MAWD on 11-12V?

Hello All,

I'm in the process of putting together the components of my video-DD e-bay. I'd like to power them with the same battery. The video system runs off of 10-12VDC @ 150ma and the MAWD specs are 6-10VDC @ 8ma. I'd like to run them both from a Lipo 3-cell 11.1VDC (nominal) which can produce 12VDC when fully charged.

So, here's the question. Will the MAWD handle 12VDC without suffering damage or a shortened life span? I've e-mailed Perfectflite's support, but haven't heard anything. I did a search here and found a few people hinting at using a 12V battery, but couldn't find anything more definitive.

Ashley

How small is the rocket going to be? It shouldn't take much space, weight, or effort to add a voltage regulator to provide the 6-10V recommended.

Hi Sylvie,

Thanks for the reply. A voltage regulator was one of my thoughts, but I don't want to add the extra complexity (and thus failure path) if I don't have to - especially to the power that will fire the ejection charges. I also looked into using a Lipo 2 cell 7.4V (nominal) to power the MAWD and step up to 12 VDC for the video system. But I could avoid all of that if the MAWD will work with 12VDC - thus my question.

I've seen some comments on running the MAWD on the 12V A23, but there were problems firing the charges because of the low current available from the A23. So there's a glimmer of hope. But I thought I'd ask to see if anyone had further info.

Ashley

If the MAWD is running off a regulator, which I assume because of the input voltage range, then the question is wattage. If the parts need 5V then with 10V in at 8 mA the regulator is dumping 40 mW of heat. For 12 V in it would be dumping 56 mW. If the regulator can handle the extra 16 mW of heat there should be no problem. If the regulator is outputting 3.3V then the heat loss will be higher. If the MAWD has ejection charges powered off the same supply, there may be other issues with higher voltage.

A simple fixed regulator (740X 3 Legs) would do the trick no capacitors or resistors needed from a voltage source that does not contain a lot of transients (I.E. a battery). Just three connections and little additional failure modes if your soldering skills are up to par. The basic circuit will handle up to 1 amp continuous current capacity but can handle a good bit more as a surge. Since the regulator chip can be had at ratshack for less that $2.00 its a simple and effective solution and cheap protection for the much more costly altimeter.

Always follow the manufacturers recommendations. Even if they are an inconvenience.

Here's a little trick you could try. Have a special LiPO pack made with a tap for two (7.4V) and three cells (11.1V). Run the MAWD off the 2 cell tap and the video off the 3 cell tap. In fact most good Lipo packs have a balancing tap that you could use to do this. The balancing tap usually brings out the internal cell connections so that you can charge a single cell. So just find the balancing tap pin that connects to the positive of the second cell, and you are good to go.

For dropping a couple of volts, you don't even need to get as sophisticated as a 3 terminal regulator, particularly if the load doesn't need a regulated voltage.

An ordinary silicon diode has a forward voltage drop of 0.6 to 0.7V, pretty much independent of current draw. Putting 3 1N4001 diodes in series with the MAWD power line will drop around 2V from the supply. Hook up like this:


+12v----->|---->|---->|----altimeter power

Thanks, guys. Both good ideas. A center tap would work, and using the voltage drop across three diodes is a nice, simple solution.

You know, as I've thought about this, I'm to the point where I've talked myself back into two separate batteries - one 11.1V Lipo for the video system, and one 7.4V Lipo for the MAWD. It seems to me that the additional three ounces on an HPR isn't worth the distinct possibility of the active video draining the battery that would be supplying the ejection juice as it sat out on the pad for an hour or so. And even though the 900mah Lipo should have more than enough power to handle a drain of roughly 210ma for a couple of hours, why take the chance?

So, I'll just charge two batteries instead of one.

Again - thanks for all of the suggestions.

Ashley