# Plzz Help! B688 become hot!!!

1. ## Plzz Help! B688 become hot!!!

Hi everyone. I have designed a power supply with high current. I used 7805 with B688 follow this diagram:

But in this diagram, when current is about 400mA, Transistor become hot a lot.
I dont know why. My transistor is B688. I try using many difference resistor but it is not effect. B688 is still hot.
Who can help solve it.
Thanks very much.

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2. ## Re: Plzz Help! B688 become hot!!!

The power dissipated in Q1 depends as much on the voltage across it as the current through it. There is no linear regulator which can defy this fundamental law of physics.
Q1 starts to conduct when the load current is about 200mA, you could reduce R1 so more current goes through the regulator before it starts to conduct but I'm guessing your real problem is lack of heat sink. The heat dissipated by the regulator and transistor combined is (Input voltage - Output voltage) x Input current. You can't change that I'm afraid, the best you can do is get rid of the heat with a heat sink.

Brian.

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3. ## Re: Plzz Help! B688 become hot!!!

Originally Posted by betwixt
The power dissipated in Q1 depends as much on the voltage across it as the current through it. There is no linear regulator which can defy this fundamental law of physics.
Q1 starts to conduct when the load current is about 200mA, you could reduce R1 so more current goes through the regulator before it starts to conduct but I'm guessing your real problem is lack of heat sink. The heat dissipated by the regulator and transistor combined is (Input voltage - Output voltage) x Input current. You can't change that I'm afraid, the best you can do is get rid of the heat with a heat sink.

Brian.
Yes, u are right. We can't change the fact that: the heat is (Input voltage-Output voltage). Input Current. But Q1 is power component. Its can work with large current throught it. My problem is when the current is about 400mA, Q1 become hot. If so, this diagram is not High Current Voltage Regulator. I agree that i have to use more heat sink.
Thanks.

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4. ## Re: Plzz Help! B688 become hot!!!

Originally Posted by buiducthang
My problem is when the current is about 400mA, Q1 become hot. If so, this diagram is not High Current Voltage Regulator.
8A sounds like a high current to me (higher than 78xx would manage). The circuit is a high current regulator. As already pointed out, you cannot change the laws of physics and must provide the means to dissipate the power resulting from the circuit.

Keith.

5. ## Re: Plzz Help! B688 become hot!!!

Yes, Maybe we can avoid the physical law. This is linear supply voltage. So the heat either disspate in Regulator or in Q1. I think i have to use Switching Power supply.
Thanks Bro!

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6. ## Re: Plzz Help! B688 become hot!!!

It's probably less about the transistor, than heat sink and
airflow. Watts is watts, and they need to go someplace
else. Quickly.

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