problem with LM317 & LM7805

[Mithun_K_Das]

I'm using LM317 to get 5V output from 56Vinput ( Serised with a 220R/5W resistor), R1=330R & R2 = 1K as calculation. But output voltage is always 20~25VDC. what is the problem?


I also used LM7805 for this purpose with a seriesed resistor of 470R/5W with input. But excessive heat created at only 70mA load current. Solution??

 

[mpathma]

Hi,

I need to double check the datasheet, but from memory, the LM317, maximum Vin is about 36VDC, you might be over-driving the chip. I think you are trying to use the 220R/5W to drop the Vin to save value, but this is not the right method. also the LM317 need to drop 36V to 5V, giving 31V Vdrop which is very high.

As for the 7805 case, assume you feed 56V via 470R and then to 7805, then take the 5V output, you are effectivly creating huge Vdrop, which will be coverted to Power Lost, that is the heat you are facing.

If you wanted to drop 56V to 5V, the only easy option is to use Switching Regulator, check out National Semiconductor (now part of Taxes) they have good online design tools.

M.Pathma

 

[Mithun_K_Das]

Can you give me any device name please?

 

[mpathma]

Hi,

Alot of choice, you need to navigate to National Semiconductor website, and use the online design tools, enter you working parameter for design (i.e component) suggestion, then look at sourcing and select suitable design.

M.Pathma

 

[Mithun_K_Das]

Can I use LM2576 with a input series resistor ?

 

[alexxx]

Can I use LM2576 with a input series resistor ?

As long as you won't violate the maximum current specifications, it seems (from maximum input voltage point of view) that you can, but only with the LM2576HV version with 60V maximum input voltage. I haven't used this part to help you more, just took a quick view at the datasheet to check maximum input voltage. This stands as long as your 56V input won't rise under some circumstances. If you are not sure that 60V will not be exceeded, then you should search for another part. Where does this 56V input comes from?

Question: What's this input series resistor all about?

---------- Post added at 11:59 ---------- Previous post was at 11:39 ----------

Alot of choice, you need to navigate to National Semiconductor website, and use the online design tools, enter you working parameter for design (i.e component) suggestion, then look at sourcing and select suitable design.

@Mithun_K_Das: You could also check TI's switcher pro application, it will help you. I have downloaded the desktop version. Unfortunately the online version does not support Google Chrome (!!!!) and personally I refuse to open it with IE. I think that our friends there in TI should take care of that...

SwitcherPro(TM) Switching Power Supply Design Tool - SWITCHERPRO - TI Software Folder

 

[Mithun_K_Das]

4*12V battery (<= 56V at fully charged) +/- 2V. A 47R/5W resistor is connected with input terminal to drop down some voltages.

 

[alexxx]

4*12V battery (<= 56V at fully charged) +/- 2V. A 47R/5W resistor is connected with input terminal to drop down some voltages.

Yes I think you are OK with your input voltage specification and without the need for a resistor.

As a last step of your research, I would tell you to search for more parts. As soon as you have gathered some that match your specifications, you could compare prices, check local availability and then make your final decision.

 

[FvM]

In my opinion, you should simply dispose all series resistor ideas. When you connect the voltage regulator to the battery, there will be initially no current and no resistor voltage drop. If the voltage exceeds the regulator's rating, it can be damaged. You need to use a voltage regulator respectively switched mode converter with sufficient voltage rating.

 

[Mithun_K_Das]

I'm really looking for that type of switching regulator but none of the items I found work over 40VDC. Can you help me in this case? LM7526HV/25HV is not available in my local market

 

[FvM]

LM7526HV/25HV is not available in my local market

Perphaps it would be easier if we know which parts are available in your local market.

But there will be always an option to design a circuit with discrete transistors of sufficient voltage rating. Of course, the discussed "burning voltage" concepts can work, too. E.g. connecting a power Z-diode in series with the input. Or a power resistor voltage divide instead of a simple series resistor. If you dodn't mind the power dissipation, you can go for a transistor+z-diode+resistor pre-regulator to e.g. 12 V with a succeding 7805. But the transistor will a heat sink accorcing to it's power dissipation of about 3 W.

A discrete transistor, or IC with voltage boosting transistors switch mode converters will be a more clever solution. Please consider, that a few 10 years ago, engineers don't had any HV ICs available and need to design respective discrete circuits. You'll still find them in application notes of basic switched-mode controllers like 3524 and others.

 

[Mithun_K_Das]

Can I use any buck converter in my case?

 

[FvM]

Yes. Buck converter is just the circuit topology for voltage down-conversion with a single inductor. It involves that the switch transistor and diode must be rated for the input voltage, and that you need to power the control circuit form the input voltage somehow.

 

[poorchava]

You may want to check regulators with bootstrap power system. They are powered from high voltage input for fractions of second and then the control circuitry begins to draw power from it's own output. This eliminates the need for linear regulation circuit for powering switch-mode regulator

 

[mpathma]

Hi,

From what I noted in your post, you wanted to power 5V circuit from 56VDC source, that mean you need to drop 51V, I did not see any output current rating, so I assume it's 1000mA, I will give three design I do not know if you can source this components locally for design (1) but design (2) is more Linear design, with bigger space, heat and power lost. (3) off the shelf design

Design(1) is using Switching Regulator from TI, using TI online tools, it suggest many option, I took the lower cost design, attached it the PDF file for the design, the TI tool is really good, and you should use it to explore other design option and see which component you can source locally, the design tools suggest component to you, you also can replace the component with your local component which you think spec. wise match and re-simulate it to verify the design. I can not be more easier.

Footnote: the design file which is attached with this posting is from TI, I just click and input-ed the parameter, I do not claim the design mine, have used this tool many time to design PSU for my company, this tools never failed me. I do not make any claim or responsibility.

Design (2),

You use some transistor Zener diode design to drop the 56VDC to lower level, i.e 24VDC, then use Lm317 to drop it to 5V, please bare in mine that at 1000mA, the stage-1 need to drop 32V, at 1A, that is 32watt, very-very hot, need large heat sink and cooling fan, the stage 2 need to drop 19V, at 1A output current that is 19W, also very very hot. Total system power is 32W+19W+system needs will be more then 51W, try holding a 60V bulb, you know how much is the heat. Please do not think that using more stage will reduce the power lost, Linear Regulator can not do that, total power lost will be equal or more. You indicate it's to be powered from battery, that mean this idea will waste 51W, which will be drained from the battery.



So, you see, the only option for you is Switching Regulator design.

Solution (3)
Get a ready made DC-DC regulator from 3rd party, check out **broken link removed** this is 72VDCin (telco rated), I think you can get MeanWell almost anywhere.

If you designing for volume, the invest to learn about Switching Regulator, it's not a friendly design, component are hard to get, PCB layout issue's but once you mastered it, you will love it for their low heat and space. (I know, because I did)

Hope this help.

M.Pathma

 

[Mithun_K_Das]

thank you for your advice. Its really helpful. I've left designing with linear regulators for excessive heat. load current is only 150mA. Now I want to check my local market then I'll select what way I need to do. But I was planning to design a small smps for my circuit. as the system have to run all the time(more than 7yrs contemptuously) so smps will be best i think.

 

[Mithun_K_Das]

Finally I got my solution. I used TIP122(for safety) to reduce voltage 60V to 44.3V using series regulator and then LM2576HVT-adj is used to take 12V (for some parts of the circuit) and then LM7805 is used to feed MCU. circuit is now fully ok and no heat is generated that can damage the devices.