Capacitor charging problem at startup

below circuit shows a 3 terminal or any type of regulator it may be switching type also.

At ouput we have large capaictance 3300uF on when power is applied a large current will be required to charge this and now if regulator is of 250mA rating then this current surge may be fatal to regulator am i right.

So what are suggestion to protect that.

I donot remeber I once saw a diode from input to ouput which serves for the purpose. I am not sure about this.

Voltage reulators have internal current limit which in this case is 250mA, so the max the 3300µF cap will "see" is ≈250mA .. and don't worry, nothing will happen to the regulator unless a higher voltage is present at the output (with reference to the input) .. therefore a good idea will be to connect a diode between the output pin(anode) and the input (cathode) ..

Regards,
IanP

As u have suggested I have drawn new diagram.

Tell me I am right in the explanation of below:
Diode will provide current for charging capacitor which will be faster then earlier.
Supose we are using LM7805 and if input is 9V and oput will be 5V.
But as diode is used at cathode of diode it will give 9V-Vd i.e around 8.5V but regulator ouput will be stuck on 5V how is this possible.
May be due to feed back circuit in regulator.

This diode has to be connected as follows:

Anode to output ..
Cathode to input ..

You have to reverse it and the output will go down to 5V ..
(see picture below)
That's for protection ..
If you need to charge the output capacitor faster and you know that the unregulated input voltage will never be higher than 9V, you can connect 5V1 zener diode between input and output ..
This will help charging the output capacitor till, say, 4V and will protect the output of the voltage regulator as if it was a standard diode ..

Regards,
IanP

I got your point.
But in few circuits I have seen schotky diodes connected.
what that is for.

The voltage regulator should have a current limiting function. Otherwise big surge current flows at startup. Alternatively you can use a series resistance at output and after capacitor charging to full output voltage it can be bypassed via a relay contact. Another solution using NTC resistor.
I think current limiting is the best solution.
Regards,

no need to be afraid
all power circuit should have an inrush current limiter ;
also in smps it has to be soft start controller.


any way u can put a resistor b4 or after the regulator to limit this current.

any way u can put a resistor b4 or after the regulator to limit this current.

Click to expand...

Alternatively you can use a series resistance at output and after capacitor charging to full output voltage it can be bypassed via a relay contact.

Click to expand...

I donot like resistor they are power wasting the are reason of voltage drops.

Voltage regulators work very well as current limiters, contant current sources etc. etc. and they don't require inrush current protection ..
However, if you really insist on connecting an inrush current limitter you can use one, and more details on these dvices you can find here:

As noted above, the large filter capacitors appear as a short circuit at turn-on. Soon after, the capacitors develop a charge of ½ CV2, where C is the circuit capacitance and V is the peak voltage. This charge is the amount of energy that must be withstood by the inrush current limiter, ex-pressed in Joules. Size and quality of construction generally determine the amount of energy that can be absorbed by a Surge Limiter. Therefore, care must be taken to select a Surge Limiter with sufficient ability to absorb energy. Failure to do so may result in catastrophic failure of the Surge Limiter.

The inrush current limiter is normally placed in series with the diode bridge, motor, or other components of a system that benefit from inrush protection. For added protection, two Surge Limiters can be placed in series.

Click to expand...

https://www.ametherm.com/Inrush_Current/inrush_current_limiters_pcim.htm

https://www.ametherm.com/Inrush_Current/minamp_inrush_current_limiters.htm

Regards,
IanP

nikhilele said:

below circuit shows a 3 terminal or any type of regulator it may be switching type also.

At ouput we have large capaictance 3300uF on when power is applied a large current will be required to charge this and now if regulator is of 250mA rating then this current surge may be fatal to regulator am i right.

So what are suggestion to protect that.

I donot remeber I once saw a diode from input to ouput which serves for the purpose. I am not sure about this.

Click to expand...

like most people who make circuits with voltage regulators

you forgot the add the surge delimiter resistor in series with the voltage supply to the regulator

this is usualy a low value in the R zone

for 5v 1amp lm7805 this is 27R

if you negate this resistor the ic{voltage regulator} will be damaged
buy excessive current

both inrush and back rush
so it will try to switch off and on very fast as it peeks in current use to supply the needed voltage

you must add a spring to it in the form of a low value resistor to add a dampening
effect to the current modes it uses

to use it hence regulate properly the voltage over a wide range of use and current paths with respect to the time its uses the current then voltage for

inrush only isnt enough

and is infact in a standby psu
what uses the current and gets hot

where a simple resistor is all that needed
and bairly gets warm so uses far less overall current

and is the right approch to regulaton using ics


a bed is hard unless it has springs

much like suspension in a car

for ac systems inrush protection is enough but dc
requires each way shuffle

VSMVDD wrote:

if you negate this resistor the ic{voltage regulator} will be damaged
buy excessive current

Click to expand...

just have a look how much voltage will drop across resistor when 500mA current is passing.