Hi

So I have built a 5v regulator using a 7805 and a power transistor (TIP42C) to carry the extra current to stop the overheating 7805.

The TIP42C is supposed to handle 6Amps constant current, i'm using less than 1.5 amps of constant current, and its overheating like crazy. Is this normal? I have a small heatsink, but I can feel the heat radiating off it, and im scared to have it running for any length of time, im sure it will melt something nearby on the circuit board.

If anyone has any advice or would like any further information about my power supply please post a message.

Thanks,
S

 

You have to have good heatsinking, which sounds like it might be a problem. What is the transistor connected too? And whilst we're gathering data, what kind of transistor case, TO220 and TO3 are probably the most common.

Most heatsinks use high temperature tape and nylon insert to prevent the transistor case from making electrical contact while allowing good thermal contact. You may need a kit, though if you pay close attention there are other ways.

 

What is your input voltage?

 

I have a 12v battery connected to the collector of the transistor, a 3ohm resistor between collector and base/input of 7805. Then i have the emitter connected to the output of the 7805. The 3ohm resistor means the 7805 draws only about 200mA which is about the max of its capabilities. Any extra current after that and the transistor switches on carrying the rest.

The transistor package is the TO220 style.
I have a small heatsink which sort of half wraps around the transistor, I also have heat paste between the heatsink and transistor, and a screw connecting them both together. Maybe I should bend the heatsink flaps outwards so it dissipates the heat better? Or just a bigger heat-sink...

 

Your transistor is connected backwards. The emitter should connect to the +12V supply.
When it is fixed or replaced then it must dissipate (12V - 5V) x 1.5A= 10.5W. With a small heatsink it can dissipate a few Watts. Use a huge heatsink and wire it like this:

 

Your transistor has 7 volts across it. Multiply that by the current through it, and you get the power dissipated. 1.5A times 7V is 10.5 Watts! Let's say that you want to keep the junction temperature below 100°C, and your ambient temperature might reach a very modest 30°C. That means you need a total thermal resistance from junction to ambient of (100-30)/10.5=6.7°C/W. The junction-to-case thermal resistance is 1.92°C/W. Your heatsink therefore needs to have a thermal resistance of (6.7-1.9)=4.8°C/W, including thermal compound thermal resistance.
Here are some TO-220 heatsinks.
Switching regulators are the best for high-power supplies, because they are much more efficient. Look into buck regulators if you want to build your own. Just be aware that they are not as simple as linear regulators.

 

7805 is capable of 1A max and adding a buck booster will extend its capability by 1 or 2 A but not for what the transistor is rated for.. Expect a maximum of 3A but never 6.. If a buck booster is used, it will heat up like crazy without any load over it to drop the excess voltage.. It is not an efficient solution but simple for us people.
Heatsinks beyond the rated size in the datasheet are not effective as it radiates as it travels across the sink and wont cover the excess area. The only difference with huge heatsinks is that you wont feel the heat radiation when u hold your hand over it.
A 7805 will heat heavily from a 12v source due to the 7v drop. And 7809 lesser and 7812 the least due the minimum volatge drop..

 

Thanks all for your replies its been very helpful.

I would love to use a switching power supply but i simply dont have the time for that (its for my final university project) also space is an issue, i need a quick fix.

So I guess ill just pump a huge heatsink, also I didnt realise I had the transistor the wrong way around, ill see if that makes a difference