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Exploding L298 stepper controlers!

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stickman_Dave

New Member
Hi all.

I'm using L298's to drive my stepper motors. I'm not using the 297 chip; just sending the 4 individual line values from my I/O card in my computer directly to the 298.

I'm driving the steppers with 24V (theoretically; it's actually more like ~21V) that i get from 2 daisychained 12V car battery chargers. The 5V logic signal comes from the I/O card.

I have 4 circuits on a single board. During testing, things were fine for a while, then all of a sudden all 4 L298 chips went snap, crackle and pop, burst into flames, and blew chunks off the chip face right above the 24V pin. The sound and light show would have been quite enjoyable had it not been my own time and energy going up in flames.

All 4 chips were enabled. Only 3 were hooked to motors, and only 2 of the motors were in use. The other motor and the unconnected circuits had all the lines set to zero.

All 4 chips have big-ass heat sinks that hardly even got warm before the blow up occurred.

After a little investigating, i found that while the the wall plug I'm using has 3 pins, the ground pin is not attacked to anything. (damn my old house and various incompetent renovators!) This means that not only are the battery chargers and my computer not grounded, but the 24 volt driving voltage and the 5 V logic are probably not relative to the same ground.

Is this likely the problem? Or is it probably that the voltage I'm getting from the battery chargers is too rough or something? Is the 100nF capacitor in the circuit insufficient?

I really have no idea what's going on here. I might be able to figure it out via trial and error, but I'd really rather not blow up any more cards, ya know?

Any thoughts on this would be appreciated. Thanks in advance.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
For a start don't use battery chargers, they don't put out 12V (it's probably a great deal more) and it's not smoothed DC either, just crude rectified AC.
 

Sceadwian

Banned
For another, they're definitely going to need a common ground, if the 5 volt side floated up relative to the driver side it could easily have enabled all the drivers at the same time, that'd smoke them in a hurry.
 

stickman_Dave

New Member
OK, so the battery chargers get the heave ho. Thanks for that. I'll dig up some computer power supplies instead.

On a related question, am i OK connecting two 12V computer power supplies in series to get 24V? And then just connecting the ground from that pair to the ground from my computer's power supply (the computer being the source, through the IO card, for the 5V powering the circuit logic)?
I'm a pseudo newbie, here. My training (a looong time ago) was in physics, and we didn't trouble ourselves with such pedestrian matters as actual applications of what we were learning.
 

arhi

Member
5V part and 24V part must have common ground

using 2 separate cpmputer PSU's can work (tried few times) but watch out how you connect them, also pay attention that computer PSU must have load in order to work.

about the blow up, it blew up because 5V and 24V had no common ground so who knows what was potential difference there .. - and it has nothing to do with lack of gnd in your house. The other possibility is that 24V from the batt chargers ... they tend to ouput much much more then 12V (I measured over 30 few times on the one I use and over 60 on some old russian one) ... you cannot even put regulator after car batt charger as most of them will burst in flames too
 

Sceadwian

Banned
You should be okay with the two 12V supplies in series, use the current output of the lowest rated supply and reduce that by 20% just to be safe. Make sure the cases never touch, and keep in mind if you trip one of safety features of one supply but not the other very weird things could happen.
 
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AlainB

Member
Hi,

In order to use 2 computer power supply in serie to obtain 24 volts you must isolate one of them from the ground connection. The black wires are grounded to the case. Joining a black wire of one power supply to the yellow of the other one will result in a short even if the boxes are not touching . They are still joined by the ground wire.

A good way of doing that would be to open one of the psu and put a piece of tape on the part of the pcb that is connecting to the metal box, under one screw. Dont put back that screw. Test to be sure that the black wires are not grounded anymore. The box itself will still be grounded and could touch any other grounded object without problem. This is for north american 110/120 volts 3 prong wire psu. I don't know how they are wired elswhere.

Use the oldest units that you can find. They are better. The newer one show high rating but it seem to be good for only few seconds before they shut down. Also connect a 10 Ohms, 10 watts resistor betwen one red and one black wire on each psu. If you have a newer psu, you may have to connect the green wire, if there is one, to a black one for him to start.

It would be better to take the 5v. from the psu, one of the red wires from the untouched psu that is.

Alain
 
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