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Do I need a Protection Diode for 78L05?

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Suraj143

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Hi I have a small 5V circuit that powering from 78L05 (TO92).The circuit needs 50mA or below.

But my supply is 12V to this 78L05.Do I need a protection diode to this 78L05 in case of reverse polarity ? Will the supply burn the 78L05?

Because i have very limited space.
 
If the input voltage to a uA78L05 is reversed then it will be destroyed. So use a diode in series with its input.

With a series diode it will have 6.3V across it and 50mA through it. Simple arithmatic calculates a dissipation of 315mW which will make it pretty hot if it doesn't have free air flow.
 
Oh I see then I'll use a protection diode.Otherwise a big problem damaging the regulator.

Thanks for that.
 
add diode is for better protection. spec usually provide suggested circuit connection to get "good" 5v at output side.
your current is very low, may need some resistor or current limitation device with your loading.:eek:
 
Hi suraj,
The 78L05 is a protected device and hard to destroy.
The minimum input voltage is 8.5 volt. A series diode will solve your problem. The diode only needs to be able to carry 52 mA (load current + 78L05 current.
The voltage drop across even a small diode should be around 1 volt maximum and its dissipation only 55 milli watt. Most small signal diodes will dissipate at least 150 milli watt.
The use of the diode will reduce the power supply input to the 78L05 from 12 to say 11 volt. As this is above the minimum of 8.5 volt, then you are OK. The reduced input voltage to the 78L05 will reduce its internal dissipation, which is good.
1N4148 is usually available.
Hope this helps.
 
If the input to the 7805 is shorted to gnd, the output voltage will be higher than the input voltage for a very short time as the capacitors on the output side discharge backwards through the '05 to gnd. This will kill it. Adding another diode across the '05 from output to input fixes that. (anode to output pin, cathode to input pin).

You mentioned that you were short on space. Some regs have protection built in like LM2937/40 and are almost bullettproof. No extra diodes needed.
 
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