I am building several battery operated circuits for a solar powered system.

One circuit requires 5v and another 10v

I am regulating lets say 14v supply down to 10v with a LM2940 due to its low dropout.
I am regulating the 14v supply down to 5v with a 7805. These are supplied seperatley but from the same battery.

I was thinking that the LM2940 has to disapate 4v in this case, and the 7805 must disapate 9v - total 12v.

Would it be more efficient to regulate to 10v and feed the 7805 that 10v as well as the circuit requireing 10v meaning 4v is disapated and a further 5v - total 9v, or do I have this all wrong and it won't make any difference?

 

You are correct - the more excess voltage you place across the linear regulator, the more power it will have to dissipate, the hotter it will get and the less efficient it will be.

I think you should reconsider your choice of design entirely. Linear regulators have their uses but battery powered applications - especially applications such as solar power - are not well suited to such wasteful devices. You'd be much better off with a switch-mode type design, perhaps you should do some reading up on these instead?

Brian

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Electronics Test Development Engineer
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ah, dont say that! how complecated would that be?

 

Ok, I have been doing a little reading and looking at efficiency I can see LDO's can be as low as 30% efficient whereas switchmode is plus 90% efficient.

The whole reason I am regulating the voltage is because the input supply could be in excess of 15v, amongst other applications I want to run some LED lighting at close to full power, so I dropped to 10v for supply headroom as I can not have the LEDS supplied at say 15v or they will pop.

Am I over complecating this? I have 3 circuits so far, one a thermostat, 2 is a charge regulator 3 is a lighting circuit with LDR.

Would I be better regulating 12v for all circuits, possibly switch mode regulation

 

You 10V consumption will have a 10/14 efficiency and the 5V will have a 5/14 efficiency. The low dropout part is useless here.

You could build 2 SMPS, one for each voltage. The likelihood of getting to 90% is low unless your current hits the sweet spot of the particular chip you choose. The datasheet will show an efficiency curve for input voltage vs current draw.

However, the most efficient regulator is none at all. It might take a little thinking but I'd try to design a circuit that is tolerant of a wide range of input voltage.

 

You have a very high supply voltage for LEDs. Their current-limiting circuit will waste a lot of power unless the LEDs are on series.

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IF max. 20mA
VF typ. 3.3V

9 LEDs: 3 Parellel rows of 3 series leds each row with a 5.6ohm resistor. all resistors dissipate 6.72 mW

 

Agreed, but I have no idea how I can overcome the voltage difference, I either design based on highest voltage and accept a lower brightness (or none at all?) when the voltage drops, or maintain a voltage with regulation and have the same brightnes all the time.

I am not sure what is the best solution, I thought the low dropout was needed so the regulation will take place at 10.5v and above.

The LEDs are just one aspect of the whole circuit, there are other circuits that have a 12v supply. Should I regulate to 12v efficiently and design the lighting based on that voltage then all supplys are stable (ish)

 

I probably wouldn't take that particular approach. I'd build one SMPS with multiple outputs, that way the efficiency would be much better.

Brian

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Electronics Test Development Engineer
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Any pointers on what IC I should be looking at? I need 5v and 12v the 12v rail will need to be at least 2A is this possible?

 

Heres a site you might like helped me out.

http://www.rason.org/Projects/swregdes/swregdes.htm

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Ah, all makes sense now, I understand how it works, but I am not sure where to start - all these harmonics etc add complexity which I simply don't understand. I see the benifit of SMPS and would very much like to include one in my implementation.

It is just where to start and with what IC?

Thanks for your help guys, its most apreciated - working with a noob is not ideal I know

 

This is going off topic now, so I am going to start a new topic for the SMPS as I have found a schematic that might do the job.

 

You could use a crude buck converter to supply the linear regulator with a low loss adjustable voltage just about it's drop out voltage. Wouldn't be as efficient as a full blown SMPS but it'd be easiser to build.

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Use a Black regulator with the current limiter.
http://www.romanblack.com/smps/a04.htm


Each LED requires its own resistor doesn't it?

Use one of the resistors as the current sense resistor, calulate it for 20mA and connect the other LEDs with their own resistors in parrallel.

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