MBED external (Vin) power problems

16 Nov 2011

Hi, I'm having trouble powering the MBED using the Vin input. I use a 12 DC power source and a 7805 regulator to feed the Vin (7805 gives ~ 4.8V). The 7805 gets instantly extremely hot (MBED is pretty cool though). Using a 7808 the same happens only this time the MBED gets hot!!.

Using the USB to power the MBED (without external supply connected) everything is cool. I measured the resistance between Vin and GND on the MBED (all other MBED pins in the air) and I saw something like 1.3K? Is there some thing wrong with the MBED or have I managed to fry it up?

I've been searching the forum but haven't seen something similar (in any case if there is a similar topic I would appreciate it if someone could provide the link).


16 Nov 2011

This might not be a fried board. A normal mbed takes about 200mA, IIRC. So your 7805 is going to dissipate that current times the voltage drop (12v - 5v), or about 1.4 watts. If the 7805 has no heat sink, it will definitely get quite hot.

The 7808 will dissipate 600mW less (since it has 3 volts less voltage drop across it); but that 600mW gets shifted to the mbed's regulator so the mbed will run warmer.

So the questions are:

  1. Does your 7805 have a heat sink?
  2. When running the mbed from the 7805, is the temperature of the mbed itself the same as when you run it from the USB power?
  3. Do you have a way to measure the current flowing into Vin?

- hb

16 Nov 2011

Hi Hexley,

Thank you for your answer. In general the MBED is cool when run from USB or when Vin is 5V (you are right about the heat dissipation).

  1. No (I didn't think I would get into so much overheating problems (In fact after 2 minutes the 7805 goes into thermal shutdown which surprised me a lot considering the current flowing into the MBED is 150 mA)
  2. Yes no problems either when 5V @Vin or when powered from USB
  3. 150 mA.

I'm a bit curious if one uses a 9V DC power supply to power the MBED and the latter gets so hot, how long would someone expect it to last? Unless there is a way to attach a heatsink on the onboard voltage regulators (I doubt so).
I need to use a single 12 Volt power supply to drive both the MBED and LEDs and motors (consumption around 600mA) to keep things compact. Any ideas what I could use (I went for the 7805 because I wanted something fast and simple)?

Thanks again,

16 Nov 2011

Hi Tassos -

If you can find a source for power resistors, that might help.

You could skip the 7805 altogether, and feed Vin through a dropping resistor from +12. If the resistor dropped Vin to, say, 5 volts, you would need (7V/0.15A) = 47 ohms. This resistor should be rated for at least 2 watts; 5 watts would be better.

Sometimes small incandescent bulbs can be used in this role. Something like a #47 bulb might work . Or you might find an automotive interior bulb that has the right filament resistance - you'd have to experiment a bit since the bulb ratings are given for 12V service and you would be using it at about half that voltage.

Good luck,


16 Nov 2011

You could go for a switched mode powersupply rather than just dissipating the energy in a resistor or in the 780x or mbed 3V3 linear regulators.

You can either use a 'pincompatible' 7805 switched mode module to convert 12V into 5V or you might try to use a 12V PSU that also has a 5V output. Note that sometimes these PSUs with multiple outputs in fact internally use a 7805 to convert 12V down to 5V! Obviously, the 7805 SMPS modules are more expensive than a normal 7805. However they are easy to use (just 3 pins, no adjustments needed) and about twice as large as a 7805. Alternatively, you can roll your own. Just follow an application note by maxim, linear technology or one of the other vendors.

Regards, Wim

16 Nov 2011

RS have a Recom 7805 SMPSU brick, 8 to 24 Volt in if I remember correctly. And it runs cold, just need capacitors on in & out.

There are also a lot of other ones available, But I have used quite a few of this one.

I (we) are assuming you are using a 'tabbed' 5 Volt regulator. Not a to92 package !



17 Dec 2011

Hexley, Wim and Ceri thank you all for your support!!

I'm really sorry for the late response (it's been a hectic week).

I started checking the LM2575 but then on a preforated board I could stumble upon design consideration as indicated in the datasheet.
Then I realized I had a spare 12Volt cell phone adapter (it was giving an 8 Volt output). I cracked it open and I found a compact pcb having an MC (I'll post the exact part number tomorrow) coil, diode and capacitors (it was a switching power supply like the LM2575 only adjustable).
I tried it and voila!! No heating problems either on the supply PCB or on the MBEd itself (this one is quite a puzzle). Measured the voltage while the MBED is operating and noticed it stays @8.1 Volts but the MBED's on board regulators are not overheating (which happened when I used an 7808 as described in my first post).Update: Actually they got warmer when the Ethernet got into operation but not as much as when using th e7805.

Anyway it has been working for at least 9 hours/day without any heating problems so I guess that the cost of cheap cell phone adapter (5 Euros) is quite a nice solution. Building one will need also finding the correct uH coils, correct PCB etc. so I guess it is quite an efficient solution for an external power supply (having range of inputs higher than 12 volts) dedicated for the MBED.