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Forum: Analog Circuits L298 exploding, after having worked perfectly


Author: Dave (Guest)
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Hi everybody

I built myself a CNC-mill (3-axis) and my "own" controller circuit. I'm 
using the well-know L297 L298 chip combination, more or less identic 
with the manufacterer datasheet.

After some trouble in the beginning the 3 controller boards were working 
perfectly. But then I decided to put the boards into a new case. After 
that the L298s exploded.
Before they did so, the motor was "stuttering" around and it only 
happended when using more than one board at the same time (over enable)

After burning some (3) L298 I decided to take a closer look at the 
circuits. Maybe I had damaged some circuit paths. But there was nothing 
obvious to see. And I checked the pcbs for hours but I couldn't find 
anything.

What are the most probable reasons for burning the L298? (sense, 
clock???)


PS: I know this is quite a imprecise question/information but I hope you 
can help me somehow. I can post the circuit (.eagle file) if necessary 
but the circuit should be ok (it worked fine for a few months).

Thank you in advance for your help

Dave

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Author: Floh (Guest)
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Dave schrieb:
> What are the most probable reasons for burning the L298? (sense,
> clock???)

Your diodes are ok? A missing diode kills a L298 with ease.
L298 mounted on Heatspreader?
Same Supply after changing case?
:-)

Author: Dave (Guest)
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Yes, the L298s are mounted on heatspreader, supply is the same as 
before. I checked the diodes with a multimeter. They were permeable from 
one side and blocked from the other. Seem to be normal, but maybe I 
remove them and check them individually.

Thank you

Dave

Author: C. H. (_ch_)
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Dave schrieb:
>...it only happended when using more than one board at the same time...
>
>...Yes, the L298s are mounted on heatspreader, ...
Are they isolated against the heat think?

Christian

Author: Dave (Guest)
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Yes, they are isolated. I mounted a seperate heatsink on every L298. 
There is no connection.

Thanks for the tipp

Author: Lehrmann Michael (ubimbo)
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Hi,

well I had these problems after having voltage reverse. Afterwards it 
worked perfectly a few seconds then it started expolding / melting.

Author: Anselm 68 (anselm68)
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Are the heatsink´s are mounted at the case of your new housing?

Author: Dave (Guest)
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Well, I think I have to reassemble the boards and make sure everything 
works fine again?? Maybe it's a wrong connected voltageconnection or a 
problem with the diodes.

The heatsinks are not connected and not mounted in the case. They are 
freestanding in the case.

Thanks for all your support, guys. I'm gonna test it again (and burn 
more L298?? ;-) ).

I can't see the problem right now, let's hope it will work again. From 
the outside everything seems to be ok!

Dave

Author: Raimund Rabe (corvuscorax)
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Since you moved the working hardware just in a new case I could guess 
that something with grounding/earthing is imperfect now. Maybe some GND 
potentials are now floating which could cause excessive high voltages on 
certain channels - at least this might match with your observation that 
a separate channel does work, but not all together.
Might be worth to look for such an issue.

Author: schodavi (Guest)
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Thank you for your guesses and help

I reassembled the boards and the cables/connections, and that time I 
checked everything more than twice (make sure that there is no unwanted 
connecion). I think too that there was a problem somehow with ground or 
just something badly connected.


I was really scared to test them again because when they explode it is 
quite load (I use a 24V/10A power supply)
but finally I did it the controllers are working perfectly again. ;-)

Thanks for all

The next time I am more carefull when changing the case.

Dave

Author: Philipp Burch (philipp_burch)
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schodavi schrieb:
> The next time I am more carefull when changing the case.
>
> Dave

And please be more careful when attaching your power supply. You sure 
have a supply with some kind of current limit, don't you? Otherwise it's 
quite likely, that you kill something if there is a mistake somewhere...

Author: schodavi (Guest)
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The power supply is an industrial supply (TRACO). It has a fuse for 10A. 
On the controller boards I have fuses too : 5A

I don't have the right fuses at home right now, but I will buy some 3A 
fuses soon.

Do you think it's better to switsch on the L298s over enable input 
instead of directly plugging in the power supply(enable always on high)?


Thanks

Author: Philipp Burch (philipp_burch)
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schodavi schrieb:
> The power supply is an industrial supply (TRACO). It has a fuse for 10A.
> On the controller boards I have fuses too : 5A
>
> I don't have the right fuses at home right now, but I will buy some 3A
> fuses soon.

That won't make a big difference.

The problem is not the fusing in normal operation. But you really 
should test your circuits using a current-limited supply after doing 
some modifications if it is possible. I guess you have seen the reason 
for this in a quite impressive form ;)

> Do you think it's better to switsch on the L298s over enable input
> instead of directly plugging in the power supply(enable always on high)?

Well, that may be a good idea. So you can disable the whole power part 
if something is wrong. An example for this could be insufficient voltage 
for the controller (brown-out).

Author: g457 (Guest)
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schodavi schrub:
> Do you think it's better to switsch on the L298s over enable input
> instead of directly plugging in the power supply(enable always on high)?

Datasheet [1] suggests (page 7/application information):
Turn-On and Turn-Off : Before to Turn-ON the Sup-
ply Voltage and before to Turn it OFF, the Enable in-
put must be driven to the Low state.

I guess weird things can happen if you don't. Like escaping magic smoke 
;-)

HTH

[1] 
http://www.datasheetarchive.com/pdf-datasheets/Dat...

Author: Dave (Guest)
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g457 schrieb:
> Datasheet [1] suggests (page 7/application information):Turn-On and Turn-Off : 
Before to Turn-ON the Sup-
> ply Voltage and before to Turn it OFF, the Enable in-
> put must be driven to the Low state.
> I guess weird things can happen if you don't. Like escaping magic smoke
> ;-)


You are right, but before/during the explosions, I was always switching 
on and off the L298 via enable input. Just today after the reassemble I 
forgot it and directly switched on the power supply. But it didn't cause 
any damage at all.

This is probably a safety feature for people who are using simpler power 
supply (which can have a voltage top when switching on/off (induction). 
Mine was really expensive (but I got it for free from a friend ;-)

PS: I have a current-limited power supply but the problem is that it 
isn't really powerful.  Maybe I have to buy an additional (better) one??

PS: There was a lot of noise from the choppers during operation, so I 
replaced some GND - wires with thicker ones. Now the noise is much lower 
(but still there of course). I think it was a problem with GND 
connection or supply voltage wrongly connected. Finally it is working 
now. I also added a mechanical switch to manually control the enable 
inputs.

That's it. Now I can do some milling again ;-)!

Author: Dave (Guest)
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Thanks for the great support and all your help.

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