Thanos h-bridge keeps frying mosfets

I build the h-bridges and had some bad connections due to hairline cracks in the traces. Fixed everything and used a 9v battery to test everything on the mosfets. Everything seemed to work great had good voltage at the motor output and was getting the voltage reversing when the motor was supposed to.
So when I tried to use a 18v battery and a Dewalt drill motor my MOSFET overheats and pops within 10 seconds of starting the motor turning. I don't know why this is happening. I figured it should be less of a draw than a wiper motor, which I have not purchased yet since I am having this problem. I don't have heat sinks on yet it should not be getting that hot and popping that quickly unless something wasn't right?

emm.. wiper motors should to be used with 12v battery! not?

I am not familiar with the mosfets, but they are probably not designed to have 18 volts going through them, that maybe why they went kaboom.

It depends on the motor. Some motors have too little resistance and draw lots of current.

The wiper motor have handy around for tests have about 7.5 Ohm resistance and draws 2A unloaded or 4A loaded. The ones I was using on my Joyrider had resistance 2 Ohm and they were draw 6A unloaded and 12A under load each!! But they were powerful and fast!

Now the Drill motor you have probably generates too much spikes due poor brush contacts backfiring to the mosfets. It has nothing to do with voltage. I tested a very expensive industrial 24V DC motor (400euro cost) on my h-bridge on 24volts and didn't blow the mosfets or overheat them too much!

Look in the wiki for more information on poor designed motors and their problems:
http://en.wikipedia.org/wiki/Brushed_DC_electric_motor

There is a second problem with this simple pole design. At the zero-torque position, both commutator brushes are touching (bridging) both commutator plates, resulting in a short-circuit. The power leads are shorted together through the commutator plates, and the coil is also short-circuited through both brushes (the coil is shorted twice, once through each brush independently).... In any case, a short like this is very wasteful, drains batteries rapidly and, at a minimum, requires power supply components to be designed to much higher standards than would be needed just to run the motor without the shorting.

Thanos

Thanks for the reply I was wondering about that. I was getting ready to go get a wiper motor to test with before I built my frame and see if it reacted different. I think i will buy a wiper motor this weekend and try again. Think will get a chevy truck wiper motor, mine seems to move my wipers close to 60rpm. I will also take a look at that link as well.
Thanks again for your input.

Well went to the yard and got me some wiper motors finally! So I hooked it up to the power supply and got it all figured out as far as fast,slow, and ground and to make sure it actually worked.
All went well with that test, hooked it up to my h-bridge and BOOM! There goes the FETs again. I'm about to give up and start over on this. It seems like it runs fine in forward but once I go to reverse the motor using the pot is when it goes bad. I'm lost on this one. I have tested everything. I've confirmed all the parts and they all seem good. With no load on the motor output of the H-bridge everything works perfectly, the voltages change polarity like they should and I get about full voltage on the output and the FETs never get warm or anything. Right when I introduce a load, GAME OVER. Any more ideas? I would love some hints of what I am doing wrong. I may just scrap this board and build a new one but I really want to know why and what is wrong.. Thanks Erik

Additional info. I am currently running with no heat sinks and I don't have the drain of the Mosfets connected with a screw. Is that important?

Ok Tronic and all you other smart kids out here, HELP ME PLEASE!!! I can't figure this H-bridge problem out. I can run small motors fine. I run a small rc car motor with a 9v battery and everything works great. It goes both directions with no problem and there is no heat build up on the FET's. As soon as I try a wiper motor the FET's get hot immediately. I took voltage readings at the output when I tap the motor and it goes to zero like there was a short. The wiper motor work fine in both directions with the power supply only. As soon as I try to put the High current draw through the FET's they heat up and I get the wiper motor to to barely shake before the FET's are fried and turned to garbage. What am I missing? Why can't I get the wiper motors to work? The motors are sitting on a plastic table and I'm only working with one motor at a time. I'm ready to tear the whole H-bridge apart and try to build a new one, maybe the yobuddy ez board next, but I would much rather make this work and learn what the f is going on.. Thanks for your time.

Buy some jrks

most mosfets don't like the gate voltage to be too high, maximun is in the order of 20volts. Motors act as a generator and if there is insufficient capasitance then the voltage will raise from the nominal 18volts and bad things can and will happen if the gate voltage rises (I assume there is no limiting circuitry on the gate, I haven't looked though)

I understand that but my problem right now is I can't get the motors to even start rotating. It would make sense if they were already in motion.

Bsft. I might have to but I hate giving up when I know I'm so close and that others have had good luck/skill with the h-bridge.

Quickly looking at the Thanos pcb layout I see that there is protection against gate overvoltage, are you sure you assembled all the parts right way around? It would be best if you could show some pics of your board

I understand that but my problem right now is I can't get the motors to even start rotating. It would make sense if they were already in motion.

Bsft. I might have to but I hate giving up when I know I'm so close and that others have had good luck/skill with the h-bridge. And I don't have the added funds to support buying more equipment.

The most critical part is when the motor is stalled. The motor actually starts stalled until develops some speed. This causes great currents. Did you measure the internal resistance of the motor windings? It should be around 2 to 5 Ohms... If less, how less?

I will double check when I get home in a couple hours but if I remember correctly I checked the one motor and it was 1.2 ohms. I'll confirm that though. And my second motor i have not pulled the electronics off of yet so haven't measured it. If that is the case, that would calc out to be 10 amps at 12v using ohms law. Of course that could go up to I'm guessing around 15 amps with the lock/stalled rotor condition. I'm an electrician and work with ac motors. Not too up on my dc motor info. What do you think would help resolve the problem if my motor are 1.2 ohms.

Edit**** the one motor is .9-1.1 ohms.

0.9ohms makes 160W @12V! That is a bit cruel for the mosfets. It's like you start the motor with short circuit. I can imagine the back-EMF would be huge trying to find it's way back to the battery. Maybe the diodes aren't large enough for that!
My motor here has 6ohms which is 24W only @12V

Well crap is there a fix I can do without different motors? Or do I go to the junk yard and start testing wiper motors til I find one with higher resistance?

hi jack time again, I run 180 watt motors well and truly overdriven on the jrks.

High watt is not always better. Its mostly wasted inside the motor in type of heat! :mug1:

Its just the some motors are more efficient than others! The $400 industrial motor that I use for my tests is so efficient that I get enough power with only 4.8A!

Here is the specs and some comparison:
For motor resistance 2.5Ohm,
Running at 12V I get 4.8A or 57.6Watt
Running at 24V I get 9.6A or 230.4Watt !!!!


Thanos

What wiper motors did you use? Year and model of vehicle. Mine are out of a 2005 Chevy truck. What if I try what they are talking about in the main thread about using both sides of the h-Bridge to power a single wiper motor? Just a thought might not be cheaper than just going back to the yard to find comparable motors to what people are using with their h-bridges

What wiper motors did you use? Year and model of vehicle. Mine are out of a 2005 Chevy truck. What if I try what they are talking about in the main thread about using both sides of the h-Bridge to power a single wiper motor? Just a thought might not be cheaper than just going back to the yard to find comparable motors to what people are using with their h-bridges