I can then throttle and monitor current, and see if the elecs get hot, or if they are OK. Then increment the current further.



I did this up to 2A, and the elecs were fine, as they should be. I didn't do anything to them yet.



I would do this before putting cells in, because in a short circuit condition the cells will dump current as fast as the chemistry will allow, which as you guys well know is a fair amount of current. Better to be safe than sorry.
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Old 2015.03.02, 06:49 AM   #22
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I must admit, I really like seeing the 'science' behind mini-z. if you every want to do a series on this, just let me know as we would love to add this type of information to sticky zones.

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Old 2015.03.04, 10:01 PM   #23
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So, I failed in trying to fix the range issue on this set of elecs that mleemor60 gave me. I thought that would be an opportunity to push the stock FET's to failure, just to see...

Using the method previously mentioned, I walked the current magnitude up in increments of 0.5A to start. I tested the stock FET's at a constant 2A, then 2.5A, then 3A, etc. and monitoring temperature vs. time...



The stock elecs impressed me in how much constant current, and high temperature that they can withstand. At temperatures well over 300 degrees F, and currents in the 6A range, the FET's just took it. This tells me that a mini-z user would be challenged to destroy them by simply using their mini-z as prescribed, with any 4-cell power source, and virtually any motor on the market.

My Fluke has a temperature probe and datalogging capability, so I charted up the temperature profile of the FET's as I apply constant current.
Here is the stock elec's temp profile for a constant current of 6A...





Here's the profile of the failure at 7A. Notice the peak. That is where they failed.....





The traces on the board got so hot from the current flow, that the solder joint holding the red battery wire to the board gave just after taking the above picture. Here's a close-up of the carnage after cleaning off the thermal compound...





So my summary on the stock elecs is that nobody would need to upgrade them for 'more' current, more than to simply lower that resistance, and to get a lower temperature profile for your operating condition.
Their power handling has my thumbs up. If you can effectively desolder your power wires as the FET's fail, that's pretty good...

**Testing does not take into account inductive motor spikes. More testing needed.....
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Old 2015.03.05, 12:16 AM   #24
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Great thread with lots of cool info. I agree with Arch2b, the science of R/C cars and Mini-Z is a fun topic. Thanks for all the pics and info Jshwaa.

The donated Mini-Z board, what stock FET's are used on it? The ASF boards I have are 3010's, double stacked, one layer/two FET's per side. Is this board a Mini-Z SPORTS PCB? I know the SPORTS bords have one side/two FET's only (Not 4 like the ASF boards have). But it looks like these aren't 3010's?
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Old 2015.03.05, 05:02 AM   #25
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Great baseline information to have on record!

Doesn't change user perception likely, especially given popping FETs is not a rare occurrence. In my experience alone, 1/2 if not more of all the cars i've ever owned have popped FETs with stock and never more than 70t motor. Maybe this simply suggests there are larger quality control issues with factory FET installations. i simply replace the FETs right out of the box on every car i own except for those that require stock FETs for class requirements as a result of the frequency in which this has happened to me.

i understand the data shows the electronics are robust enough that this should not happen, experience tells you this does happen often enough that its not an anomaly.

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Old 2015.03.05, 07:37 AM   #26
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Great baseline information to have on record!

Doesn't change user perception likely, especially given popping FETs is not a rare occurrence. In my experience alone, 1/2 if not more of all the cars i've ever owned have popped FETs with stock and never more than 70t motor. Maybe this simply suggests there are larger quality control issues with factory FET installations. i simply replace the FETs right out of the box on every car i own except for those that require stock FETs for class requirements as a result of the frequency in which this has happened to me.

i understand the data shows the electronics are robust enough that this should not happen, experience tells you this does happen often enough that its not an anomaly.
Good feedback, arch2b. I would only add that a motor is a little more of a noisy load to the FET's, and I did not take into account the massive inrush of current that braking causes. Your motor essentially is a generator at that point, and could be producing spikes of current/voltage that have damaging effects, which my testing would not have brought to light. I had a soft gentle, quiet, 0.5ohm resistor as a load through most of my testing.
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Old 2015.03.05, 10:40 AM   #27
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I was going to bring up the spikes from the motor but you already mentioned it. What usually blows fets is the huge spikes on startup or a stalled motor. You need to test this with a modified motor attached. Also, with no load on the motor, it will not look the same.
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Old 2015.03.05, 12:34 PM   #28
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It is also possible the N and P channel fets get turned on at the same time due to poor hardware design, firmware bugs, extreme noise from the motor or static. This will generally kill fets instantly. I don't believe it happens that often in Mini-Z's but am sure it has claimed boards over the years.
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Old 2015.03.05, 12:43 PM   #29
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contrary to that theory, most of my FET failures were mid stream driving and not at a start off or pinned at a rail. i've had no less than 2 F1's pop FETs on long straights. it looked awesome but not fun to DNF.
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Old 2015.03.05, 02:21 PM   #30
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contrary to that theory, most of my FET failures were mid stream driving and not at a start off or pinned at a rail. i've had no less than 2 F1's pop FETs on long straights. it looked awesome but not fun to DNF.
Do you recall the cell/motor composition on the setups that you blew FET's on, and whether or not the motors had capacitors on them? I believe the caps are what's supposed to help suppress the huge spikes that occur; as the commutation switches poles and you have large current flowing through the motor windings.

Also, built into most FET packages is a diode...



This diode is there to protect the FET from large spikes, and to shunt the large potential to a source, whether it be ground, or the Vcc rail depending on which FET you are referring to (N-channel or P-channel), in the event of an inductive spike seen at its load.

This is one huge advantage to stacking, as you are putting 'more' of these diodes in parallel as well, giving 'more' of a channel to shunt large spikes, and each individual diode is helping protect the sum.

In the case of a FET-failure when running an actual motor, it starts with the breakdown of these diodes. So it would be worth noting that the previous testing did not essentially stress these diodes at all. I was merely running clean current through the FET's drain-to-source channel.

I would give this another whirl with stock FET's AND a loaded motor, but you see what happened to my test specimen. When I 'finally' get my inertial load wheel from my machinist, I will try this again, with say the Si4562's or the 8858's. I got a 10-strip of each. Anyone prefer one over the other?
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