Need 8V, 20 amp switching regulator for Nightflyer

[RicVaughn]

Hi All,


I'm a novice in electronics, but this is what I want. Need a 8 volt switching regulator that will have a 20 amp continious output taking it's input from a 10S Lipo pack, apx 35 volts. This is to power 300 lights on my Nightflyer. Is this a DIY project that will not cost a zillion dollars?


Thanks,


Ric

[Wayne V]

What kind of lights are you using?

[RicVaughn]

They are "regular" 6 V bulbs. I run some at 6V and some at 8V. Both bulbs have the same part number, but the size and brightness are different. You can see my plane on the old Neat Fair thread. It's 114" span, and 3100 sq. in. I also have a 25 watt landing light. Right now, I run the lights off an 8 cell Nimh pack.


Ric

[Wayne V]

I know this sounds easy but why would you run a 10s pack, can you wire the pack to a 2s5p so you get lots of MAH's and 7.4volts out, there's no sense in increasing voltage just to bring it back down again.

[AndyOne]

Ric,

Why not just connect your bulbs in a series/parallel network to match the voltage of your battery.

Andy.

[RicVaughn]

Hi,

As for the 10S, that's what I need for the motor, and 7.4V is just not enough for good brightness of the bulbs. The plane is 17 lbs.

All bulbs are in parallel now running on 6 or 8 volts. All 300 of them!


Ric

[jeffs555]

If all you are going to drive is light bulbs, you don't need a switching regulator, a simple pwm driver will do(ie a light dimmer). The bulbs will average the current, so you don't need the inductor of a switching regulator. You just need to generate a rectangular wave with a duty cycle of 8/35 and drive a mosfet with it. It would be close to 100% efficient(only loss would be in the mosfet). You could even use an ESC driven by a servo tester, or a spare channel on your receiver. You would just have to make sure that it never went much above 8/35 or you would blow all your lights.

PS You could use an electronic servo travel limiter like this one designed by Andy to limit an ESC to a safe value. http://www.rcgroups.com/forums/showthread.php?p=3942769
Put it in front of an ESC, and control it from a spare channel if you have one, and you could control the light intensity remotely.

[jeffs555]

I did a quick search for high voltage brushed ESC's and it seems there aren't any. You could build a simple 555 based dimmer like this one. http://www.electronics-tutorials.com/devices/555-light-dimmer.htm

[Bruce Abbott]

If all you are going to drive is light bulbs, you don't need a switching regulator, a simple pwm driver will do(ie a light dimmer). The bulbs will average the current, so you don't need the inductor of a switching regulator. The bulbs will 'average the current' with respect to filament heating, but the peak current will still be 35/8*20 = 87.5 Amps! Adding a diode, inductor and capacitor (classic Buck convertor configuration) can drop the peak current to just over 20A, which is much easier to switch and kinder on the battery.

Another thing to consider is that the resistance of a cold light bulb filament is only about 1/10th of its operating value, so inrush current could be as much as 200 Amps! (without an inductor the current could theoretically exceed 800 Amps!!!). You will need a current limit and/or soft-start circuit to avoid blowing up the switching transistor.

Linear Technology's LT1339 (http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1042,C1032,C1 092,P1551) should do the job.

[rcjunkie]

I have tried using a speed control set to limit the current, but the problem is that it strobes the lights. It still applies the full voltage to the cells but just pulses it on and off to average out the currrent. It doesn't work well with night flying.

Ernie

[rcjunkie]

Bruce,

I'll be working with Ric to help build the unit. I've got a couple questions on the LT1339. It shows that you need a 12V supply to run the IC besides the 48V input and it shows that it puts out a reference 5 volts, Can it be changed to put out 8V and how do you run the IC on 12V without another DC/DC converter to supply it?

Thanks,
Ernie Schlumberger

[jeffs555]

You would need another DC/DC converter to supply the 12V, probably just a linear regulator. The current needed is only about 50ma, so with 36V input it would dissipate about 1.2 watts and could possibly get by without a heat sink. You need to check the spec on the regulator as most linear regulators will handle less than 35V, and 10s lipo would start out at 42V. The TL783 from TI will handle 125V input. The output voltage is determined by the feedback resistors(RFB1,RFB2) and could be changed to 8V.

I still think the simple PWM dimmer is workable and would be simpler, more efficient, lighter and have fewer RFI problems. If you are getting a strobe effect from the lamps, just need to raise the switching frequency. Don't know where my head was, but the pulse on time would be (8/35)^2 not 8/35. The points raised by Bruce would need to be addressed, but could be handled. The bulbs heat rapidly, so the cold current could be limited by a simple RC on the mosfet gate drive with a diode in series with the cap to prevent discharge during off period. Mosfets that will handle 80amp pulses and 20amp average are cheap and readily available.(average current would actually be less than 5 amps) The 80 amp pulses from the battery could be moderated by a capacitor on the input. It would need to be selected carefully with respect to ESR and ripple current. Higher switching frequencies would help there also.