MOSFET turned on and burned without gate bieng connect to signal

Thread Starter

idrbur

Joined Jan 11, 2018
64
Hi,
I am using a cj3400 30v 5A mosfet.
I build a 555 pwm controller to control its output. 555 gives a 5 v output and is connect to mosfet get with a 1k and 1k voltage divider. While it's drain is connected to loads and load is connected to a 24v supply.

When I used the circuit it worked perfectly.
But later I mistakenly turned on the 24V supply while the 555's supply was disconnected. And the mosfet somehow turned on and now the source and drain only have resistance of 40 ohm.
I didn't notice if until the load started burning and turned off the supply.
The load is a 18 ohm heating element.

What went wrong with the Mosfet and how can I prevent it?

Thankyou
 
Last edited:

shell.albert

Joined Jul 23, 2014
21
Could you show a brief circuit? Your description confused me.
MOSFETs are voltage sensitive, for a N-Channel, pull down the Gate with a big resistor to give it a stable level.
 

geekoftheweek

Joined Oct 6, 2013
1,256
You will need a pull down resistor between the gate and source to keep the MOSFET fully turned off. It is possible if the gate is left floating that it could develop enough of a charge to start conducting, but not be fully turned on. When this happens it can burn out and short the source and drain pins together.
 

Thread Starter

idrbur

Joined Jan 11, 2018
64
It's
Leaving the gate floating probably placed the MOSFET in its linear region.

Side question:

Why are you dividing the 555s output?
Sorry I made a mistake.
It's not a potential divider it's just a current limiting resistor and a pull down resistor.
I am a beginner in electronics so please forgive any mistakes.
 

StefanZe

Joined Nov 6, 2019
191
It's
Sorry I made a mistake.
It's not a potential divider it's just a current limiting resistor and a pull down resistor.
I am a beginner in electronics so please forgive any mistakes.
It is both.
Move the current limiting resistor between the pull down and the gate, if you don't want the lower voltage at the gate
 

sarahMCML

Joined May 11, 2019
425
It's
Sorry I made a mistake.
It's not a potential divider it's just a current limiting resistor and a pull down resistor.
I am a beginner in electronics so please forgive any mistakes.
You don't need a current limiting resistor for a MOSFET, just a 10 to 100 Ohm resistor to stop any potential ringing at the gate, and a pull down of, in this case 10k Ohms in case you disconnect the 555. Just ensure you don't drive the gate past its maximum allowed in the datasheet!
 

sagor

Joined Mar 10, 2019
927
5v output to a 1k/1k voltage divider probably means that the gate voltage is not high enough to fully turn on the mosfet. Even logic level mosfet needs at least 5v to turn fully on. Most mosfets like 10v on the gate to reach minimum RDSon.
So, it looks like the mosfet resistance was high enough to generate lots of heat and destroy itself. (At least a possibility)
 

Irving

Joined Jan 30, 2016
4,074
This device quotes a maximum on resistance of 52mOhm at Vgs = 2.5v so in theory should have been ok with a 5v feed and a 2:1 divider so that suggests it was a residual voltage <2.5v but given the 1k to ground its hard to see why it turned on. However, when on i = 24/18 = 1.33A so an Rds of only 0.2ohm or 350mW would have overheated the junction which gives credence to the turn on in the linear region theory.
 

Attachments

Thread Starter

idrbur

Joined Jan 11, 2018
64
I built a new circuit and replaced the 1k ground gate-source resistor with 10k and now it's working perfectly.
Thankyou.
 
Top