Going from 13.8V to 9V safely

Hello,

I have an astron power supply laying around and I would like to power about 5 arduinos. From what I understand I need to get the voltage down as the rest is convereted to heat.

Can someone please tell me how I can go from 13.8V to 9V safely?

Thanks in advance

EverydayDiesel:
Hello,

I have an astron power supply laying around and I would like to power about 5 arduinos. From what I understand I need to get the voltage down as the rest is convereted to heat.

Can someone please tell me how I can go from 13.8V to 9V safely?

Thanks in advance

If you don’t plan to draw more than 1A total, (200mA per Arduino), a 7809 regulator with heatsink maybe? (Which will also convert it to heat, but not in the Arduino regulators.)

Or a switching DC-DC converter for higher current, if you can find one with a 9V output. You could get a 5V output DC-DC converter, then power the Arduinos via the 5V pin and bypass the Vin pin, too.

Pololu has lots of good switching regulators. Use one of them to go straight to 5V.

MorganS: Pololu has lots of good switching regulators. Use one of them to go straight to 5V.

MorganS: Pololu has lots of good switching regulators. Use one of them to go straight to 5V.

Yep, that's the best option if he doesn't want the 9V for anything. I only mentioned it last because he specifically asked about 13.8V to 9V, so covered that first.

MorganS: Pololu has lots of good switching regulators. Use one of them to go straight to 5V.

I was going to put one before every arduino. What are the advantages of pololu over a 7809 regulator. The price difference is quite a bit.

I do have 2 sensors that pull about 20ma each that need 9v. My input source will be 13.8

  1. Power. If you're on a battery then the power saving of the switching regulator is significant.

  2. Heat. The linear regulator will need a heatsink to keep its temperature within allowable limits. Yes, it will get hot enough to burn itself, or at least go into thermal shutdown.

  3. Size. The heatsink will be quite large.

  4. Weight. The heatsink is also heavy.

A 7809 would be appropriate for two 20mA 9V sensors as the power loss will be quite small.

I was going to put one before every arduino. What are the advantages of pololu over a 7809 regulator.

A switching regulator is more efficient. With linear regulators going from 13.8V down to 5V, you are dissipating more energy (and heat) in the regulators than is being consumed by the Arduino. (That's not always a terrible thing.... I normally take advantage of the Arduino's on-board linear regulator.)

A switching regulator converts the energy rather than wasting it as heat. For example, with a step-down switching regulator you get less voltage out than you feed-in, but you get more current-out than you feed-in.

Since it doesn't need to dissipate as much heat, typically you can get more current/power out of a switching regulator and you can probably use one for all of your Arduinos.

I actually built something where I used three 7805's to power three identical circuits, but that was a Mickey Mouse solution. The regulators still got hot and I had to use so additional Mickey Mouse tricks... I now have the switching regulator and I'm going to upgrade "real soon now..".

The price difference is quite a bit.

Switching regulators require an inductor, and usually a couple other components (besides the main regulator chip). Plus, I assume you are comparing the price of the 7805 chip to an assembled circuit board from Polou.

DVDdoug:
Plus, I assume you are comparing the price of the 7805 chip to an assembled circuit board from Polou.

So you have to be careful when you talk about cost.

When you include the heatsink, the mounting hardware and how you mount the capacitors as well as the outer casing as against the assembled switchmode regulator, the latter is likely to be a far better bet.

Linear regulators like the 7809 have one advantage over any switching regulator - they don't produce any switching noise, so are much preferred for audio or sensitive analog sensing.

Switching regulators can easily be generating 50mV or more noise, linear regulators noise output is in the microvolt range by comparison.

Thanks for all the replys. I am thinking that my best option would be to power the arduino at 5v with a pololu step down voltage regulator to take the voltage down from 13.8v down to 5v regulated.

Since my sensors are 9v analog sensors, I think my best option is to use the 7809 chip suggested with a heat sink. Each adruino will have 2 9V sensors, one that is 7mA and another that pulls 3mA. My guess is operating these analog sensors (vegatronix soil moisture and temperature sensors) at 9 volts is important (not sure they will work accurately at 5v).

EDIT: It seems that if I bypass the voltage regulator then I will lose the ability to give the software updates so I do not think that is a good option for me. Also arduino recommends 7v input minimum so maybe it is better to bring it from 13.8v down to 7.5v.

EverydayDiesel: It seems that if I bypass the voltage regulator then I will lose the ability to give the software updates

Why? You just disconnect the external 5V from the board and plug into USB as usual. I do it several times a day. [/quote]

EverydayDiesel:
It seems that if I bypass the voltage regulator then I will lose the ability to give the software updates so I do not think that is a good option for me.

The more important question here, is what makes you think that? Please explain your thought process - I suspect that along the way you will figure it out.

EverydayDiesel:
Also Arduino recommends 7v input minimum so maybe it is better to bring it from 13.8v down to 7.5v.

Again, what make you think that? You do not feed 5 V into the regulator, you feed it into VCC.

Hello again. It seems that I would just need to find a usbcable and cut the 5v from the cable.

I have the pololu 5v regulator as suggested and now I am looking to bypass the voltage regulator on the uno board.

Is this the way you do it (marked in red/black) |500x379

EverydayDiesel:
Hello again. It seems that I would just need to find a USB cable and cut the 5v from the cable.

No.

EverydayDiesel:
I have the pololu 5v regulator as suggested and now I am looking to bypass the voltage regulator on the UNO board.

No.

You just connect your 5V regulator to the VCC and Ground pins on the UNO. All you have to do is to ensure that you have the polarity correct and it is a reliable 5V regulator that your are using, and that it is connected securely before you apply power to the whole unit.

EverydayDiesel: Also arduino recommends 7v input minimum so maybe it is better to bring it from 13.8v down to 7.5v.

I think that's a far better idea than you realize. 8V into the barrel jack and "you're gold", kid.