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I am going to Purchase LM7805 for 5V supply for my school project and I now know how to connect it. Now I want to know that will excess current damage my circuit? As per datasheet output current is 1.5A. Can you tell that will my IC work on it? ICs are -: 7404,7432,7438. And how do I know what is max current limit for a certain IC? Is that is related to iol and Ioh?

Shubham Wagh
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  • @MattYoung Nope it is not – Shubham Wagh Dec 31 '16 at 18:18
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    @ShubhamWagh it is a duplicate - the linked thread is what we immediately refer all similar questions to. If you read the first answer, the bit under the heading "Current Rating", you'll get your answer. – Tom Carpenter Dec 31 '16 at 18:20
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    @ShubhamWagh: Yes, it is a duplicate, and a very common question. The 7805 _can_ deliver up to 1.5 Amp (with appropriate heatsink, and other qualifications), but your circuit will only draw the current it requires. – Peter Bennett Dec 31 '16 at 18:22
  • go carefully through the question and answer @Matt suggested. You would find the answer you are looking for. – MaNyYaCk Dec 31 '16 at 18:25

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The ICs will draw whatever current they want to (if the power supply can deliver it). The regulator just needs to be able to deliver the maximum current your circuit wants to draw.

Some 'headroom' is necessary to ensure that the circuit won't be starved of current, but too much makes it more vulnerable to damage if overloaded (eg. due to incorrect wiring or a short circuit). Therefore you should choose a regulator that can supply more than the expected peak current draw, but not a lot more.

For experimenting I recommend using an adjustable current limiting power supply, or at the very least an ammeter in series with the supply. If that is not possible then use a regulator with a lower current limit (eg. 78L05 or 78M05) and check its temperature during operation of the circuit.

Finding the maximum current draw of a particular IC can be a bit tricky. You can add up IOL of each output, but that may exceed the maximum current or power dissipation rating of the entire IC. The IC will draw some internal current even when the outputs are disconnected, and this internal current may go up or down in different logic states. Often the only way to find the actual current draw is to put the IC in a circuit and measure it.

Bruce Abbott
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Short answer: No. The 7805 will not damage your circuit.

Long answer: The current specification for a three-terminal regulator is the maximum current it will provide. If you stop and think about it, you can either regulate for voltage (very common), or regulate for currrent (less common) but not both.

Do the following thought-experiment: What happens when you connect the output of the 7805 to nothing but a 5-ohm resistor. It cannot provide 5 V and 1.5 amps without violating Ohm's law. It's either going to provide 5V at 1A (which it does), or it would provide 7.5 volts at 1.5 A (which it won't).

There are regulator circuits that only guarantee regulation when they have some specified minimum load, but the common three-terminal regulators are not of those types.

The answer to your second question can typically be found on the datasheet for the part in question. Icc - and it will usually be stated as typical and maximum values for some specified conditions. For example, I have the 7438 (BTW: Are you actually using ancient TTL parts? The answers will be very different for 74LS38 or 74ACT38 or 74foobarbaz38) datasheet in front of me, and "with all outputs high, the maximum power supply current is 8.5mA and with all outputs low, the maximum current is 54 mA"

Lawrence NK1G
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