Voltage regulator for LiPo

[rysium]

Hi,

I just read the post about using 7805 to get 5V of higher voltage pack:

The 7805 is a poor choice. Most will not operate correctly if the voltage drops below 7.0V (that is surely to happen in with your 6-cell packs).

What I need is a power for CH ignition module for gas engine. I'm using 2s1p Lipo 1050 but CH ignition needs 5V +/- 0.2V

I planned to use 7805 for that. I was thinking that 2s LiPo cant go below 7V as if they go as low a 7V they wil be empty anyway. CH Ignition module takes abaut 0.1A - 0.2A so it is not much load for 7805 and 1050 LiPo.
I have included piezo buzzer (from RS) as indicator that ignition is "hot". The buzzer is before 7805. I also added capacitor 16V 220uF on 5V site in case the Ignition module creates some voltage spikes back to the regulator.

Is anything wrong with that design, that might shut down my plane?

RysiuM

[Happy Hobit]

Hi rysium,

There is nothing wrong with your design, but here are a few observations.

The recommended ‘Low Voltage’ for a LiIon cell is 3 volts. Two cells = 6 volts.

The 7805 has a ‘Drop-Out’ voltage of 2 volts. This is the minimum voltage that it will drop at the rated current. A regulator rated for 1.5 Amps will have a lower dropout at 200 ma.

There are other regulators with lower Drop-Out voltages such as the LM1086-5.0 with a Drop-Out voltage of 1.3 volts. (MAX)

Jay

[rysium]

The recommended ‘Low Voltage’ for a LiIon cell is 3 volts. Two cells = 6 volts.

Thanks Jay, for the this. After this post I started to look around for possible flaws. And there are couple things I discovered. I think it's worth to share with this.

1. On CH-Ignition website:
The CH Ignitions unit draws between 350-600ma., depending on the
application. A 1/4X32 single is around 350ma. A 14mm single
is around 400ma. Some twins will draw 600ma. A 1200ma pack
should last at lease two hours. We recommend an 800ma pack
as a minimum.

I'm going to use single cylinder engine so 400mA will be the right number. I should be good for at least 2 hours of flying.

2. On BalsaProducts.com website there is a discharge characteristic for 1020 LiPo (see the picture below). For my setup it is 0.5C (red line). Frem there I see the battery gets 3.5V at about 950mAh. So for the voltage dropp-off I should be rather on the save side. If LiPo voltage goes below 7V at 400mA I shouldn't be flying anyway.

3. At 400mA I need to dispoze a little over 1W of heat that will radiate of the regulator (8V-5V= 3V at 400mA it is 1.2W max, 7V-5V = 2W at 400mA = 0.8W). Some heat sink should be in place.

RysiuM

[Happy Hobit]

Hi RysiuM,

Looks good, but you shouldn’t need much of a heat-sink.

The regular TO-220 style package (221A) can dissipate 2 Watts with no heat-sink in an Ambient Temperature of 25 degrees (C). Keep the tab in the air flow and it should be fine.


Jay

[rysium]

Looks good, but you shouldn’t need much of a heat-sink.

Good., I don't need any extra weight :D Thanks for the info.

RysiuM

[vintage1]

TO-220 package will do about 600mW in free air. Just a light clip on the package and a bit of forced air will see you right.