7805 high input voltage problem.

Dear Tpetar,
I already mentioned the reason for LM257x failure..here in our part of Asia market is flooded with inferior Chinese parts... so in this situation you never know when you have the right original part or the low grade one... infact an original LM257x should work well and up to specs... the part itself is not to be blamed but its source/origin..hope you now got my point...

BR
Saifullah Khalid

Do you have option to order from other shops over Internet, from Europe or US or Australia ?

Dear Tpetar,
No no now they are not needed... what i told you was a past story...

BR
Saifullah Khalid

aashitech said:

Dear Keith ,
I think you missed something..i already pointed out in above post to replace R1/D1 with 300++ ohms Resistor(2w/5w+) and a 12-14 V Zener with at least 1 watt rating... if the input is in range of 30 Vdc i think this raw solution will work... original poster can experiment with resistor values as per his requirement...

Click to expand...

The input voltage is 40V. If it was 30V then this thread wouldn't exist.

You suggested 100/220/330 ohms with 12-14V zener. With 40V input, 100 ohm resistor and 12V zener the power dissipation in the resistor will be 7.8W and 3.4W in the zener. The actual values really need some proper calculation, but only when the requirements are know. As yet, I don't think we have heard the current requirement from the 5V and 12V supplies.

Keith.

@ aashitech , Tpetar, keith :

Acctually i am fresh graduate and don't have great practical experiance and want to know and experience maximum techniques and ideas using electronics components; i see all electronic components as my tools and want to know work with them practically. so i will work with all ideas given by you all and others in the forum. and i am very thankful for every one who is giving me any idea.

I am also feeling the use of glasses while working with Power Electronics Projects as i have blown out a 2200uf 50V capacitor with a soundly explosion.. some one may have inverted it with bridge that have 30V when i walked out of office for Friday prayers so now i am catious about that.

About zener is that i've only quater watt diodes because higher wattage diodes are not available in our market.. :-( so i will try and see what happend with that.

Please do tell me about any other solution and idea if you have, i must perform experiment to increase my practical knowledge. please also tell me the formulas to calculate values.

and please guide me if inductors are availabe commonly to use with LM2576-adj or i have to make inductors by my own?? if i have to make inductors by my own then how can i make an inductor with specific ratings?? and Capacitors connected to it are Ceramic or Electrolytic.

actually such a large transformer is the requirement of my project related to power electronics i am acctually using 30V, 20A transformer and these volts boosted up to nearly 40V after passing through 3300uF 50v capacitor.. all i want to do is to use the that output for small and low power part of that high power project. i don't want to use separate transformers and bridge and capacitor for that purpose as it will increase the cost of the circuit that will be very redundant.... i want low cost and effective solution ...

Can i use LM741 to reduce voltage i.e; to divide input voltage by 3 ??? as i have given the immage of that circuit in my last posts?


Best and warmest regards to all
M Qaisar Azeemi

---------- Post added at 10:20 ---------- Previous post was at 09:51 ----------

i tried your suggested circuit Mr Keith but i noticed that it is highly unregulated. i used 5.6k and 2.7k as voltage divider and used BD135 npn transistor for this purpose but each time i increased the load the output of emitter decreased. so i think the only solution is using LM2576 buck convertor. if you have any other idea besides this i will be thankful to you.

Qaisar Azeemi said:

i tried your suggested circuit Mr Keith but i noticed that it is highly unregulated. i used 5.6k and 2.7k as voltage divider and used BD135 npn transistor for this purpose but each time i increased the load the output of emitter decreased. so i think the only solution is using LM2576 buck convertor. if you have any other idea besides this i will be thankful to you.

Click to expand...

The circuit I posted is not intended to be a "regulator" - it is intended to drop the voltage down to a safe voltage for the 7805. You do need to choose your resistors to suit the transistor current gain and the load current as well.

I agree a buck converter would work but it is far from "the only solution". There are linear regulators around that can take 100V or more.

Keith.

Another alternative that hasn't been mentioned is to use an LM317 to get the 12v and a 7805 to reduce the 12v to 5v.
LM317 has a max input-output difference limit of 40v (as opposed to an absolute input voltage) so it can be used but be prepared to get much lower efficiency that the switching reg solution and more heat on the regulator.

Alex