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[SOLVED] Dropout voltage concerns.


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I'm trying to build a basic power supply for a few of my electronics, which include my desktop speakers. My concern at the moment is the voltage dropout rating. I've never built a 12V regulated power supply that runs off of a 12V battery.

I was looking at a LT1085CT-12 from Digi-Key. It has an input range of less than 25V (I guess that means I don't even need a battery) and a dropout voltage of 1.5.

This is to run off of a battery that will have a voltage range of about 10.5V to about 14.5V.

The maintenance charge is about 13.5V. So, assuming a voltage dropout of 1.5, if I hooked up this regulator, would I get 12V?
What happens when I remove the battery maintainer and the voltage drops to about 13V? Do I all of the sudden get no voltage?
Is the actual input range 13.5V - 25V and less than 10.5V?

Thanks!

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You are correct, they are running at 15.5V. If I where to hook them up to the battery directly though, wouldn't they get quieter and quieter as the battery drained?

I was curious, so I checked the power supply's of my modem, router, and answering machine as well.

The modem and router are both rated at 12VDC.
The modem is 17.2V.
The router is 13.7V.

The answering machine is rated at 7.5V, but the power supply is supplying 13.2V.

What I find most interesting about this is the voltage of my incoming power, which is 112V AC. I know a lot of houses, in America anyways, are running closer to 120V AC.
I guess that means most electronics aren't as picky as I thought they where.

Still though, my modem is always really hot, maybe I should run it on a lower voltage?

Should I just get a 15V regulator and call it a day?

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You are correct, they are running at 15.5V. If I where to hook them up to the battery directly though, wouldn't they get quieter and quieter as the battery drained?

No, it's not that simple.

A good audio amplifier is designed so its volume is independent of the power supply voltage - a characteristic known as power supply rejection ratio.

If the volume is set high, it will stay exactly the same until the battery voltage is too low to give the required peak voltage to the speaker, causing the output signal to become clipped and distorted. In other words, it will sound the same until the voltage drops past a certain point at which the sound quality will deteriorate rapidly. Turning the volume down should improve the sound quality but it will oviously be quieter.

If the volume is set very low, the amplifier will become unstable when the battery voltage drops below a certain point. It will either stop working completely, the sound will become distorted or it will oscillate and make a funny noise. Turning the volume down further won't do any good.

I was curious, so I checked the power supply's of my modem, router, and answering machine as well.

The modem and router are both rated at 12VDC.
The modem is 17.2V.
The router is 13.7V.

The answering machine is rated at 7.5V, but the power supply is supplying 13.2V.

What I find most interesting about this is the voltage of my incoming power, which is 112V AC. I know a lot of houses, in America anyways, are running closer to 120V AC.
I guess that means most electronics aren't as picky as I thought they where.

Still though, my modem is always really hot, maybe I should run it on a lower voltage?

Did you measure the output voltages of the power supply with the load connected or disconnected? Some poor quality unregulated power supplies give a higher voltage unloaded than when loaded.

Should I just get a 15V regulator and call it a day?

No, an audio amplidier doesn't require a regulated power so adding one is a waste of time, just connect it to the battery via a fuse with roughly the same rating as the power supply.


I'd still like to know what happens when you run a 12v regulator on a 12v source.

I guess I'll just have to buy one an find out the old fashion way...

Or you could read the datasheet before you buy it. If it's a linear regulator, the output will be as specified until the input voltage drops below a certain level equal to the rated output voltage plus the drop out voltage.
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No, an audio amplidier doesn't require a regulated power so adding one is a waste of time, just connect it to the battery via a fuse with roughly the same rating as the power supply.

I was actually referring to a 15V regulator for everything.

Did you measure the output voltages of the power supply with the load connected or disconnected? Some poor quality unregulated power supplies give a higher voltage unloaded than when loaded.

I don't have an effective means of measuring the voltage under load. I doubt any of them are regulated, but the router, because of its 13.7v, might be.
I'll just get a 12v regulator and run the modem, router, and answering machine off of that, and the speakers off of the battery.

Or you could read the datasheet before you buy it. If it's a linear regulator, the output will be as specified until the input voltage drops below a certain level equal to the rated output voltage plus the drop out voltage.

I did read the datasheet; that is where I learned of the dropout voltage. What I don't under stand is if it is a 12v fixed regulator with a 1.5v dropout, how can it have an input range of 5v to 25v? Does the voltage dropout when you give it a 12v input? I'm guessing the current drops, not the voltage, right? There are two graphs that mention the dropout voltage in the datasheet, which can be found in the link above or here: LT1085CT-12, but the graphs don't mean anything to me. They are comparing the input output differential to the output current, but I'm not really sure how that helps. That is where I got the idea of the max current dropping and not the voltage.
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I did read the datasheet; that is where I learned of the dropout voltage. What I don't under stand is if it is a 12v fixed regulator with a 1.5v dropout, how can it have an input range of 5v to 25v?  

The datasheet covers a whole familly of regulators with the same architecture but differing output voltages and current limits.
http://cds.linear.com/docs/Datasheet/1083ffd.pdf

Does the voltage dropout when you give it a 12v input?

Yes, if the regulator has a dropout voltage of 1.5V and an output voltage of 12V, the output voltage will drop to 10.5V. If the output voltage is 5V then, not it'll remain at 5V until the input voltqage drops below 6.5V.


I'm guessing the current drops, not the voltage, right?

The voltage drops and so will the current, assuming the load is resistive. If the load current is constant then the voltage will drop and the current will stay constant. If the load is constant power then the current will increase and voltage will decrease.

There are two graphs that mention the dropout voltage in the datasheet, which can be found in the link above or here: LT1085CT-12, but the graphs don't mean anything to me. They are comparing the input output differential to the output current, but I'm not really sure how that helps. That is where I got the idea of the max current dropping and not the voltage.

The dropout voltage is dependant on other factors: mainly the current and the temperature of the regulator chip. The graphs show how the dropout varies depending on the current and temperature.


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Thanks, that answers my question.

Now I'm wondering how to get around this problem of 12v in to 12v out.
I know they make low dropout regulators, but even if the dropout voltage is .01v, I will still run into the same issue.
It doesn't seem wise to use a 20v regulator to power a 12v regulator.

Are there any other options?

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The solution is just to power everything differently off the battery.

I don't see what all the fuss is about, you've got a range of appliances which either have built-in voltage regulators (this applies to the answer machine, router and modem) or are designed to run off a wide range of voltages (the audio amplifier inside the speakers). If you open them up, you'll probably find regulator ICs such as the LM7805 or LM7808 which are fine for up to 35V, although they'll probably overheat and shut down if you throw that at them as the heat sinking will be inadequate but note I said shut down, not smoke and blow up as they're thermally protected.  The audio amplifier inside the speakers will probably be slightly more sensitive, although I'd be very surprised if it's rated for anything under 25V  and it'll certainly take 15V, minimum.

Going from what I've read about the USA electrical system, the mains voltage can be as high as 126V in which case even the voltage to the answering machine will be 14.85V which is much higher than the battery when it's hot off charge and the modem will be running off over 19V.

In short all of the appliances you've listed should be able to run from 10V to 15V without any problems and will probably run much cooler and efficiently off the battery than the mains adaptors. The audio amplfier will certainly be better as the battery voltage will certainly be cleaner than the mains.

The only problem you may have is noise created by a ground loop which will form if a device connected to the speakers' audio signal input is also powered from the same battery as the speakers. The solution to this problem would be either an audio isolation transformer or powering the device off a a supply isolated from the speakers e.g. a DC-DC converter.

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I don't see what all the fuss is about, you've got a range of appliances which either have built-in voltage regulators[...]

Well, the fuss is, I don't want to break my electronics!! However, it never occurred to me that they would have a built in voltage regulator. Of course, now that you pointed that out, it seems stupid that they wouldn't. That's why it was so strange to me that there was such a large difference between what the devices input was rated for, and what the wall adapter was pumping out. Now I just feel stupid...

Thanks!

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