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Exlpoding MOSFETs!! What the problem!?

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GTechno13

Member
OK so I am using MOSFETs to switch on and off short pieces of ni-chrome wire. I am using a 5A 12Vdc power supply. I have one end of resistance wire (about .6ohm) connected to +12 the other end connected to drain. The source is then grounded. I will eventually be using a 12v decoder (VDD will be the same 12v supply) but for now I am just connecting the gate to 12v via a 10k resistor and then grounding the gate to shut it off. This all works fine and dandy with a indicator light or in a simulation but once I hook up the resistance wire the mosfet gets hotter than the wire!! Whats wrong!!!

Also, I'm having a hard time understanding the equation for this but what is the opinion on the size of smoothing cap(s) for the above mention supply. 12v 5A

Thanks!
 

Sceadwian

Banned
What mosfet are you uusing?
 

GTechno13

Member
uhhhh ya forgot to mention they are 7.5A i think uh FQP7N10L

My best guess at this point is that the resistance in the mosfet is possibly higher or at least too high compared to the resistance wire.
 
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Sceadwian

Banned
What's the resistance of the nichome wire?
 

tcmtech

Banned
Most Helpful Member
At .6 ohms you should be getting about 20 amps of current going through the wire.

First, Your power supply is not big enough for starters!
Second, For actual on off switching of loads you should have at least a 2x current rating for your Mosfets.
Third, I would still be putting a heat sink on a mosfet thats being driven the way you are running it.
 

MrDEB

Well-Known Member
IMHO use a TRIAC or SCR

or even a relay maybe??
just my opnion
look at it this way = exploding mosfets or SCR's??
 

Sceadwian

Banned
tcm he's using 22 gauge wire for leads, .6 ohms for the heating element, and the FET itself is .35 ohsm. By my calculations it's 9.6 amps (this was mentioned in chat) He's losing 50% of his power to the FET and leads. he's gonna try to parallel the FETS to see if he can avoid over heating them. The leads area loss but not important.
 

tcmtech

Banned
Most Helpful Member
Ah... Getting info from chat is cheating! How is anyone supposed to come up with the right answer when the questions are being loaded! :confused: :(

I think thats cheating! :D Sort of like insider trading info, only you have insider design info! ;)

I will go and play some where else then. ..l.. :rolleyes::D
 

Sceadwian

Banned
So drag yer sorry behind into chat =) It's there for everyone, so don't complain about cheating ;)
 
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Sceadwian

Banned
Good idea gramo, but chat is often.... off topic most of the time.
 

tcmtech

Banned
Most Helpful Member
I have often wondered if it would be possible to have a little indicator by the chat button that showed how many people were in the chat area at that particular moment.
 

GTechno13

Member
Haha yea this migrated into the chat right away... I believe my main problem was concluded that I didn't know the 12v 5A supply could deliver more than 5A! Apparently the supply will drop the voltage and increase the amperage leading to my superheated mosfets. So like he said I am doubling them up and then using a relay to separate the 30 pairs into two groups of 30 giving 60 like I originally intended.

I cannot use the mosfets on the positive side (before the wire) correct? (hence using a relay instead)
 
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tcmtech

Banned
Most Helpful Member
Mosfets work just fine on the high side of you have a high side driver IC.

This is just one example of many high/low side driver IC's. They make running Mosfets and IGBT's on H-bridge setups very simple and reliable.
 

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carmusic

New Member
for your project i would use a IRFP4004 with have only 1.7 mohm resistance it is almost better than a mechanical switch
 
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