12V output from board


#1

Hi everyone,

I need to power an external pressure sensor with 12V. Is there a pin/port on the board that can give me the battery voltage? I have a v.2.6 board.

Many thanks
Thomas


#2

Schematic which shows power circuitry:

https://github.com/OpenROV/openrov-electronics/blob/master/OpenROV%20Controller%202.6/OpenROV%20Controller%202.6%20Schematic.pdf


#3

There actually isn’t any regulated 12V on the controller, the closest is the V_BAT levels which will drop to 7-8V as your batteries discharge (depends a bit on the batteries you are using). If this isn’t an issue for your pressure sensor you could hardwire it to any of these points, alternatively if you want to be able to have switching control you could use one of the PWM outputs (you will need to solder in a jumper to select the V_BAT level and write some code for the control).

Otherwise I suspect you best option would be to buy a cheap 5V/12V boost converter and connect it up to one of the 5V lines on the header sockets (i.e. J3-1/J3-3 for +5V/GND)


#4

D’oh, yea my bad. I’m used to thinking about the battery voltage for the multirotors I worked on sitting at around 12.5V. Forgot that the OpenROV sits at around 8.5V usually. Post redacted (save for the schematic link, just in case it is still useful to him).


#5

I am using LiPo batteries at 12V. Are you suggesting that V_BAT will be lower than this?

My other option is to solder directly on the battery in the DB25 connector. Is this safe? Is there any difference if I choose P or SB battery?


#6

The LiFePO4 batteries that can be purchased through OpenROV have a nominal cell voltage of ~3.2V, with three of these in series, the nominal voltage from the battery packs is around 9.6V. This will drop to around 7.5V (or 2.5V per cell) when the batteries are discharged

If you are planning on attaching anything to the ROV batteries (whether directly to the DB25 or to V_BAT on the controller card) it will need to be able to work over at least this range (say 6-12V to add some safety margin). If your pressure sensor works with this voltage range, I would connect it to V_BAT on the controller board rather than the battery connections on the DB25 as doing this bypasses the tether power switching and would mean that you batteries will be slowly drained by the pressure sensor even when the ROV is not powered up.

Your pressure sensor probably wont work down to 6-7V, in which case my suggestion would be to buy a cheap 5 to 12 volt dc converter (often referred to as boost converter), this can then be powered from the 5V outputs on the control card and will provide the 12V for your pressure sensor.


#7

Why not just add a small voltage regulator like this: https://www.pololu.com/product/2117


#8

You have a good point there.

I don’t use the LiFePO4 any more I switched to LiPo batteries and they operate at 10.8 - 12.2V. The sensor operates at 10-30V so I will be ok with the battery output. My concern is if there is a voltage drop in V_BAT. I will check it with the output for the lights.

Probably I will try use the PWM_3 channel. I can see the jumpers in the board and the resistors are missing. Do you know what resistor is required there or just a solder connection would work??


#9

I am a bit cautious using voltage regulators. Do you think the board can handle the extra current?


#10

Check the specs for the pressure sensor, I wouldn’t expect it to draw much current. If that is the case, then a low wattage voltage regulator would be fine since it would not need to draw much current.


#11

Hi Thomas:

I think I’m with Stretch and Jim in recommending the boost converter. The 5V supply on the 2.6 board doesn’t have a huge amount of spare capacity (this is something that was fixed in 2.7), but it should be able to handle the power draw of a pressure sensor.
Alternatively, you might try running the converter off of battery voltage (just tap in someplace after the main power switch), and that way, when the motors are drawing lots of power, your sensor will still get 12V. With freshly-charged LiPo batteries you’ll have a bit more than 12V, but I think in that case the boost circuitry just shuts down and it passes the full voltage on to your sensor.

-W


#12

Thank you all for your advice. I see that a boost converter is quite recommended so I will give it a try.

I was thinking to use PWM_3 to get the V_BAT level, but I will try a converter as well.
The sensor can operate from 10 to 30 Vdc power, so it will not affect it to go slightly above or below 12.