[NSRCA-discussion] Regulators and heat

Sat Jun 17 06:09:21 AKDT 2006

Wow, I must have missed a bunch of email the past couple of days.  John's 
right with all the analysis here - it's not a good idea to use a 3 cell Lipo 
to run a flight pack, it would require a heatsink unless the servo load is 
very light.  This is why ESC's have such stringent requirements on the 
number of itty bitty servos they can run - a large percentage of the current 
that those servos draw are turned into heat in the regulator on the ESC 
board.  It's simple: Power in watts equals current times voltage.  So if you 
are sourcing around 12V from a 3 cell LiPo and only putting 5 or 6 V across 
the servos, about half of the power delivered to the servos is being wasted 
as heat in the ESC flight pack regulator.  Take a "3D" foamy actively 
flailing around.  Those little servos are drawing up to a combined 1/3 to 
1/2 amp.  At 1/2 amp, there's 3W being dissipated on that little ESC. 
Things can get a little toasty if you do that without any let-up.

So you would only want to run a 2 cell Li-Ion or LiPo with any linear 
regulator, no matter who makes it.  Look at what Jim Oddino makes with the 
Perfect Switch regulator as a good example of what to expect in a pattern 
bird with a regulator.  If you run his regulator at 6V on a LiPo, it's going 
to just barely get warm.  Even at 5.1V, it's not going to get very warm  You 
only need one to handle airplanes the size of ours.  Two is nice just for 
that extra security in case a wire breaks, a battery dies, whaetever.  Two 
is also nice in the event that a high torque digital servo locks up and 
draws an amp or more continuously until you detect it and land (if you are 
able).  One regulator will still work, but will be heating up a fair amount 
until you take off the large sustained load of a stalled servo.  Two is a 
little added margin of safety, that's all.

I've flown Jim's and mine either single or dual, no problems.   Actually, I 
did a quad arrangement with something that Jim made for me a few years ago 
for an IMAC bird and that worked fine as well.  I just wanted something for 
myself and a few buddies here in NJ with a very fine adjustment capability 
for dual battery setups, so as to have the batteries draw down more evenly. 
The regulators used these days do a very fine job of holding a precise 
voltage, so if you use two in parallel and one is set just a little lower 
than the other, the lower one will only deliver current when larger current 
spikes come along.  That's OK, but you do wind up having one battery go down 
earlier than the other unless you match them better.

Ed

----- Original Message ----- 
From: "John Pavlick" <jpavlick at idseng.com>
To: "NSRCA Discussion" <nsrca-discussion at lists.nsrca.org>
Sent: Saturday, June 17, 2006 1:00 AM
Subject: [NSRCA-discussion] Regulators and heat

> Guys,
> Some more interesting things about regulators. There are basically two
> types of regulators: linear or "shunt" type and switchers. Each has
> advantages and disadvantages. Most people are familiar with the linear 
> type.
> They get warm when they have to dissipate power. That's why it's not a 
> good
> idea to use a 3S Li pack with a linear regulator. The excess voltage is
> absorbed by the regulator and makes it get warm. In a switcher, the
> regulator actually varies the duty cycle (ON vs. OFF time) to regulate the
> voltage. Here's the cool part (pun intended) it doesn't give off much heat
> because it's not shunting the current, it just turns off when the voltage
> gets too high. It has an inductor and a diode so the output doesn't turn
> off, just the current draw from the battery. The other neat thing about a
> switcher is this. If you use a 3S 730mAh pack, you'll actually draw less
> current than if you used a 2S pack. Why? It's a complicated procedure
> involving lasers... No, not really but it's a little hard to explain in a
> few words. Take my word for it, it's true. Now before you go out and buy a
> magic switcher, there is a downside. Actually there are a few. First, a
> switching regulator circuit requires more parts than a linear regulator
> circuit. More parts = more weight. Weight is bad. The other problem is
> "noise". The regulator turns the power on and off and this can create 
> havoc
> with your radio. There are a few good switching regulators for R/C use so 
> it
> is possible to use one, but you have to be very wary of the potential for
> noise.
> Matt mentioned a linear regulator that Ed built for him. He may not have
> mentioned that this regulator has an adjustable output and it's very 
> light.
> By having an adjustable output, you can buy one regulator and set it to 
> what
> you prefer. You don't need to buy another regulator if you want to run on
> 5.6V now and then 6V later for example. The other cool thing about having 
> an
> adjustable output is if you run 2 packs w/ 2 regulators in parallel. You 
> can
> fine tune the voltage output so that both packs are supplying power or you
> can favor one of the packs by setting the regulator on the other pack 
> lower.
> Ed chose the linear topology for reliability and "clean" output. Because
> it's a linear type it does generate heat, but for the power that's 
> required
> to run the equipment in a pattern plane, it's not even breaking a sweat if
> you run a 2S Li pack.
>
> John Pavlick
> http://www.idseng.com
>
> --
> No virus found in this outgoing message.
> Checked by AVG Free Edition.
> Version: 7.1.394 / Virus Database: 268.9.0/368 - Release Date: 6/16/06
>
> _______________________________________________
> NSRCA-discussion mailing list
> NSRCA-discussion at lists.nsrca.org
> http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
> 