Mysterious Self Destructing LM317s

[icedvolvo]

Hi People,

have a technical Q about 317s if someone can help!

I wanted to make a new "smooth" variable power supply for a welder drive motor, motor uses about 0.5A average and about 2.5A inrush and works form 3-24V. OK so I think OK a 317 with a 5K pot will work fine. BUT every 317 gets destroyed!! So I kept adding capacitors and protection diodes all around the thing to protect it but still gets destroyed after a few seconds! Even a 338 only lasted ~10 seconds.

They do not smoke or blow up they just die!!! So far I have a dozen dead 317s and a dead 338. Circuit is stock standard LM317 design as shown in attached gif.

Any and all suggestions appreciated

 

[Externet]

Do you know the pinout for sure ?
What is connected to the heat sink ?
Is there an insulator on the heat sink?
Is there a plastic washer anywhere ?
Is it a TO-3 or a TO-220 ?

 

[strantor]

What is connected to the heat sink ?
Is there an insulator on the heat sink?
Is there a plastic washer anywhere ?

taking one step back, is there a heat sink?

 

[KeepItSimpleStupid]

Are you using a heatsink?

 

[icedvolvo]

Hi People thanks for the replies,

Are you using a heatsink?

1: yes the pinout is correct; the device works fine until the load is applied.

2: yes there is a heat sink, its a TO220 package but shouldn't need one for two reasons:
- the avg current is only 0.5A with a 5V differential which is only 2.5W
- the internal thermal protection should shut it down b4 damage occurs ???

 

[DerStrom8]

Do the 317s get warm to the touch before they "die"? I don't think you're calculating the wattage correctly. I think you need to take the voltage in, subtract Vout, and multiply the result by the current. What is your power source? If you have a 12 volt supply and are using 5 volts and drawing 0.5 amps, you get (12-5)*0.5 = 3.5W, which is already more than the 2.5W you calculated. I'm looking at your circuit, and if I'm reading that correctly, it says 24 volts as a source. That gives you a wattage of (24-5)*0.5 = 9.5W. You'll need a fairly large heatsink for that. I think I'd need a little more information here before going on.

 

[Externet]

A spike in current when the motor starts would kill the regulator on overcurrent before any raise in temperature can be sensed.
Your "about 2.5A inrush" is probably much, much higher.

 

[DerStrom8]

A spike in current when the motor starts would kill the regulator on overcurrent before any raise in temperature can be sensed.
Your "about 2.5A inrush" is probably much, much higher.

I would second that!

 

[ronv]

Congratulations, I didn't think you could destroy an LM317.
Since it is well protected as far as current goes, I would think it is some type of over voltage.
Are the diodes schottky diodes? What size are the filter caps? Are you changing direction on the motor? How is the load switched.
I'll play with it in a simulation, but ......

 

[ronv]

I think the 317 is short circuit protected. My best guess is that the clamp diode across the motor is missing or bad.
Are you sure they are bad? Or is it possible they just current limit and won't drive the motor.

 

[unclejed613]

what kind of motor is it? if it's a commutator type motor, the AVERAGE running current might be 0.5A, but the peak currents might be much higher

 

[KeepItSimpleStupid]

Let's go an entirely different route. Changing the voltage is not what you really want anyway. This part, which I have used https://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_120539_-1 would probably make a very nice inexpensive speed control.

By using the rated voltage of the motor and changing the pulse width the torque will remain essentially constant. You may even have trouble starting or the inrush current is lasting too long at the lower voltages.

 

[icedvolvo]

Hey thanks for all the replies, there were a few Qs so here goes:

1: Do the 317s get warm to the touch before they "die"?: No not that I can feel

2: I don't think you're calculating the wattage correctly.? Vout -Vin x I = 24 - 17 x 0.5 = 7 x 0.5 = ~3.5W. Should be OK with a heatsink. But in any case the 317 should shut down if it gets too hot not self destruct.

3: A spike in current .... Yes this is what I am thinking too but shouldn't the diodes negate that?

4: inrush could be higher than 2.5A. Yes this is true, my Agilent DMM has sampling at 16kHz so should see any real spikes in Hold/Max mode. But may be you are correct. If this is the case then I will put this down to experience. However note that I can drive the motor with my home made EA piddling 1.2A variable bench supply with no problems!

5: Congratulations, I didn't think you could destroy an LM317. One!! Aw shucks I have a pile of dead ones on my bench as I type!!

6: Since it is well protected as far as current goes, I would think it is some type of over voltage. Are the diodes schottky diodes? No

7: What size are the filter caps? : combo of 100+10 elctro + 470n greencap + 1n poly both sides!

8: Are you changing direction on the motor? No, same direction.

9: How is the load switched. Welder trigger shuts a relay to 240V --> transform/bridge->24VDC-->317->load

10: what kind of motor is it? Hmm, don't know that, I suspect commutator cos I can see sparks, drives the wheels that force the wire through the conduit on a MIG welder.

11: if it's a commutator type motor, the AVERAGE running current might be 0.5A, but the peak currents might be much higher. But still the 317 should handle this no???

12: Let's go an entirely different route.... By using the rated voltage of the motor and changing the pulse width the torque will remain essentially constant. You may even have trouble starting or the inrush current is lasting too long at the lower voltages. Yes PCM is looking more likely!




Thanks to all who replied,

Like many I thought these 3 term voltage regulators indestructible but obviously they are not. It's sort of looking like the most reasonable answer is the 317 can't handle the inductive kickback in the motor and the diodes don't react quick enough to kill it off. If I had a DSO I could try to capture the point of failure but alas I don't :-(

 

[tvtech]

Got icedvolvo up to speed. Repped him too for being gracious with thanking people here that tried to help with his problem....LM317's blowing up..crazy stuff.

In all my years with 317T's ....I have never managed to blow even one up. And I have REALLY tried (within reason). Normally bullet proof beauties of note...

Who has an answer....?. I know I don't.

Regards,
tvtech

 

[picbits]

My money is on large input spikes. The outputs tend to be fairly bomb proof but spike them at the input and you will easily kill them.

 

[Jaguarjoe]

Two thoughts:

If the protection diodes in and around the 317 are slow diodes like many 4001's, reverse spikes from the terribly(?) noisy commutator could shoot the 317 dead before those diodes ever had a chance to act. I've had some 100/$1 diodes that were so crappy they had a hard time at even 60hz.

Look at fig 6 & 7 in this Microchip app note:

https://www.electro-tech-online.com/custompdfs/2012/06/00905a.pdf

See the explanation for R1/R2. I have no idea how to implement a 317 this way but its something to investigate.

Ultimately I think KISS is right. Just buy/build that $22 PWM unit and move on to some other bulletproof IC that needs to be spanked hard

 

[tvtech]

My money is on large input spikes. The outputs tend to be fairly bomb proof but spike them at the input and you will easily kill them.

Unqualified chirp from me...but yes that would make sense.

Best regards,
tvtech

 

[strantor]

what kind of motor is it? if it's a commutator type motor, the AVERAGE running current might be 0.5A, but the peak currents might be much higher

Just on a note, not that I think overcurrent is the problem, but I was playing around with a wheelchair motor last night and ended up having to parallel 2 of my biggest supplies to get it started. my Fluke DMM on min/max mode tells me it peaked at 10.75A. Running current was .85A. This was all with no load. So for the brushed commutator motor I observed ~13X inrush current over running current.

 

[colin55]

I suggest you place another 3-terminal regulator before the circuit and convert the 24v to 12v.

 

[bountyhunter]

Hi People,

have a technical Q about 317s if someone can help!

I wanted to make a new "smooth" variable power supply for a welder drive motor, motor uses about 0.5A average and about 2.5A inrush and works form 3-24V. OK so I think OK a 317 with a 5K pot will work fine. BUT every 317 gets destroyed!! So I kept adding capacitors and protection diodes all around the thing to protect it but still gets destroyed after a few seconds! Even a 338 only lasted ~10 seconds.

They do not smoke or blow up they just die!!! So far I have a dozen dead 317s and a dead 338. Circuit is stock standard LM317 design as shown in attached gif.

Any and all suggestions appreciated

Wow, I'm pretty stubborn but I think I would have stopped when the body count got to five.....

Motor is likely creating a back EMF that is swinging the LM317 output below ground. The protection diode may not be able to do the job.