Create a +5.00V source from a +5.15V power supply?

Hello.
I´ve a 5.15V power supply (in fact,is my laptop USB port V+) and I´d like to create a +5.00V supply from it.I can´t use a LM317 regulator because it needs some volts of input/output difference.I´d like the source to provide 100mA maximum (or so).
Is there an easy way to get this? (I have no 5V Zener diodes)

Is whatever you're supplying that fussy? Does it have to be precisely 5.00V?

Why?
5V Zener will not work!
The next USB computer you use will not be 5.00V. It might be 4.91 volts.
Use a longer cable and the voltage will drop with load.
If you really need 5.00 volts, (knowing the USB voltage could be 4.5 to 5.25 volts) then you are looking at a boost buck supply.
When you say 5.00V what do you really need? 5V+/- 1%, 2.5%, 0.1%????

Thanks for the answer.
In fact,I´m measuring temperature with a 12bit ADC,and I´m afraid my max sample value 4095,could depend on the V+ supply value.

What is the hardware device? I use devices like this one for ADC all the time running off USB on my laptop. Devices like this have internal references for the ADC. Just a matter of what the external hardware device is.

Ron

Is a uChamaleon.

http://www.starting-point-systems.com/index.html

I can´t be sure if it uses a reference voltage or not.I can set a LM385 2,5V reference,and use one of his 8 inputs to set the 2048 level.

You'd be better off with a 4.096V (or perhaps 2.048V) voltage reference, such as these from Microchip: https://www.electro-tech-online.com/custompdfs/2012/04/21653b.pdf

They are available in through-hole and SMD packages and require only one external component.

That linked unit is right along the lines of this type unit. It will work fine off any USB ports voltage of about 5 volts. Any USB or even a wall wart will power it. Measuring temperature is just a matter of signal conditioning your temperature sensor output and scaling it to 0 to 5 volts.

Flexible powering schemes

powers from USB
powers from external transformer (internaly regulated)
powers from external +5 Volts
automatically switches from USB to external supply (and back)
provides power to external circuits
overcurrent and overtemperature fault protection
external transfomer polarity protection

The µChameleon powers itself from USB by default. It is also possible to power it from an external wall-mount transformer, thanks to its internal linear regulator. In both cases, the supply voltage is brought out to a screw terminal, and lets you power your external circuits easily. If you already have a regulated voltage available, you can also use it to power the unit. And the power circuirty inside the µChameleon will automatically switch between its power sources without any mechanical intervention: if an external voltage is present, it is given priority, and when it is removed, the unit falls back to USB power, all this with over-current and over-temperature protection.

Click to expand...

It will work just fine.

Ron

Yes I know it works that way.But I´m not sure if 4096 level is exactly +5.00V or the voltage I measure after connecting to my laptop,which is 4.77V in regulated output and 5.13 or so in unregulated output.
So,I think i can make a 5.00V from those 5.13v at my USB and feed with that both the uChamaleon and my circuit.
But how can i do 5.00V from 5.13V (as easy as possible)?

If you have the uChamaleon sitting there try it. Feed a known voltage into one of the analog channel inputs. Install the driver disk.

From the manual:

1.7. Test application: μChameleon control
1.7.1. Overview
On the install CD you received with your μChameleon 2 (also available
as a download on our web site) you will find an utility called
“μChameleon Control”, that lets you perform simple tests with a
graphical user interface, and that allows you to exert all the features of the
μChameleon, as well as show you it is properly connected (if multiple
μChameleons are connected to your computer, you will be able to select
the one you want to talk to).
Another interesting feature is that for every action you perform in the user
interface, the software shows you the command string that is sent to the
μChameleon (along with the response if applicable) so you can learn
along the process, making for a very short learning curve. In a few
minutes, you’ll understand how to perform most of the tasks in your own
applications.
Additionally, an entry box will let you type random commands that will
be sent as is to the μChameleon, again showing the eventual answer.
To install this software, simply launch the “setup.exe” in the
“μChameleon Control” under “Utilities” directory of the install CD, or
grab it on our “downloads” page on our web site.
This software full source code is available on the install CD and web site.

Click to expand...

Use the test software. If you are really curious about the bit count apply a known 5 volts and write a script so you can read that channel and see what you get. The unit does not need to be powered by a precision 5.000 volts.

There is nothing (nothing simple) that will convert your 5.1 volt USB power to a perfect 5 volts. According to USB specifications Supplied voltage by a host or a powered hub ports is between 4.75 V and 5.25 V. Your device is designed to work from the source. If I supply the device with 4.8 volts or 5.2 volts and apply 5.00 volts to an analog input and read the bits for that channel I will see 4095. The USB input voltage has nothing to do with the Vref of the A/D as long as the input power is in normal USB specifications. Have you read the manual?

2.2. Power supply circuitry
2.2.1. Power circuitry overview
• Powers from USB
• Powers from wall-mount transformer
• Powers from local regulated +5 Volts
• Switches automatically between its power sources
• Provides power to your external circuitry
• Multiple protection schemes ensure high reliability
The power circuitry of the μChameleon is extremely flexible, and has
been designed to adapt to as many real-world situations as possible.
First, it can be powered directly by the USB port of your computer, which
is the default configuration most users are satisfied with.
In some instances however, for example when using an bus powered hub
that is not capable of providing enough current, or to conserve the battery
of a laptop, or to get an accurate 5 Volts level for some analog
applications, the μChameleon has an internal linear regulator.
2.2.2. Power circuitry protections
The power circuitry is protected against the following situations:
• Polarity inversion of wall-mount connection
• Board power short circuit
• Current consumption over 500mA (protects your computer)
• Over-temperature of linear 5Volts regulator

2.2.3. Using an external wall-mount transformer
Simply connect a standard wall-mount transformer, with a typical output
voltage between 9Volts and 12Volts to the black connector beside the usb
connector. The external voltage will be internally regulated to a clean
5Volts, and the μChameleon will switch from USB power automatically.

Click to expand...

The unit is designed to work from a USB port and as long as that port meets USB specifications it will work just fine. You do not need a precise 5 volts for the power source.

Ron

po210 said:

Hello.
I´ve a 5.15V power supply (in fact,is my laptop USB port V+) and I´d like to create a +5.00V supply from it.I can´t use a LM317 regulator because it needs some volts of input/output difference.I´d like the source to provide 100mA maximum (or so).
Is there an easy way to get this? (I have no 5V Zener diodes)

Click to expand...

the normal way of doing things would be to get a very good reference. measure the reference and calculate a supply gain error factor

5.15V will be fine for what ever your using it for. No need to trifle about .15v

Yes,i read the manual and that why i bought the thing.What it says is

<<or to get an accurate 5 Volts level for some analog applications, the μChameleon has an internal linear regulator>>

but if i measure 4,77 in the output marked as "regulated" i don't see a 5volt level.
If i connect an analog input to that line, I get values from 4085 to 4095.
So,I´m going to "sacrify" an input line to measure a 2.5V reference voltage derived from USB source,and then scale my measurement appropiately.

I admit I just don't get it. Devices like this are pretty common and I have never seen one behave that way. The attached is the open view of your unit and most units like this all look the same. It looks like USB power is directly fed to the main chip and the external DC is run through a common regulator chip. The main chip should take care of its ADC reference.

Seeing some bounce in the straight A/D count is not unusual. There will always be some noise so even if you apply a precise 5 volts you will not always see the exact expected count. Generally in devices like this I right justify the data a few bits to get reliable stable readings. Doing the math in my code.

Several years ago I did an experiment to measure several computer case temperatures using a real simple 4 channel A/D device. The results can be seen here. I actually calibrated the thing.

I even went to the forums for your device but did not see mention of anything of value. The USB voltage on the system I was messing with was about 4.8 volts and no problem with similar devices. Matter of fact, thinking about it the one in the link was real old as I used RS232 port for communication.

I just do not see, using that device, why it isn't working for you. Beats me.

Ron

po210 said:

<<or to get an accurate 5 Volts level for some analog applications, the μChameleon has an internal linear regulator>>

Click to expand...

The Chameleon does not regulate the USB power. It says in the manual:

Simply connect a standard wall-mount transformer, with a typical output
voltage between 9Volts and 12Volts to the black connector beside the usb
connector. The external voltage will be internally regulated to a clean
5Volts, and the μChameleon will switch from USB power automatically.

Hi Mr T

He wants to run off a laptop. If the analog inputs need a precision 5 volts for the thing to work off a USB port that sort of sucks.

Ron

Thank you very much for your opinion and experience.
So, 4095 should be exactly 5.00V and i must not worry.
What confused me is that if I measure 4.77V in an output marked as "regulated" it could be,i though,the one used to reference the samples. This phrase "The external voltage will be internally regulated to a clean 5Volts" is not true, if that regulated voltage is the one i measured,but maybe that voltage has nothing to do with the one used to reference the ADC samples.thas was my reasoning line,but surely,the real reference is inside the chip and i must not worry.(although then, what are those 4.77V for? )

This is a guess but when they say "The external voltage will be internally regulated to a clean 5Volts" I believe they refer to if you apply an external DC voltage to the DC PWR connector. I am guessing IC2 in the picture to be a 5 volt regulator. IC2 only comes into actual play when an external DC voltage is applied through that DC PWR connector. Diodes D1 and D2 look to be polarity protection diodes. Again, I am guessing but it looks that way.

Next, I seriously doubt the actual uC is using that voltage as a Vref. For example one similar little device I have been playing with is USB powered and has an analog input range of -10 to +10 volts. The lower end units have a fixed analog input range but the slightly better units allow the use of code to change the input ranges.

My guess looking at the image is those 2 output connectors are used to provide 5 volt power to any other small devices you may wish to power. That was based on what I did read. However, the actual manual really doesn't make mention of them. I would venture if you for example externally powered the device and as a test just used a 9 volt battery you would see about 5 volts on those terminals. Simply because the 9 volts would exceed the dropout voltage of IC2 which I guess to be the regulator. The reason for the 4.77 when you USB power with just over 5 volts would be loss. Again, a best guess case.

Overall, I would not worry about it. I would feed the thing a few known voltages and see what I read. Also, remember what I mentioned earlier as to noise so do not expect to see a dead stable count if you look at the bits.

Ron