## Vin range

02 Oct 2009 . Edited: 02 Oct 2009
Question :
> On your website "take the tour"
> you show a picture of mbed Microcontroller.
> The "VIN" range is 4.5 - 14.0 In
> On the quick reference for the NXP LPC1768
> the VIN range is 4.5 - 9.0v IN
The maximum recommended value of Vin was reduced to 9.0v to give a margin of safety for worst case load conditions. 14.0v will work in some cases - the design hasn't actually changed. However, sticking to new recommended Vin will ensure proper operation for all designs.
I'll make sure this gets changed to 9.0v in all other refernces.

Thanks,
Chris
02 Oct 2009

You say "in some cases"... can you give a bit more details? Could I burn the device by giving e.g. 12v?

02 Oct 2009

Hi,

Yes, it was a bit of a vague thing to say.. sorry about that.

The main power rail on the board is 3.3v, and so any voltage applied to Vin is regulated down to this using an LDO, which dissipates power as heat. For a given load current (mbed usually draws about 150mA) you can calculate how much power has to be dissipated as heat as Vin-3.3v * 150mA.

For Vin at 9.0v and nominal load current of 150mA, this equates to 855mW, which doesn't lead to significant increase in heat.

However, at 14.0v this power dissipation is 1.6w which is more significant in terms of temperature rise. When you then add in the fact that you can be drawing up to 400mA through the IO pins, the power dissipation of the LDO can lead to thermal shutdown - the LDO switches off so it can cool off.

While datasheets and a bit of maths can show you the power dissipation and heat rise for a given load current and a supply voltage, we are suggesting that unless you are willing to experiment, and work the numbers, you should keep Vin at 9v or below.

I have successfully run my mbed at 12v knowing that I will not be drawing any power through the IO pins, and understanding that the LDO will get hotter than usual.

If you are not sure, keep it below 9v.

Thanks,

Chris

02 Oct 2009

04 Oct 2009

Yesterday I was building a LCD shield for my mbed. I also used my xbee module to create a wireless connection to act as a serial monitor for my computer. It worked great, I only had one problem regarding powering the board. Everything was working when the usb cable was connected to the computer (for power), but as soon as I used a battery pack (first 6V then a 9V) and connected it in to VIN. The board powered on with no problems but the LCD screen would not turn on.

I had a MintyBoost from adafruit industries (http://www.adafruit.com/index.php?main_page=product_info&cPath=10&products_id=14) which had some chips but only ran with with two AA batteries. When i pluged it in to the mbed everything worked perfectly including the LCD screen.

I was just wondering how I could get the LCD screen to work with the battery pack.

Im assuming its a power/current issue so that is why I posted it here.

04 Oct 2009

I am assuming you are using the Vu output of your mbed to power your LCD? Vu is basically the power from the USB cord passed out onto a pin (with some protection first of course!), so you only get 5v on that line when it is plugged in to your PC. The minty boost thing sounds like it might have a switch mode supply on it that makes a 5v supply from two AA batteries... i'd be surprised if your mbed ran on 3v.

Vout (3v3) is dervied from whatever source is used to power the mbed (Vin or USB), so is always powered while mbed is powered. The best thing you could do is to power your LCD from Vout (3v3), is possible.

The 2x16 character display http://mbed.org/projects/cookbook/wiki/TextLCD works rather nicely from 3v3.. so you might be lucky :-)

Cheers,
Chris

04 Oct 2009

Chris Styles wrote:

Vout (3v3) is dervied from whatever source is used to power the mbed (Vin or USB), so is always powered while mbed is powered. The best thing you could do is to power your LCD from Vout (3v3), is possible.

The issue occurred when I tried powering the board from 3v3 (I have a 2X20 display), it powered the backlight and text area but nothing was showing. I removed the solder and moved it one down to VU and presto it worked. So as soon as you remove the USB plug everything goes down.

One Question: Can I leave the power wire going to VOUT and attach the battery pack there(assuming the USB is not plugged in) or should I create a switch to direct where the power is coming from (wether i use the usb cable or not)

28 Nov 2009

When using a 9V battery pack like this one, do you need a 9V tension regulation before linking to VIN or does the LS33 on the mbed does the job ?

28 Nov 2009

Hi Stephane,

Vin can be 4.5v - 9.0v with the onboard circuits taking care of the voltages for you.

If it helps, there is a schematic of the mbed NXP LPC1768 microcontroller module at the bottom of this page :

http://mbed.org/nxp/lpc1768/technical-reference

Best regards,
Chris

28 Nov 2009

Thanks,

Yet I may have missed something as it did not work.

28 Nov 2009

Hi Stephane,

I've read back through the rest of the post, and I have realised that I'd not asked you to describe what you are trying to do!

My assumption was that you were simply trying to power the mbed module, in which case a 9v battery connected between GND and Vin should work nicely. You should also get 3.3v on Vout, but you will *not* get 5v on Vu, and Vu is only derived from USB. If you are powering the rest of your circuit from Vu, it'll not work when powered from Vin. If this is the case, can you confirm that at least the status LED come on?

If it doesnt, my suspicion would be the 9v battery. They do not have a huge amount of capacity to start with, and a power hungry circuit could quickly drain them. I tend to use a 4xAA battery pack when powering my mbed from batteries.

Cheers,
Chris

29 Nov 2009

I just wanted to power the mbed without the USB just with an RJ45 plugged for testing the HTTPServer. The battery was fresh but I think I fried the LD1117S33 (IC5) even after double checking polarity... It explains why I can not have the mbed back with USB...

29 Nov 2009

HI Stephane,

So your mbed is completely dead now, even when you plug in to USB? I'd be surprised if you fried the LD1117S33 with a 9v battery. They are pretty sturdy and are rated to 16v.

One thing you could do is check the output of the LD1117s33's (IC4,5). You cna check IC4 by measurng Vout. IC5 you can check by measuring between the ground pin and the Tab (which is also Vout).

Have a look, and post back what you find, and we'll work to the cause.

Thanks,
Chris

29 Nov 2009

Yes, 16V, that's what I read from the PDF LD1117S33 documentation.

I measured Vout for both (IC4 and IC5) and got something around 0.10V I am not very precise for Vout value because I was not very lucky yesterday : my voltmeter is a very old and imprecise analogic one. I bougth a fancy digital one yesterday that never worked when I turned it on (batteries are good as is the fuse...). I have to change it tomorrow.

IC3 seems to be good because Vout is +5V

If I am refering to the mbed ref manual, I should have IC5 Vin at 5V and I get something like 0.15V

29 Nov 2009 . Edited: 29 Nov 2009

Correction from previous post : IC3 Vout is 0,15V, not 5V

29 Nov 2009

That's a good point. Is there any reverse-polarity protection implemented? I see that there is reverse-polarity protection on the Vin (the diode) and Vbatt is also has protection, but Vext doesn't look like it has anything. How succeptable are the voltage regulations to damage? The datasheet does not say anything regarding this.

If you burned out your regulators, it doesn't look too hard to change them..

29 Nov 2009 . Edited: 29 Nov 2009

Igor Martinovski wrote:

That's a good point. Is there any reverse-polarity protection implemented? I see that there is reverse-polarity protection on the Vin (the diode) and Vbatt is also has protection.

If you burned out your regulators, it doesn't look too hard to change them..

That's what I tought so I do not understand how I burned it. I do not want to do this experiment again with my other mbed....

Igor Martinovski wrote:

If you burned out your regulators, it doesn't look too hard to change them..

I am not equiped to solder such tiny pieces, unfortunately

30 Nov 2009

Hello,

I changed the dead digital multimeter with a working one so here are more precise values :

When USB link is on :

IC3 (FPF2123) => Vin = +5.0V and Vout = between 120mV and 200mV (and not +5.0V)

That's why IC4 and IC5 have a bad Vout value. So it seems IC3 is the problem.

Am I right ? Is it possible I did something wrong with my battery or there was a initialy a problem with IC3 ?

30 Nov 2009

Hi Stephane,

Even if the FPF2123 has been fried, it should still be possible to power the mbed from the Vin pin.

Does the FPF2123 get hot when you plug the mbed in via USB? I am wondering if one of the LD1117s33 has been fried and is a short circuit. If it is, then i'd expect the excesive current to make the FPF2123 get hot. When powering by Vin, it might just empty the battery real quick.

Can you describe what you were doing immeditely before it all stopped working? What other circuits do you have attached?

Cheers,
Chris

30 Nov 2009

mbed is on USB for testing since the beginning of this evening. Nothing is hot on the board.

My test saturday was simple. I compiled a program playing with the 4 LEDs. It worked on USB. I wanted to see it working in stand alone mode. There was just the mbed and the 9V battery connected between GND and VIN. Nothing else. I wanted to test that before attaching the RJ45 for a stand alone HTTPServer. After connecting the battery IC4 was very hot so I removed the battery swiftly. I put back the mbed on USB and saw it was damaged.

Using the ohm meter I comparerd the two LD1117s33 and get differents values :

If USB is OFF

IC5: Gnd to Vout is 290 ohm / Gnd to Vin is 1160 ohm / Vin to Vout is 1300 ohm

IC4: Gnd to Vout is 640 ohm / Gnd to Vin is 1825 ohm / Vin to Vout is 2240 ohm

I have not connected another source to Vin, been scared to do more damage to the mbed.

04 Dec 2009

Hello,

The fried mbed is working again !

Just to let you know, I plugged in Vin 4xAAA (4.8V) and the mbed started (It did not with 9V) and it worked but the LPC1768 was heating a lot so I removed the batteries.

Then plugged in USB. It did not work again.

So after letting the LPC cool off a bit, I did it again : plugged in the AAA in Vin (mbed started) and plugged the USB together. The mbed was still working with both inputs. Then I removed the AAA but kept the USB and the mbed kept runnig the LED program !

Then I removed the USB and put it back. The mbed is now back to business as if nothing happened !

It ssems it was stuck in some weird status in one of the IC for some reason.

So case closed.

Cheers

18 Dec 2009

I just had a similar experience!

I stupidly connected the 3.3V out to 12V of my supply and the mbed died. I remembered this thread and did what Stephane did and got it working :) Needless to say, always be careful on those breadboards.

09 May 2012

I had a similar issue a couple of days ago.. I disconnected the USB cable from the mbed and then went back to it after some minutes. The mbed was not recognised and the blue led would not turn on. I tried a different USB port but no luck. I tried a different PC and the led flashed on but was very weak. I only had two potentiometers on p16 and p17 when the problem occurred so I can't see how I could have damaged the mbed. Then I supplied 9V from the Vin and the led flashed on normally. I then tried a different USB cable and everything was working again but since then the Vout is lower than normal (3.1V) when powering from USB. Also all analog input pins give me a different reading than before, as if the internal reference voltage (3.3V?) had changed, and the noise on the reading is much higher. When I connect both a 9V battery on Vin and the USB to the PC I get 3.3V on the Vout and the analog pins are working like before the problem occurred. In order not to waste money on batteries I thought of using a 9V voltage regulator (connected to my 20V supply) and then feed that to the Vin but the reg got hot very quickly and the potentiometer reading started drifting so I disconnected that straight away. I'm not too sure what to try next since I'm afraid of damaging the mbed even more.