Voltage fluctuation when transmitting over wifi on qbcan compact

[MrOwens]

Hi,

I am the teacher supervising the Falcan9 Cansat team in Craigmount High School, Edinburgh.
The team noticed that some of their readings from sensors were fluctuating when there should have been no change, on investigating they found that there is a fluctuation in the regulated voltage when transmitting over wifi, it drops initially then oscillates for a short time.
To over come this they have introduced a delay in the code after transmitting data over wifi, so it is not a show stopper.

Is this a known effect or is there a problem with the board we have?

Any ideas for testing or investigating would be appreciated.

Thanks.

[jordi]

Hi David,

It seems that your wifi module might be drawing more power than what the regulator can supply when starting, this would explain the fluctuation and stabilisation afterwards if the continuous power was to be smaller than the starting power.
The sensors would then see a fluctuation in the power provided to them and hence their measurements would fluctuate as well.

You can put a multimeter in series at the power line with your wifi module and measure the current that is going through, you will then see if when starting draws more current than when already started and in nominal mode.

The qbcan regulator can provide up to 500mA as shown in the wiki:
http://doc.open-cosmos.com/qbcan_modular#Power
http://doc.open-cosmos.com/qbcan_compact#Power_buses

If your payload components draw more current than 500mA, it is advised to take power directly from the battery.

Let us know how you are getting on!

[MrOwens]

Hi Jordi,

Thanks for the reply.

I do not believe it is power drain of the the external sensors as this happens when the code is only reading the on board BMP180 sensor and transmitting the values from that.
I may well be wrong but I had read that the wifi board had a current draw of about 140mA at max transmit power, the on board regulator should easily be able to provide that.

Perhaps I misunderstood your point but could you clarify how I would connect an ammeter in series with the RFM69HW wifi module on the qbcan compact board in order to measure the current being drawn.

Thanks,

[jordi]

HI David,

Ok, when you mentioned WiFi I thought that you were using an external WiFi module. The RFM69HW is a radio transceiver in the UHF amateur band and does not use WiFi frequencies which are much higher.

You won't be able to connect a multimeter to the RFM module without desoldering the modules from the board, so better not to.

When module starting to transmit could cause a surge in power that the converter takes some time to respond to. Introducing a delay to the transmit would indeed be a good solution.

Do you see the fluctuation of the sensors with the qbcan example code?
Also, could you explain a bit more the fluctuations? like we are talking 0.1ºC fluctuations every second or more like 5ºC?very small fluctuations (0.1-0.5ºC) are expected as the power supply is not perfectly stable and BMP180 sensor characteristics.
Bear in mind as well that the board does heat up a bit when being used just due to the components present not being 100% efficient.

Regards,