We are using L7805CV , Input voltage = 33V output expected = 5V with 20mA of current requirement But we are getting 7.5V, the L7805 may know the reason? according to datasheet input voltage is 35V

it may be oscillating. check with scope or put voltmeter in AC mode. DVM in ac mode - if it meaures a voltage then it's oscillating.

Add 10uf to the input pin. you have a high input source impedance, and the 7805 may not "like it" :)

You can lower the value of the big output cap too. - with 20mA load current, the 220uf is overkill. IMHO.

Dear Mr.Bryan Speed,

Just now we have tested across the 7805 input pin, there is no AC voltage observed at DVM,

and changed the input capacitor value to 10uF and out put capacitor also added 10uF(220uF cap also no difference) and check the output of the 7805, the put is 8V was observed,

Please suggested be wave forward to solve this problem,

do make the AC voltage measurement on the output pin :)

Since it is working so close to max, maybe there was a input ac turn-on surge or a momentary high line condition... so try replacing it if you have not already.

FWIW, the part requires a minimum load of 5ma to regulate. So if you are measuring it with no load and only the 10K and led..... =)

 so try it with a load resistor for 5mA+

Hello,

Point1:

We have connected the CRO today, the ripple volt observed in 750mV the waveform attached,NO AC voltage observed at output pin

Point2:

We have tried with load resistance value 1Kohms at output pin of the 7805, the out voltage is 5V regulated we are getting it,

Point3:

According to datasheet the maximum suggested is 35V,

But datasheet

So 5mA to 1A, the recommended voltage is 7V to 18 Volts, please let us know the reason why cannot we give the 30V

and for our application requirment is 5mA to 20mA max,with rein put voltage of 30V to 7805, is there any risk to go head further after connecting 1K ohm to regulate the 5V output as mentioned in point2.

please let us know is there any risk to go further with 30V input?

I dont know why they used the 7 to 18v input range to spec the output voltage min/max.

Possibly it wont meet the output tolerance spec at higher input. if thats the case, keep in mind that it applies to the fact we millions of these parts.

You can use it with 30v input, for 5 to 18v out as mentioned in abs. max specs.

But here is my overall opinion - redesign.

Choose a different, more applicable part, and get rid of the power resistor voltage divider... wasted power, cost.

ST only has parts that go up to 40V input - though with short term transient to 60v : LM2931

But there are other parts out there with input specs of 65 to 100v and very cost and space effecient.

such as this 30ma output part, with 100v input. Lots of margin on input!

ZXTR2005Z-13

Hello Team,

Thanks for your suggestions..we will think of redesign.

We wanted to understand the what are the risk may foresee,

if input voltage is 30V ​for out put current 20mA to 100mA?

and at what condition we can give 30V input?

​

And what is Vin 7V to 18V for 10mA to 1A mentioned in datasheet..is this means we should not provide greater than 18V to 7805?​

Thanks