A constant 5v regulator?

[Op-Ivy]

Hi, I'm new here and I'm just starting to get into electronics, but I was wondering if someone would be able to provide me with a bit of info.

I'm currently building a solar powered charger and need a constant 5v output for the charge to be recognized by my device. However the part that I'm using, the LM7805 5v regulator, is not working exactly how I would like.

My problem is that because I'm using power from the sun, unless I have complete direct sunlight, I won't get enough of an input to create an output of 5v. The output will often drop to 4.90, even when the input is still 6v or so, which my device won't recognize.

It would seem that this type of regulator reduces output as a percentage of input once the input drops below a certain value.

My question: Is there a regulator that can give me a constant 5v output? As long as the voltage is above 5v of course. But what I am looking for here is a 5v output even if the input drops to 5.5v for example.

Thanks a lot!

Matt

 

[dknguyen]

It's called a low dropout regulator. They have a more limited maximum input voltage, but the overhead voltage required between input and output is much lower. The "dropout voltage" is lower.

 

[Op-Ivy]

Awesome dknguyen! Thanks fellow Canadian! Rep added!

I just did a quick search for them and a lower max input would be ok. My input would peak at 8v. (I have yet to see it reach 8)

Thanks again!

 

[Op-Ivy]

Hmm another question:

When looking at the data sheets it says output current max. Amperage will work the same as voltage, regulating what comes in, correct? So if I have 500ma coming in and it's output current is 200ma, the output will be reduced to 200ma?

I basically need 500ma at 5v. I'm not exactly sure where to look. I'm looking on mouser.com but I can't find anything that works.

 

[dknguyen]

I don't understand your question. The device being powered will only draw the current it needs. Just because the regulator can output 500mA doesn't mean it will output 500mA whether the circuit wants it or not (that would require increasing the voltage but the regulator is a voltage source so it's contradictory and impossible if it did both).

For a given load, you cannot regulate both current and voltage at the same time. THe combination of voltage and load will decide the current. In general: voltage, current, load- fixing the value of any two will automatically decide the third. And usually you can't control the load- it is what it is.

 

[Op-Ivy]

So what I need is a voltage regulator with a 5v output and 500ma output?

Also, when it says max dropout voltage, what would make it reach the max dropout amount? Input voltage decreasing to a level below desirable input? Is there a way of telling what voltage this is?

Thanks again for all of your help!

 

[dknguyen]

I'm not sure if you understand what dropout voltage is. It's the MINIMUM amount of voltage that the input voltage must exceed the output voltage by for the regulator to keep regulating. It does change however with output current, however. Higher currents tend increase the dropout voltage. So if you are drawing more current you need more voltage overhead. In other words, higher currents will raise the amount of input voltage overhead required over the output voltage and as a result, increase the minimum input voltage allowable.

You can see it from the dropout voltage listed for various currents. It's almost always in the datasheet for low dropout regulators (not always the case for standard linear regulators). It may be in the form of a few points on the graph listed in a table, or you might get a graph.

The regulator current rating is just the max current it can supply. You just need the regulator be able to supply more current than you need and the load will draw whatever it needs. However...that rating is for ideal conditions where the input voltage is *just* high enough for the regulator to keep regulating. THis is where it runs the coolest. Linear regulators work by burning off excess voltage as heat. THat's why current in equals current out. They don't work on power in = power out. That means they don't convert "excess input voltage" into "extra output current" which would result in "high voltage and low current in = low voltage, high current out". No, no. That means that for the same output current, the higher the difference is between input voltage and output voltage, the more heat will be dissipated. Obviously more current increases heat too. But this heat limit comes very quickly and is much more likely to limit the current output and the maximum input voltage of your linear regulator than what they have written on the datasheet.

 

[Op-Ivy]

Ahh I see. So a regulator with a 5v output and a .2v dropout would need 5.2v input in order to maintain 5v out.

I think I've got this now! Thanks again Dknguyen!!

 

[dknguyen]

Temperature Rise = (Vin-Vout)*I*Thermal Resistance

I is current in and out which are the same. THermal resistance is from the datasheet and if you use no heatsinks is junction-to-ambient heatsink.

 

[colin55]

How many mA will your panel produce?

 

[Speakerguy]

I was feeling helpful so I went and looked for a part for you.

Digi-Key - 576-1136-ND (Manufacturer - MIC2940A-5.0WT)

Fixed 5V LDO regulator in an easy to use TO-220 package. Two bucks or so. About .25V dropout at 500ma.

 

[Op-Ivy]

Each panel produces 166ma and they are wired in parallel. So just under 500ma.

 

[Op-Ivy]

Thanks Speakerguy! I was looking on mouser.com at something similar.

Hopefully I can find something locally.

 

[amando96]

You should really get a switching regulator, linear ones waste tonnes of energy, which matters a lot when powering from the sun, with very inefficient solar panels.

 

[colin55]

What is the voltage of the solar panels?

 

[Op-Ivy]

The panels create just over 7v when in direct sunlight.

 

[colin55]

What is the voltage they produce when delivering 100mA each.

 

[Op-Ivy]

I have no idea how to figure that out. Here is the panel I'm talking about. 750-00030 Parallax Power Management Modules & Development Tools

 

[colin55]

It tells you quite clearly that the output voltage is 6v @ 160mA.
You really don't have much headroom.
You need some form of buck/boost converter.
Something that will convert an input voltage of 5v to 7v to an output of 5v. You can try buying a $3.00 car converter for a mobile phone, from a $2.00 shop.

 

[ben7]

# of panels connected in paral

How many of these panels do you have connected in parallel?
NOTE: You need to have 4 or more solar panels connected in parallel to get 640mA (160 x 4) which should be fine to power a low drop-out voltage regulator with a load of 5 volts at 500mA.