Current limiting circuit for dynamic load

[HagenK]

So I have this 9V 2.5A power supply and a flash unit which needs 6V. I use LM7806 to drop voltage, but when the flash is charging up it overload regulator, thus further dropping voltage to megger 2.5V. I think I need to somehow limit max current draw to 1.5A. Surprisingly I couldn't find a solution in google, so maybe you guys can help me out?

 

[D.A.(Tony)Stewart]

7806 is not the best LDO to use here.

edit**
Reason? LDO will drop 2.5V at 1.5A at 85'C , which means you need 8.5 min anyway but chances are your load demands more than 1.5 and that's causing shutdown on starting.
Power dissipation (9-6V)*1.5A= 4.5W is a big problem
A heatsink will improve overtemp condition, but not enough. if overloaded. thus use a 1.5A PTC in front is a bandaid fix.

A better solution is use a better adjustable LDO with 0.5 Dropout and lose 2.5V*1.5A=3.75W in a power resistor or 3 power diodes.

The best solution is a 99% efficient Buck regulator which can output >3.5A @6V = 21W from 9V*3,5A = 22.5W

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It may be possible to bypass LDO with a Pch FET with a heatsink
Using LDO drain current to bias Vgs=-1V (8V)

e.g. https://www.digikey.com/product-detail/en/STD30PF03LT4/497-3158-1-ND/654490

Or better use a suitable 3A 6V LDO with 0.8V dropout @3A but still needs heatsink
https://www.digikey.com/product-sea...=0&page=1&quantity=0&ptm=0&fid=0&pageSize=500