Solar flux is considerably higher at stratospheric altitude and there is much less cloud cover, so solar is a more reliable option.
Matt and the crew have launched solar powered balloon payloads before - but never mechanical stuff...
The idea of the relays would be to switch from the solar arrays to a conventional battery if the solar supply falters (so I guess I need a double-pole relay)
The next trick would be how to power the relay direct from a microcontroller, since the pins on say, a PICaxe give only 10mA. We need a relay that can handle either the 1A or so from the solar array or the massive ampeage from a battery shorting through a SMA (offscale on a 30A multimeter!). But relays often need 100mA or so to switch... So we probably need a transistor to switch on a 100mA current from a PICaxe pin.
Why do we need this?:
At Black Rock we ran our battery ragged well before landing - the last coms I got was just before the balloon burst (when we were near Frog Pond), which was only about 90 mins (about half way?) in. The intense cold probably played a part but we also had a constant power demand of a couple of amps at all times anyway.
The LiFePO4 battery I got lasts longer but Matt said they don't like balloon flight conditions at all.
So any other options would be worth trying.
It looks like the cost driver for solar arrays is voltage - say £70 for a 12V, 5W, which is a measly 400mA or so. For SMAs we only need amps, so a cheap low V, high A array might buy us something.
It's a bit the same with battery charging. For a 6V battery we need a 6V array and a decent ampeage to get a decent charge rate, which means in practice an array too expensive and too big to fly.
Besides, as far as I can tell, the only practical battery for trickle-charging from a solar array is a lead-acid one, which might burst at high altitude (since they are sealed to prevent leaking) and are bulkier. They are no good burst-wise for rocket flights either, for reasons HAB 1 well knows!
So, it might be fun to play with solar to extend the balloon flights, but have a relay to switch back to battery if needed.
In general, good DC sources for use in the field would make a lot of sense, since as it was we were constantly running AC inverters off car batteries to power power supplies to supply DC to power laptops and so on... Whereas a 19.2V battery and/or solar array could have done it directly. We spent a lot of time angsting over generator resources etc similarly. It might also be worth digging up an old low power PDA, say an Atari Portfolio, for the serial coms... Don't really need a laptop at all! Would run for hours on AA batteries...
Subject: Re: Ideas for switching low volts, high amps?
Sent: 22 Feb 2011 20:01
I think a mechanical relay would be the only way to do what you ask. Maybe not suitable for rockets (acceleration might trigger it) but ok for balloon.
But you could maybe switch each separately (0.4A) and combine them afterwards, which might get you back into the FET range.
Solar sounds a bit unreliable though, given there's no real problem with a battery? Solar usually gets combined with a battery / large capacitor anyway to take out the peaks & troughs...
On 22 Feb 2011, at 19:22, email@example.com wrote:
> Can I ask you an electronics question?: What is the best way to switch a low voltage but at highish current? I'm looking at about 0.45V, 1.2A. I've played with the SSRs before (basically big MOSFETs) but they need at least 3V for the circuit being switched. Same for Darlingtons. Should I use a mechanical relay or is there something else?
> This is for a H.A.B advance project - I found some cheap 0.45V, 0.4A solar cells so, intriguingly, three of these in parallel might be enough to trigger a SMA in good light. SMAs only need current, not volts.
> Quite big, expensive solar arrays - enough to charge a battery for instance - seem to be high volt (say 12v), but not necessarily high current. So by contrast we might be able to do a solar-powered HAB quite cheaply?