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strange problem with 7805

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Hi
I want to simply drive a relay(5V) via a tiny45 (and of course a 2N3904 to provide relay current). the point id that the power supply is 12V and I used a 7805 to convert 12V to 5V. the problem is that the circuit works properly when I use a linear power supply5V(and omit thr regulator) and the circuit consumes 100mA,but when I use the regulator and drive it with 12V,the circuit does not work properly,although the 5V output is present at the regulator output!!!it seems that the regulator can not provide the circuit with this current but why???
 

I don't think so. Could you post the schematic ?
 

7805s come in different verities - 7805C can supply 1 Amp, 78L05 --100mA, 78M05 --500mA. Put 0.1mfd capacitor on the output side and 10uf capacitor in input side of 7805.
 

Three terminal regulators are as foolproof as it can be, but do require proper decoupling capacitors. The problem is that a transient voltage could cause an instantaneous voltage drop, and your microcontroller will reset.

In addition to the 0.1uf at the output of the regulator itself, I like to add a 1uf electrolytic and a 0.1uf ceramic in parallel, right at the microcontroller's Vdd and Gnd pins.
 

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