[SOLVED] power supply problem in micro controller circuit please help

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Hello
i am using SMPS +24,2.1 Amps then by using series connection of 78012+7805 powered the micro controller and toggle 8 relays (24 @1100ohm) at same time using ULN 2803.
when relays are toggle then output of 7805 swing from 4.75 to 5.01. what i can do to over come the load of circuit
thanks in advance.
 

You can increase electrolytic capacitors at input of 7812 and 7805 regulators, and also can add ceramic types e.g 100nF in parallel, in order to improve its overall decoupling capacity response for spikes becoming from bobbin relays.


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but you said you already use 2.1A supply?
do you have enough capacitance at output of LM7812 and LM7805?...AS well as at there inputs... can you put a current sense resistor in (small value) and see the current spike that gets pulled when you switch on those relays...maybe there is much interwinding capacitance of the relay coil...............but it sounds like you don't have enough caps
 

Another useful proceeding could be insertion of a bead inductor at input of LM7812 on order to reduce spike on supply bus.
Even a small resistor could perform an little additional dumping on disturb effect caused by bobin of relay.



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You just used slight number of the rated power,so i don't think the problem is less output capacitor.But trying add cap is a good idea to ensure that.And try what andre_teprom said.If it doesn't ,make your SMPS on a condition of working well,check the output routing of the LDOs if it is too long or making a large current loop.Just my opinion,hope it can help.
 

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