7805 Voltage regulator????

[alexan_e]

I don't see the point of continuing this conversation about the value of the led resistor, the main subject of this thread is the 7805 regulator so please lets post messages relevant to that.

Alex

 

[tushki7]

@ cameo -- Actually if he is an engg. i Dont think he will go for mega ohms,(Tahmid also cleared the way how he can choose the value of rsistance) . . may be he joined this forum now, it doesnt means thats he's so new to electronics that he cant choose value of resistor after giving such reference!!! NOw did you got my Point ?
And i really think alexan is right!!!
And elimethew who needs help about 7805 Voltage regulator got my point!!!

 

[cameo_2007]

whether he is an engg or not, the same post and thread could be useful to anyother who visits later and probably new to electronics. Conditions and ratings should be specific.

In the interest of not taking this thread off the topic, I stop here.

 

[bagusdj]

hey guys, i have problem with my 7805. the first time create adapter using it, it works fine (i get 4.9x Vdc). i gave 9ac to rectifier, and it reduce to around 8V (entering 7805).
after i join the circuit with mcu, npn optocoupler (for zero crossing), adc 0809, tgs 2600 smoke sensor, an 16x2 lcd, my 7805 is heating. at first is not but today its heat sink is warm and going hotter.

i check with my Voltmeter and i got increasing voltage after diode bridge (before is reduced). i gave 9 to tranformator to diodes, and i got 10vac after the bridge (entering 7805). but the output of the 7805 is only 4.5vdc. way too far from it should be. my lcd is kinda dim, my sensor is not working well (it needs 5vdc/vac +-0.2 for heating element to clear smoke inside sensor).

i installed it currently in 3 breadboards. whats wrong with my device? is it 7805? is it the circuit? or is it the breadboard? circuit attached.

please help me

 

[rfredel]

Hello bagusdj,

first, calculate all current you need with 5V. Is it more the 1A you need a voltage regulator that gives more than 1A like 78S05.

That the 7805 is going hot is normal. Calculate the power dissipation.

V_dc = V_AC * sqrt 2 = 9V *1,414 = 12,73V

P_dis = (V_DCin - V_DCout) * I_out = (12,73V - 5V) * 1A = 7,73W

This is only heat, that must take away with a heatsink.

Look for the attached circuit diagram.



I hope that helps.

Regards

Rainer

 

[bagusdj]

Hello bagusdj,

first, calculate all current you need with 5V. Is it more the 1A you need a voltage regulator that gives more than 1A like 78S05.

That the 7805 is going hot is normal. Calculate the power dissipation.

V_dc = V_AC * sqrt 2 = 9V *1,414 = 12,73V

P_dis = (V_DCin - V_DCout) * I_out = (12,73V - 5V) * 1A = 7,73W

This is only heat, that must take away with a heatsink.

Look for the attached circuit diagram.

View attachment 85958

I hope that helps.

Regards

Rainer

but my transformator is 1 Amp max. is it still possible to get current flowing more than 1 amp to 7805?

what do you mean by "first, calculate all current you need with 5V"? how to do that?

and what about Vdc out that not reaching 5V? how to test that my 7805 is still works fine?

 

[rfredel]

Hello bagusdj,

the 7805 is limited to 1A output. If you take more current the voltage drop down, but the current is constant. So the power dissipation rise up and the heatsink is going more hot.

what do you mean by "first, calculate all current you need with 5V"? how to do that?

I mean, add all max. currents of the devices shown in datasheets. For example 3mA for ADC + 60mA for smoke sensor + 20mA for optocoupler + 100mA for mcu = 183mA

and what about Vdc out that not reaching 5V?

That's normal. The output voltage can be between 4,75V and 5,25V. Look for datasheet

how to test that my 7805 is still works fine?

You can test it with a resistor. Put a resistor of 5Ω / 5W resistor at the output and measure the output voltage. It must be 5V ± 0,25V.

In Moment I see

i gave 9ac to rectifier, and it reduce to around 8V (entering 7805).

It looks like you don't have the smoothing capacitor (C1 in my diagram) in your circuit. That makes possible, that your regulator is going very hot and your voltage dropped.

Regards

Rainer