What happens if you put too high voltage across a voltage regulator ?

Thread Starter

Aero787

Joined May 4, 2019
2
Hi everyone, i started a project that includes regulating a voltage down to 5v.
I'm using the ST LM317 the input voltage must be lower than 36v, what happens if it is higher than that ?
 

Wendy

Joined Mar 24, 2008
22,141
It dies, your parts die, everything dies. You might even let the magic blue smoke out of most of them...

Failure modes are usually unpredictable, That's why you try to avoid then...

Wendy's Index
 
Last edited:

bertus

Joined Apr 5, 2008
20,535
Hello,

Never exceed the maximum input voltage of a device.
When a LM317 is used at a high input-output voltage difference, the output current will be very limited:

LM317_electrical_characteristics_current_limit.png

Bertus
 

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OBW0549

Joined Mar 2, 2015
3,401
Hi everyone, i started a project that includes regulating a voltage down to 5v.
I'm using the ST LM317 the input voltage must be lower than 36v, what happens if it is higher than that ?
Evil-smelling smoke, fire and woe. Or just "pffffft!"
 

kubeek

Joined Sep 20, 2005
5,724
Typically there are two operating regions that are stated in the datasheet. Here for supply voltage you got a recommended supply below 36V, and an absolute maximum of 40V. The distinction is that above 36V the other parameters stated in the datsheet might not be true, such as accuracy, supply current etc. Above 40V, be it a small spike in voltage or operating at that all the time, the manufacturer tells you that the reliability will be very low.
Edit: I don´t see the recommended voltage in this datasheet, but other components typically state that.
 

Wendy

Joined Mar 24, 2008
22,141
I have seen chips cratered from pureline voltage applied to them back when I was in tech school, It was a 741 someone accidentally hooked up to line voltage. If you are interested I'll tell more of the story. The silicon under the epoxy was completely vaporized.just a conical crater in the package was left.
 

Thread Starter

Aero787

Joined May 4, 2019
2
Thanks a lot for your quick answers !
The input voltage will be around 24v if i'm not pedalling too fast (I try to recharge batteries with my bike)
Seems like i will have to experiment and maybe use gears to make sure the motor won't spin too fast
 

Wendy

Joined Mar 24, 2008
22,141
You could make a preregulator or a voltage limiter if there is a real chance of overvoltaging the circuit. I'd be tempted to put a cheap voltmeter and see how high it goes. The LM317 is a first generation chip, there are better out their for about the same price ranges.
 

narkeleptk

Joined Mar 11, 2019
461
I have seen chips cratered from pureline voltage applied to them back when I was in tech school, It was a 741 someone accidentally hooked up to line voltage. If you are interested I'll tell more of the story. The silicon under the epoxy was completely vaporized.just a conical crater in the package was left.
You see a lot of this in automotive modules. Especially the ignition drivers.

20190107_202941.jpg
 

Wendy

Joined Mar 24, 2008
22,141
Since you are talking about generated voltage a simple zener circuit in front to prevent overvoltaging the regulator.
 

Wendy

Joined Mar 24, 2008
22,141
My then instructor had this bright idea of usin AC power plugs for ±15VDC. He forgot about fools (him being the prime one).
 

narkeleptk

Joined Mar 11, 2019
461
Do you see a lot of automotive systems that had been abused with line voltage (120-240Vac based on your location) instead of 12Vdc? ;)
Can't say I have ever seen that. For things related to this topic its either from the ignition coils, hooking 12v systems in to 24v, a short in wiring or the alternator/regulator over charging .
 
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