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Author Topic: I keep burning my Sabertooth´s  (Read 844 times)

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Offline CorpzeTopic starter

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I keep burning my Sabertooth´s
« on: January 21, 2014, 02:54:45 AM »
Hi, i am posting on this forum because i don´t know where else to go, Dimension engineering doesn´t know, my distributer doesn´t know... i don´t have a clue, why i am burning my sabertoot.

Basicly, i have two 12v, 63Ah batteries @ 24v, one sabertooth 2x25 and two regular wheelchair motors.
I have driven the robot fine for several weeks with two smaller 10Ah batteries without any problem.

The problems started when i switched to the bigger, brand new, lead-acid 12v car batteries.

I have a 16mm2 cable with a master switch (200A) on the positive side, connected to a distribution block, and a 4mm2 cable in to the sabertooth

The negative side is connected to the steel frame with a 16mm2 cable. The sabertooth is connected to the frame as well (not on the same place though)

The batteries are hooked up from + to - via a 5cm 16mm2 cable.

The motors are hooked up to the sabertooth via 4mm2 cables.

The sabertooth is hooked up to a Spektrum AR6200.

The batteries has about 24.6V output at the time i measured (after the insident(s))

This is what happend:
I turned the master switch - the Sabertooth went poff... basicly. Resulting that the differential mode went non existing, just barely i could move motor B in micro controller mode. According to Dimension engineering, the RC-chip might have gotten fried.

Second time i tried, i checked all the wiring (again) with no short circuits or anything abnormal in my eyes, it went completly black. nothing, nada.... the R/C reciever just blinks in 1/10th of a second and then it´s black again...

Now to what i have heard that "might" cause these problems:
I have not my + and - cables twinned witch results in high inductens? (spelling)
A severe power rush due to my large batteries?

I don´t know what to do any more :(
pls help // Daniel

Offline Tommy

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Re: I keep burning my Sabertooth´s
« Reply #1 on: January 21, 2014, 05:19:45 AM »
Corpze, you never mentioned the electric motor brakes, are they removed?

posting pictures of your application would also be helpful.

Tommy
« Last Edit: January 21, 2014, 05:21:21 AM by Tommy »

Offline CorpzeTopic starter

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Re: I keep burning my Sabertooth´s
« Reply #2 on: January 21, 2014, 08:42:14 AM »
Corpze, you never mentioned the electric motor brakes, are they removed?

posting pictures of your application would also be helpful.

Tommy


Thanks for reply :)

Yes the brakes are removed, just tried to run the motors directly on batteries and they run fine in both directions...

Here is a picture that i hope explains the connections. The sabertooth is returned to dimension engineering for repairs.. but i have laid out the cables where the sabertooth where mounted.


Offline Cristi_Neagu

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Re: I keep burning my Sabertooth´s
« Reply #3 on: January 21, 2014, 10:35:11 AM »
Your Sabertooth is 2x25A, and you have 2x30A fuses... If you have a short, or your motors draw more than the Sabertooth's rated current, the fuses won't protect it (that's not to say those fuses would burn faster than the Sabretooth, but anyways...).

Offline CorpzeTopic starter

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Re: I keep burning my Sabertooth´s
« Reply #4 on: January 21, 2014, 01:14:13 PM »
Yes i now, i will skip those fuses and replace them with one 100A fuse before the main switch, Dimension engineering recomended that. The fuses between the sabertooth and the motors are there to protect the sabertooth if the motors gererate more power in a steep hill, than the sabertooth can handle.

Offline jwatte

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Re: I keep burning my Sabertooth´s
« Reply #5 on: January 21, 2014, 04:14:14 PM »
If all the connections are right, then inductive spikes is the most probably cause.
To tame that problem, you can:
1) Add a 220 uF to 1000 uF electrolytic capacitor between + and - right at the connection to the Sabretooth.
2) Add a 1 kW or higher TVS diode with a standoff voltage of 25V or more in parallel to that capacitor.
3) Add a similar TVS, but bi-directional, across the output to each motor, also on the Sabretooth.
4) Perhaps add a 60V or better ceramic capacitor across the output to each motor -- I don't know how much capacitance -- I'd try a 1 uF first and look at the scope to see what that does.

Offline CorpzeTopic starter

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Re: I keep burning my Sabertooth´s
« Reply #6 on: January 21, 2014, 11:31:48 PM »
If all the connections are right, then inductive spikes is the most probably cause.
To tame that problem, you can:
1) Add a 220 uF to 1000 uF electrolytic capacitor between + and - right at the connection to the Sabretooth.
2) Add a 1 kW or higher TVS diode with a standoff voltage of 25V or more in parallel to that capacitor.
3) Add a similar TVS, but bi-directional, across the output to each motor, also on the Sabretooth.
4) Perhaps add a 60V or better ceramic capacitor across the output to each motor -- I don't know how much capacitance -- I'd try a 1 uF first and look at the scope to see what that does.


Should i do all of that or just pick one? ;)

Would this capacitor work? http://www.ebay.com/itm/New-1000uF-25V-Electrolytic-Capacitor-10-20-Radial-10PCS-/201021962776?pt=US_Car_Audio_Video_Capacitors&hash=item2ecdd7ba18#ht_877wt_1153

What about the grounding point in tha frame? should i skip that and draw a wire directly to the sabertooth?
« Last Edit: January 21, 2014, 11:37:35 PM by Corpze »

Offline waltr

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Re: I keep burning my Sabertooth´s
« Reply #7 on: January 22, 2014, 07:36:21 AM »
With the problem you are having you may need any one or a combination or all of Jwatt's suggestions.
But then none of these may help if the problem has a different cause. This is due to making guesses as to why the SabreTooth board is dieing.

Offline jwatte

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Re: I keep burning my Sabertooth´s
« Reply #8 on: January 22, 2014, 11:20:29 AM »
The voltage rating of that capacitor seems a little low to be good for the given application. I'd go with a 50V or better rated, and perhaps less capacitance instead if cost or size is a factor.

And, yes, IF the problem is one of those that these things help against, then you may need one or all of these. IF the problem is something else (like, oops, you switched the power leads :-) then no, these will not help. Similarly, if the problem is overloading, then this will not help.
Well, the unidirectional TVS might actually help, because it will forward conduct and hopefully blow the fuse if you hook it up the wrong way. (And if the fuse is between battery and TVS!)

The single 100A fuse is not great in my mind, because if you stall one channel of the Sabretooth, and stall current is > 25A (say, even 60A) and the other channel is not stalled, then you won't draw the full 100A, and the Sabretooth will not be protected. I believe Sabretooths have some kind of current sense and protection built in, though -- if that's true, then individual fusing becomes less important.


Offline CorpzeTopic starter

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Re: I keep burning my Sabertooth´s
« Reply #9 on: January 22, 2014, 11:37:52 PM »
Yea, that sounds right... so a 50v 1000uf would do better?
Does this capacitor work in cold weather? it´s the heat that breaks them huh?

The 100A fuse is not to protect the sabertooth, it has already heat protection, but the batteries... That was my intension :)

Is there somehow i can measure if i have a power rush without an oscilloscope?

Offline gerard

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Re: I keep burning my Sabertooth´s
« Reply #10 on: January 26, 2014, 10:54:01 AM »
so from your explanation, the sabertooth got fried when turning the power on, not trying to move the motors, correct?
do you know what stall current your motors run at?
this is a strange issue because the sabertooth does have overcurrent limiting protection, overheating protection, and auto shutdown.
did you try the sabertooth again with the previous 12V SLA battery to see if it was completely fried?
with those motors and batteries i would recomend going for a 2x60A sabertooth, or possibly a high current roboteq motor controller (which are very expensive)

Offline jwatte

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Re: I keep burning my Sabertooth´s
« Reply #11 on: January 26, 2014, 12:30:59 PM »
Just turning on power may cause an inductive spike. See for example: http://www.pololu.com/docs/0J16/all

Buy a capacitor rated from -40 C to +85 C and you'll be fine down to -40 C :-) Aluminum electrolytics don't do great in cold weather, so you may need a fancier kind -- again, you may have to trade off some capacitance.

Offline CorpzeTopic starter

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Re: I keep burning my Sabertooth´s
« Reply #12 on: January 26, 2014, 10:05:27 PM »
so from your explanation, the sabertooth got fried when turning the power on, not trying to move the motors, correct?
do you know what stall current your motors run at?
this is a strange issue because the sabertooth does have overcurrent limiting protection, overheating protection, and auto shutdown.
did you try the sabertooth again with the previous 12V SLA battery to see if it was completely fried?
with those motors and batteries i would recomend going for a 2x60A sabertooth, or possibly a high current roboteq motor controller (which are very expensive)


Yes thats right, the same second i turned the switch something happend.
I tried the sabertooth with the SLA-batteries and it lit up, but didn´t work so well... i presume the damage allready had been done.
I might go with the 2x60 in the future...

Just turning on power may cause an inductive spike. See for example: http://www.pololu.com/docs/0J16/all

Buy a capacitor rated from -40 C to +85 C and you'll be fine down to -40 C :-) Aluminum electrolytics don't do great in cold weather, so you may need a fancier kind -- again, you may have to trade off some capacitance.



What kind of capacitor do you recomend? i have just ordered one 0.1uf and a 1000uf electrolytic... i can evaluate the function with theese and switch to better one´s after that.

I have also skip my ground point, I will have a cable directly to the negative terminal.

Offline jwatte

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Re: I keep burning my Sabertooth´s
« Reply #13 on: January 27, 2014, 10:30:44 AM »
The 1000 uF electrolytic will probably work fine, and while it will degrade in cold weather, it will probably be sufficient as long as it doesn't freeze-destroy itself. Check the data sheet/ratings for the specific capacitor you ordered.

A very large ceramic capacitor, or multiple parallel ceramics, with a small resistor (0.5 to 2 Ohms) in series would also work, and might be less cold sensitive. Cost and mounting starts becoming a problem, though, as all affordable large-capacity ceramics are surface mount these days.

Offline CorpzeTopic starter

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Re: I keep burning my Sabertooth´s
« Reply #14 on: January 27, 2014, 04:41:05 PM »
To add extra protection, someone mensioned that i can "preload" the sabertooth via a small switch with a resistor in front of it, like the picture, but what size of the resistor would fit for a application like this?

Offline jwatte

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Re: I keep burning my Sabertooth´s
« Reply #15 on: January 27, 2014, 08:16:01 PM »
Typically I believe you will use between 1k and 10k Ohms. The point of pre-loading is largely to avoid arcing in the switch, but it will also help some with capacitors. Make sure the device on the other end is okay with this kind of pre-loading, though, as some devices are very unhappy when they are fed too weak a current source.

Offline mallster

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Re: I keep burning my Sabertooth´s
« Reply #16 on: January 28, 2014, 10:32:55 PM »
What about the negative wire coming from the sabertooth supply? Looks to be terminating somewhere on the frame away from the battery ground. Could this be a problem due to the painted frame?

Offline CorpzeTopic starter

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Re: I keep burning my Sabertooth´s
« Reply #17 on: January 31, 2014, 10:42:34 AM »
Typically I believe you will use between 1k and 10k Ohms. The point of pre-loading is largely to avoid arcing in the switch, but it will also help some with capacitors. Make sure the device on the other end is okay with this kind of pre-loading, though, as some devices are very unhappy when they are fed too weak a current source.

Can the arcing itself cause voltage spikes? Will adding more capacitors cause a even higher spark?
I ordered 100ohm 7watt resistor... Just to be safe.

Offline jwatte

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Re: I keep burning my Sabertooth´s
« Reply #18 on: January 31, 2014, 12:58:25 PM »
I'm pretty sure 100 Ohms is too little! It will drain your battery, and will likely keep the Sabretooth "on" even though the power switch is "off," with instability as a result.

The point of the electrolytic capacitors is that their ESR is about an Ohm. Thus, when the L-C resonance swings, that 1 Ohm will dissipate the energy of the spike, making it much less severe. If, instead, you use ceramic capacitors (or no capacitors,) then the ESR is a lot lower and thus the spikes will swing more. If you use no capacitors, there will still be some stray capacitance, as well as possible de-coupling capacitors on the Sabretooth, that causes spike ringing, but at a higher frequency.

Thus, if you use ceramics for spike prevention, you actually want a 1 Ohm resistor in series with the ceramics, and then put the ceramics in parallel.

Another useful widget might be a 5 kW TVS diode with a stand-off voltage at or just above your battery fully-charged voltage. Mount it "against" the proper polarity (cathode to plus, like a Zener) and it will do two things:
1) short out the battery, and thus blow the fuse, if you hook it up wrong
2) clamp any spikes over the stand-off voltage, such that they aren't more than the clamp-voltage rating of the diode
You'll want to mount this as close to the Sabretooth as possible, but "after" the capacitor (although "after/before" doesn't matter that much at these frequencies :-)


 


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