too much current thru transistor

xplod1236 · 26th May 2004, 12:19 AM

what will happen if I try to pull too much current through a transistor?

ex: transistor is rated at 5 amps. I try to pull 7 amps thru it. What will happen?
Will the transistor overheat and fry? Wil it limit the current going thru it?

Tensop · 26th May 2004, 12:20 AM

it will overheat, die and burn your house down, voiding your house insurance

xplod1236 · 26th May 2004, 12:25 AM

what if I install it on a really big heatsink so that it doesn't overheat? what will happen then?

amb · 26th May 2004, 01:06 AM

The internal junction sizes of the transistor are rated only for the specified amount of current. Even if you install large heat sinks, the junction will be destroyed if you apply too much current. You will end up with either a open circuit or worse, a short, causing other parts of the circuit to fry along with it.

-Ti

xplod1236 · 26th May 2004, 01:53 AM

what will happen to diodes if too much current is pulled thru them?

amb · 26th May 2004, 02:02 AM

Same thing... the diode will be destroyed.

-Ti

dmfraser · 27th May 2004, 12:27 AM

These things are so cheap. If you need 7 amps but a 15 amp device. Never pull more than 1/2 the rated current through a semi-conductor. If you need more, buy more. There is a reason why a 5A device is rated at 5A. If it could run safely at 7A, it wold be rated as a 10A device.

xplod1236 · 27th May 2004, 12:56 AM

that was just an example. I have an amp that was designed for 8 ohm speakers, and im running 3 ohm subs off it. I calculated the current to be 16+ amps going out to each speaker. The transistors are rated at 17. The diode bridge is rated at 5 amps. The power going thru it is 30+ amps. I had the amp running like that for a while now, and nothing has fried yet. I'm planning to replace the diodes sometime this week.

I just wanted to know what could happen to transistors and diodes.

amb · 27th May 2004, 06:01 AM

Your amp survives because the over-current condition are short duration transients while playing music, and you're probably not grossly exceeding the specs of the parts (how loud do you play your music anyway). I'll bet if you try to play a continuous sine wave test tone at max output, or stress the amp with prolonged high-volume playback, those devices' life could be on borrowed time.

-Ti

Christer · 27th May 2004, 06:01 AM

Always be careful, especially with prototypes. Semiconductors
can really blow up into pieces if you do something wrong so
you go above the max ratings. I know. Always wear protective
glasses, or ordinary glasses if you use them anyway, when
working with prototypes.

26th May 2004, 12:19 AM	#1
xplod1236 diyAudio Member Join Date: Dec 2003 Location: On the couch	too much current thru transistor what will happen if I try to pull too much current through a transistor? ex: transistor is rated at 5 amps. I try to pull 7 amps thru it. What will happen? Will the transistor overheat and fry? Wil it limit the current going thru it?

27th May 2004, 12:27 AM	#7
dmfraser diyAudio Member Join Date: Dec 2003 Location: Orange County, CA	Current These things are so cheap. If you need 7 amps but a 15 amp device. Never pull more than 1/2 the rated current through a semi-conductor. If you need more, buy more. There is a reason why a 5A device is rated at 5A. If it could run safely at 7A, it wold be rated as a 10A device. __________________ Dan Fraser

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26th May 2004, 12:20 AM	#2
Tensop diyAudio Member Join Date: Jul 2003 Location: Chapel St.	it will overheat, die and burn your house down, voiding your house insurance

26th May 2004, 12:25 AM	#3
xplod1236 diyAudio Member Join Date: Dec 2003 Location: On the couch	what if I install it on a really big heatsink so that it doesn't overheat? what will happen then?

26th May 2004, 01:06 AM	#4
amb diyAudio Member Join Date: Apr 2004 Location: Sunnyvale, CA.	The internal junction sizes of the transistor are rated only for the specified amount of current. Even if you install large heat sinks, the junction will be destroyed if you apply too much current. You will end up with either a open circuit or worse, a short, causing other parts of the circuit to fry along with it. -Ti

26th May 2004, 01:53 AM	#5
xplod1236 diyAudio Member Join Date: Dec 2003 Location: On the couch	what will happen to diodes if too much current is pulled thru them?

26th May 2004, 02:02 AM	#6
amb diyAudio Member Join Date: Apr 2004 Location: Sunnyvale, CA.	Same thing... the diode will be destroyed. -Ti

27th May 2004, 12:56 AM	#8
xplod1236 diyAudio Member Join Date: Dec 2003 Location: On the couch	that was just an example. I have an amp that was designed for 8 ohm speakers, and im running 3 ohm subs off it. I calculated the current to be 16+ amps going out to each speaker. The transistors are rated at 17. The diode bridge is rated at 5 amps. The power going thru it is 30+ amps. I had the amp running like that for a while now, and nothing has fried yet. I'm planning to replace the diodes sometime this week. I just wanted to know what could happen to transistors and diodes.

27th May 2004, 06:01 AM	#9
amb diyAudio Member Join Date: Apr 2004 Location: Sunnyvale, CA.	Your amp survives because the over-current condition are short duration transients while playing music, and you're probably not grossly exceeding the specs of the parts (how loud do you play your music anyway). I'll bet if you try to play a continuous sine wave test tone at max output, or stress the amp with prolonged high-volume playback, those devices' life could be on borrowed time. -Ti

27th May 2004, 06:01 AM	#10
Christer diyAudio Member Join Date: Sep 2002 Location: Sweden	Always be careful, especially with prototypes. Semiconductors can really blow up into pieces if you do something wrong so you go above the max ratings. I know. Always wear protective glasses, or ordinary glasses if you use them anyway, when working with prototypes.

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