Hi guys, I have questions about the LM7805, how much input current can it withstand? for example if I have a large input current of 20A the output how much is it?
The regulator and its load draw whatever is required up to the maximum load current rating of 1A rating of the regulator.
The input current equals the load current PLUS the small current taken by the regulator (a few milliamps).
The input current equals the load current PLUS the small current taken by the regulator (a few milliamps).
It is not the input current that is important, you could supply it with a 1000VA transformer.
It can only supply a max of 1.5 - 2.0A depending on variant to the output.
It can only supply a max of 1.5 - 2.0A depending on variant to the output.
The output current is more dependent on the V drop across the regulator and the capacity of the heatsink attached.
Could be less than 1A.
The current supply capacity pre-regulator is of no importance.
Could be less than 1A.
The current supply capacity pre-regulator is of no importance.
The nature of the question compels me to ask...are you trying to do something, or are you just curious?
Mike
The nature of the question compels me to ask...are you trying to do something, or are you just curious?
Mike
me too.
So if you need a 20A at a regulated Voltage you need other components too e.g. some output transistors. the forum is full of that solutions.
data sheet:
max Rating at the input 35V, max 1A - impulse up to 20A with big heat sink etc.
Vout = Vin - Vdrop(min. 2-2,5V)
chris
The nature of the question compels me to ask...are you trying to do something, or are you just curious?
Mike
Thanks guys, no it was just curiosity to understand how LM7805 works, but actually how voltage regulators work in general
Some info from the data sheet.
7.3.2 Current Limiting Feature
In the event of a short circuit at the output of the regulator, each device has an internal current limit
to protect it from damage. The typical current limits for the LM340 and LM7805 Family is 2.4 A.
7.3.3 Thermal Shutdown
Each package type employs internal current limiting and thermal shutdown to provide safe operation area
protection. If the junction temperature is allowed to rise to 150°C, the device will go into thermal shutdown.
See foldback current limiting here:
http://www.ti.com/lit/an/snva558/snva558.pdf?ts=1588205812570
7.3.2 Current Limiting Feature
In the event of a short circuit at the output of the regulator, each device has an internal current limit
to protect it from damage. The typical current limits for the LM340 and LM7805 Family is 2.4 A.
7.3.3 Thermal Shutdown
Each package type employs internal current limiting and thermal shutdown to provide safe operation area
protection. If the junction temperature is allowed to rise to 150°C, the device will go into thermal shutdown.
See foldback current limiting here:
http://www.ti.com/lit/an/snva558/snva558.pdf?ts=1588205812570
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Most linear regulators like the 78 series work by having an internal fixed 'bandgap' reference voltage produced internally a bit like a super stable Zener diode. Its low in value at around 1.22 volts.
The regulator contains a series pass transistor and an error amplifier (opamp).
The reference voltage is applied to the opamp input and the opamp gain set accordingly to give whatever the voltage of the regulator is.
The output of the opamp feeds the series pass transistor which is needed to deliver lots of current if required.
The feedback loop for the opamp is connected to the output of the pass transistor so that is now in the loop.
The input voltage must be at least a couple of volts higher than the regulator voltage so that the pass transistor can operate correctly.
There's a turn-down valve and a bunch of little demons inside a regulator.
One is ordered: "if output exceeds 5V, turn down the power". (Which is why, if you put in 4V, you can't get 5V. The demon can only turn-down, not pull power from thin air.) (Actually there's always losses, and you need over 7V in to get 5V out.)
Another is ordered: "If the chip gets hot, turn down!" This generally limits the useful current when input voltage is high.
Another is commended: "If there is over 1.5 Amps, turn down!" This limits current for very low input voltages where heat may not be extreme even when bond-wires are smoking.
There is a little-known demon. If input is over 35V, this one shuts-off all current. However if the voltage *drop* is over 40V, or the input voltage is much over 40V, the turn-off valve (transistor) will burn-short no matter what the demon does. Don't tease this guy.
There is no demon to detect/correct reversed input connections. What would he do? Jump out and swap the wires? (Yes, today we could make a chip this smart, but with other trade-offs, notably price and losses.)
One is ordered: "if output exceeds 5V, turn down the power". (Which is why, if you put in 4V, you can't get 5V. The demon can only turn-down, not pull power from thin air.) (Actually there's always losses, and you need over 7V in to get 5V out.)
Another is ordered: "If the chip gets hot, turn down!" This generally limits the useful current when input voltage is high.
Another is commended: "If there is over 1.5 Amps, turn down!" This limits current for very low input voltages where heat may not be extreme even when bond-wires are smoking.
There is a little-known demon. If input is over 35V, this one shuts-off all current. However if the voltage *drop* is over 40V, or the input voltage is much over 40V, the turn-off valve (transistor) will burn-short no matter what the demon does. Don't tease this guy.
There is no demon to detect/correct reversed input connections. What would he do? Jump out and swap the wires? (Yes, today we could make a chip this smart, but with other trade-offs, notably price and losses.)
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Your question makes no sense.
Input and output teminal are **IN SERIES** so current entering one must be exact same as curent leaving the other.
That said, a tiny tiny amount of curent does not reach output, it´s branched to gound through the tiny voltage regulator "brain" for its own use.