Onboard DC/DC Converter

Hello everybody

I have a question regarding the onboard DC/DC Converter on the Arduino.
I'm planning to use my Arduino for switching solenoid valves (as many did before). Because the valves get really hot I bought a small RC fan (max 7.4V, draws approx. 230 mA when connected to the 5V Pin on the Arduino). Due to the fact that I want to use one power supply for everything (including valves, fan and Arduino) I purchased a DC/DC Converter (24V to 12V, max 1A). This works pretty well but the point is that the onboard Converter (12V to 5V I guess) starts getting hot when I connect the fan to the 5V pin on the Arduino. Moreover the ON-Light as well as the L-Light starts to flicker after a while.
Now my question:
Should I stop powering the fan over the Arduino? I wont be connecting that much to my Arduino on the Breadboard (two Hallsensors, three Mosfets for switching the Valves and the LED Screen JHD659 M10 1.1 (the one that comes with the Starter Kit, which draws 0.6 mA?))
Is this still to much for the onboard Converter?

Kind regards and thanks for your answers

Benjamin

You realize that you posted into a German forum?

12V is very much for the little on board voltage regulator. Why don’t you use a 12V fan with a 12V power source and load?

Eventually you can connect the fan to 12V through a 22 Ohm 2W resistor. or between 12V and the Arduino.

Let's calculate how much power you can get from the Arduino UNO's +5V pin. The UNO itself uses about 30-40 mA. Let's call it 50 mA to be safe.

On USB power the limiting factor is likely the 500 mA Polyfuse. That means you can draw about 450 mA from the +5V pin before the fuse opens. (Disconnect power briefly to reset the fuse.)

On DC power (Vin or power jack) the limiting factor is the 5V regulator on the board. The maximum current the regulator can provide is 1000 mA, but in practice the limitation is how much power the regulator can dissipate before overheating.

Maximum power dissipation (PD) is maximum junction temperature (TJ(max) =150°C) minus the ambient temperature (TA, typically 25°C) over the thermal resistance, junction to ambient (RJA). The thermal resistance is 67°C/W for the DPAK package used on the Arduino. Fortunately the big copper pad on the PCB acts as a tiny heat sink and lowers the thermal resistance to 47°C/W. This makes the power limit 2.66 Watts (125°C / 47°C/Watt).

The current through the regulator is the same as the current drawn from the regulator. The voltage drop across the regulator is the source voltage minus the output voltage (5v). The power (Watts) dissipated in the regulator is the voltage drop times the current.

To draw the full 1000 mA current from the regulator you have to keep the voltage drop low enough that the power dissipation doesn't go above 2.66 Watts:

2.66W / 1000 mA = 2.66V dropped = 7.66V source

So if you want to draw the maximum (950 mA) from the +5V pin you should use an input voltage below 7.66V (and above the 6.2V (Vin) or 6.8V (power jack) minimum). That is why 7V is an excellent choice for source voltage.

Available power goes DOWN as you increase the source voltage above 7.66V. For example, if you feed 12V to the Vin pin the voltage drop across the regulator is 7V (12V-5V). To keep the power dissipation below 2.66 Watts you can draw at most 330 mA from the +5V pin.

I had the impression that the mounted regulator supports .95W only. But this may vary across boards.

Your solution for the original problem of hot solenoids should have been to add heat sinks to the solenoids, then perhaps a continuously running fan to keep air moving over the heat sinks.

In addition, are your solenoids getting hot because they are drawing too much current or because they are set on too often? Are they operating beyond the design specs?

Paul

johnwasser:
To draw the full 1000 mA current from the regulator you have to keep the voltage drop low enough that the power dissipation doesn't go above 2.66 Watts...

1watt for an Uno is wishful thinking.
Did you test that 2.66watt yourself?
Leo.

Did no one remember that each pin on the micro supplies 40mA MAX and really needs to run only 20mA for the sake of the micro.
Also recomended that the total power through the board beonly around 200mA.

Warnings

The Arduino Uno has a resettable polyfuse that protects your computer's USB ports from shorts and overcurrent. Although most computers provide their own internal protection, the fuse provides an extra layer of protection. If more than 500 mA is applied to the USB port, the fuse will automatically break the connection until the short or overload is removed.

Power

The Arduino Uno board can be powered via the USB connection or with an external power supply. The power source is selected automatically.

External (non-USB) power can come either from an AC-to-DC adapter (wall-wart) or battery. The adapter can be connected by plugging a 2.1mm center-positive plug into the board's power jack. Leads from a battery can be inserted in the GND and Vin pin headers of the POWER connector.

The board can operate on an external supply from 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may become unstable. If using more than 12V, the voltage regulator may overheat and damage the board. The recommended range is 7 to 12 volts.

The power pins are as follows:

Vin. The input voltage to the Arduino/Genuino board when it's using an external power source (as opposed to 5 volts from the USB connection or other regulated power source). You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this pin.5V.This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.3V3. A 3.3 volt supply generated by the on-board regulator. Maximum current draw is 50 mA.GND. Ground pins.IOREF. This pin on the Arduino/Genuino board provides the voltage reference with which the microcontroller operates. A properly configured shield can read the IOREF pin voltage and select the appropriate power source or enable voltage translators on the outputs to work with the 5V or 3.3V.

Hope this helps. Google Arduino Specifications for more.

Arduino pins should only be used to drive transistors, fets or other ICs.

Daz

Wawa:

1watt for an Uno is wishful thinking.
Did you test that 2.66watt yourself?

Are you saying that the TJ(max) and RJA in the datasheet are overly optimistic? What values have you measured?

Daz1712:
Did no one remember that each pin on the micro supplies 40mA MAX and really needs to run only 20mA for the sake of the micro.

Daz

That limit applies to digital output pins, not to the power supply.

Daz1712:
Also recomended that the total power through the board beonly around 200mA.

If supplied with less than 7V, however, the 5V pin may supply less than five volts

Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don’t advise it.

A 3.3 volt supply generated by the on-board regulator. Maximum current draw is 50 mA.GND.

Google Arduino Specifications for more.

Arduino pins should only be used to drive transistors, fets or other ICs.

You, or the article you got that from, mixed up ‘board’ and ‘MCU’.
e.g. max current from the DC socket to V-in can be <=1Amp.

May…, but not true for the V-in pin. 6volt on that pin will still produce a constant 5volt.
The ‘USB disconnect switch’ is set for 6.6volt though.
The DC socket needs 0.7volt more than the above, because of an inline polarity protection diode drop.

The 5volt pin is a rail, and can be input (if you know what you’re doing) or output.
The 3.3volt pin is output only, and has nothing to do with the rest of an Uno.

They took the 50mA from an older generation, where the 3.3volt came from the FTDI chip.
The current Uno R3 has a dedicated 150mA regulator.

Right… never mind the many errors.

Not important what a pin drives. As long as you keep pin/port/chip current/voltage below the limits.

johnwasser:
Are you saying that the TJ(max) and RJA in the datasheet are overly optimistic? What values have you measured?

Datasheet is probably correct, but board/heatsink is the problem.
I know for sure that a Mega, with a bigger regulator and board/heatsink, could shut down (depending on ambient temps) after a while with 12volt on the DC socket and a ~200mA ethernet shield.
That’s ((12-0.7) - 5) * 0.2 = 1.26watt in the regulator.
Leo…

benireloop:
I have a question regarding the onboard DC/DC Converter on the Arduino.

What Arduino has an onboard DC-DC converter? The ones I have and know of use a linear regulator, very different.

If I draw 100mA of 5V from a 7805 regulator supplied with 12V, it will draw 100mA of 12V and drop 7V at 100mA straight to ground, the heat generated requires current to make.

If I draw 100mA of 5V from a DC-DC buck converter supplied with 12V, it will draw (100 x 5 / 12 + appx 5%)mA of 12V and make much less heat than the regulator, that appx 5% inefficiency x 100mA drawn.

I'm a fan of DC-DC converters but IMO you should put a bypass capacitor across the output PWR and GND contacts to help level the output at least if you use much power. Look into boost converters as well.

GoForSmoke:
What Arduino has an onboard DC-DC converter?
The ones I have and know of use a linear regulator, very different.

A DC/DC converter converts one DC voltage into another DC voltage,
so a linear regulator is also a DC/DC converter.
A switching DC/DC converter just does it in a more efficient way.
Leo..

The difference matters when you go to buy one. Who sells linear regulators as DC-DC converters?

As far as I see, converter is more than just a word describing input to output, different sites compare converters to regulators.

Just because I swim doesn't make me a fish.

Linear regulators sold as DC/DC are not that hard to find on ebay.
But then again, most sellers there don't know what they sell.
And sometimes you just don't know what you're getting.
I like to add 'linear' or 'switching' to DC/DC, to make things clear.
Leo..

johnwasser:
Are you saying that the TJ(max) and RJA in the datasheet are overly optimistic? What values have you measured?

See Figure 22, the PD(max) is below 1.2W for the minimum pad size, less than 1.5W max.

Sorry, i missed the 5v pin and thought 'Pin' hence the 40mA

Info came from here,

If its out of date then someone should update it or delete it.

Went with UNO as could not see what type of Arduino the poster used.

Daz

Daz1712:
If its out of date then someone should update it or delete it.

It took 10 years to partially update a mistake on the ShiftOut page.
Not going to happen I think.
Leo..

Makes it hard to offer advice when you are still learning your self.

Biggest problem with the internet today is there is just to much darn info out there to sort through!

Thats why we have Boards!

Thank You.

Daz

DrDiettrich:
See Figure 22, the PD(max) is below 1.2W for the minimum pad size, less than 1.5W max.

Note that the chart specifies an ambient temperature of 50°C (122°F). I used 25°C (77°F) which seems more likely for hobby use.
There is a disconnect between that chart and the specifications. Let’s try a 17.5 mm square copper pad where the two curves cross the scales neatly at: RJA = 50°C/W and PD(max) = 1.4W. We calculate the junction temperature as 50*1.4 = 70°C above ambient. That would put the junction temperature at 70+50 = 120°C which is 30°C below the maximum junction temperature. Are they using the wrong maximum junction temperature? Are they using the wrong ambient temperature (should be 80°C=176°F?) Does the datasheet have the maximum junction temperature wrong?

All produced heat goes into the ambient, and must be moved out of it. Without a fan or other forced air circulation an ambient temperature of 50°C is very likely for a cooling plate, even more if covered by protective paint. And we all know how the junction temperature influences the life time of a chip, so that it's not a good idea to run it by design at its limits.

The figures in a data sheet can be interpreted in several ways, as optimistic or pessimistic, or as marketing vs. safe limits. Add production deviations, which become visible in min/max values. And there may exist hidden dependencies, like the threshold temperature of the temperature cut off.

With power semiconductors also the internals (mounting...) of a chip become very important. And who can be sure nowadays to really get the original brand, and not some cheap clone?