too much current thru transistor

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#4
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
 
#7
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.
 
#8
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.
 
#9
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
 
#10
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.
 
#13
cowanrg said:
hum, after reading the posts, its almost like im getting the feeling the manufacturers put on limits because their parts fail beyond those limits. interesting. 🙄
Click to expand...

It's actually the other way around. The amp was designed to work with a certain max output/current conditions, and they choose parts that will allow fail-safe operation plus a little headroom. Although it is not unusual for cheaper equipment to skimp, since real-world music don't stress the parts as much as test tones do, and most people don't play test tones at max output.

Also, parts choice is a bit more complex than just failure limits. For example, a higher-current transistor sill have bigger junctions but that has more capacitance, which leads to poorer Ft (high frequency response) when compared to a lower-current device. This could lead to lower performance. So the choice is a tradeoff. It's not always the right thing to use the biggest part there is. An engineer who designs the circuit must carefully examine the circuit performance requirements and choose the devices based on the various specifications.

-Ti
 
#19
amb said:


The wattage output rating of an amp is dependent on the load impedance, amongst other things.

-Ti
Click to expand...

Sorry, I guess that may have been a little sarcastic for most of the non UK residents out there. What I was simply trying to state is that I doubt he was actualy getting the claimed 1400+W of power from said amplifier, especialy as it has now been described as a 125W x 2 into 8 ohm speakers (you could possibly get up to about 1000W if you bridged it and ran it into a four ohm 'speaker, but I doubt the amp would cope if it has the ouput devices already mentioned)
 
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