Volts For
Dolts
A Dummies
Guide to Voltage Regulators
Voltage regulators
come in a variety of configurations. The one you choose can affect the
performance of your wireless video system. So, let's talk about the basic
technologies that can be used.
And best of all,
you can learn how to build a high performance DC-DC switcher supply
for your wireless video system for less than $10!
Power at a Price
There are several ways
to power your wireless video gear. In its most basic form, you can brute
force it with carefully selected batteries that meet your voltage and current
needs. This works fine if you use NiCD/NiMH batteries and have unified voltage
needs for your camera and transmitter.
But with mixed voltage
installations, such as when the transmitter is 12V and the camera is 5V,
things get a bit messy. Or, if you wish to use other battery technologies,
like Lithium Ion/polymer (which are ideal for low weight airborne installations),
the odd voltages can create installation hardships. That is because the Lithium's
3.7V per cell voltages do not align well with 5V or 12V equipment.
Methods to improve
the flexibility in powering the video gear comes from using VRegs (voltage
regulators). The common solution involves starting with a higher voltage
and regulating it down. Perhaps the most recognized voltage regulator in
the world is the
LM78xx
series. These are linear regulators that come in a variety of
fixed output voltages (5V, 6V, 9V, 12V, etc.).
However, linear VRegs
have low efficiency. They often waste battery power and create a lot of heat.
For example, if you used an LM7805 (5VDC output) with a 12V battery, there
will be 3.5 watts converted to heat when 500mA is drawn from it. That is
a serious amount of wasted energy and it is a pain to manage the resulting
high temperatures.
But there is a solution
to the linear's deficiencies. The undesirable issues are eliminated by DC-DC
regulated switching power supplies. These power miser designs are no
longer the difficult-to-implement solution they used to be. But the increased
efficiency comes with a price. Switching supplies can emit EMI/RFI interference,
so poor installation techniques are going to invite trouble. If severe enough,
it can cause video noise or R/C interference.
Linear Voltage Regulators
Linear VRegs
have been around for decades. As mentioned, the
LM78xx
series is perhaps the most familiar 1 amp VReg IC (integrated circuit)
in the electronic industry. It is a simple 3-terminal device that offers
good regulation at low cost. It comes in a variety of packages styles, but
the one most useful for our application is the TO-220 tab format. It can
easily bolt to large metal heatsinks to handle the wasteful heat that sometimes
must be accommodated.
With
78xx parts, the input voltage must be at least 2V higher than the chosen
output voltage. For 5V use, a 7805's battery needs at least 7VDC. For 12V
operation, a 7812 VReg will need at least 14VDC on its input. Allowing the
input voltage to go below this spec can result in odd behavior. That means
you must consider the full voltage span of your battery pack as it is discharged.
Other than installing
two decoupling capacitors (see above schematic and the
data
sheet), LM78xx VRegs require very little effort to implement. Despite
appearing as optional parts, the input and output caps are very important
for good performance. I have personally seen these IC's oscillate at RF
frequencies, and cause harmful interference, when the caps were omitted,
installed wrong, or poorly chosen.
Beyond
the 78xx, there are better choices for battery operation. The preferred linear
types are called LDO (low dropout) regulators. They work well with batteries
because the input voltage does not need to be 2V higher than the output voltage.
Most will work with input voltages that are just a few hundred mV above the
required output. One old example is the
LM2940T.
It comes in 5V, 12V, and adjustable versions.
With linear VRegs,
the current on the input side is about the same as the current on the output.
This may seem like an obvious relationship, but you will soon see that other
VRegs behave differently. It is also the basis for the linear VReg's poor
efficiency. The reasons for the wasteful operation and heat production
will become painfully obvious once you calculate the input and output power
used by your linear VReg.
Boost Switching
Regulators
Linear VRegs will never
go away, but they certainly have their drawbacks. So here is where regulated
DC-DC switching supplies step in. Switchers can provide a lower voltage (buck
mode) or higher voltage (boost mode) than the input voltage. Efficiency of
a good DC-DC switching VReg is usually 85% or higher. Heat issues are minimal
or non-existent.
Several years ago the
RC-Cam site
introduced
the
PT5041N DC-DC Boost Switching VReg as an efficient way to obtain 12VDC
from a 4-cell (4.8V) NiCD/NiMH pack. The PT5041 accepts 4.7VDC to 11.0V on
its input. In return, it will provide up to 1 amp at 12.0VDC on its output.
Weight is just a few grams.
That old RC-CAM project
used the raw voltage for the 5V CCD camera. However, the video transmitter's
12V was provided by the stepped up (boosted) voltage from the PT5041 VReg.
This arrangement was an immediate hit among the R/C video hack community.
It is still a very popular Boost Switching VReg for R/C video applications.
Digikey.com stocks
them.
The
PT5041 is easy to use. It sort of looks and mounts like a standard 3-terminal
voltage regulator. For reliable operation, three additional capacitors must
be installed and their leads must be as short as possible. Do not omit these
caps!
As mentioned, well
designed switching VRegs are very efficient. If you calculate the input and
output power you will see that they are nearly the same. The difference between
the two is the efficiency factor, a specification that can vary among
switcher designs (so consult the data sheet). The magic behind this power
conversion process results in input and output currents that are vastly
different. This is often a source of confusion for some folks. Doing the
math offers a better explanation.
For example, let's
say you have a 6VDC battery connected to a PT5041 that powers a 500mA load
at the rated 12V. Output current would be 500mA, of course. But input current
would be about 1150mA. The 2X increase in output voltage requires 2X increase
in battery current. If you compute the efficiency you will discover that
it is about 85% {(12Vx500mA) / (6Vx1150mA)}. That means the battery supply
will last nearly twice as long than an equivalent Linear VReg solution. And
the high efficiency means that the wasted current is not converted to ugly
heat.
Buck Switching Regulators
You may have noticed
that many wireless video transmitters and cameras operate on 12VDC. That
is because they were originally designed for security/surveillance installations.
But, the latest micro sized equipment is now arriving in 5VDC flavors. That
is a good thing. What is handy is that many of these systems will tolerate
4-cell (4.8V) NiCD/NIMH packs without the need for a VReg at all.
But high capacity NiCD/NIMH
packs are often too heavy for some R/C models. Thankfully there are alternatives.
The latest micro weight battery technology is based on Lithium-Ion (Li-ION)
and Lithium Polymer (Li-PO) chemistry. Their 3.7V per cell voltage requires
that we use two cells and a 5V regulator. The low pack voltage will not reliably
support a LM7805, but the LM2940T-5.0 linear VReg will work fine. However,
that means we will have a lot of wasted power and heat to deal with.
The solution is to
use a DC-DC Buck Switching Regulator. Unlike the DC-DC Boost switcher
that steps up voltage, the Buck switcher steps it down. Yes that's right,
we can have it both ways! And just like a Boost switching VReg, the Buck
VReg is very efficient. If you calculate the input and output power you will
see that they are very similar (less the efficiency factor).
For example, let's
say you have a 10VDC battery connected to a good Buck VReg that powers a
1000mA load at 5VDC. Output current would be 1000mA as noted. But input current
would be about 585mA. The 2X decrease in output voltage allows a 2X decrease
in battery current. If you compute the efficiency you will discover that
it is about 85% {(5Vx1000mA) / (10Vx585mA)}. And as mentioned, the high
efficiency eliminates the heat issues.
There are dozens of
buck regulator VReg modules and IC's. Enough so, that I will leave the search
for them up to you. A good place to start is with Texas Instrument's
Power
Trends modules.
A very low cost 5-volt buck
switching VReg is available from the surplus market. And when I say Low Cost,
how does $5 sound? Here are the specifications:
-
6VDC to 25VDC
input
-
5.0VDC output
-
Continuous 1.25 Amp
rated output
-
Low output ripple
-
> 85%
efficiency
-
10 grams weight
-
30mm X 36mm overall
dimensions
So, do I have your
attention? This diamond-in-the-rough comes from
allectronics.com and is part number
MB-90. It is described as "PACTEC PROJECT BOX WITH
5VDC UPS." As of Nov 2004, the supplier has hundreds in stock. But you better
hurry if you want to beat the rush.
However, as a surplus
item there is some hacking required. This gadget is a small plastic box that
has two 4.8V 1400mAH NiCD batteries and a custom 5V DC-DC Switching VReg
in it. The actual DC-DC switcher is based on a popular
Linear Technologies IC. All it takes is one hour
to convert it into a perfect two to five cell Lithium-Ion/Polymer battery
powered 5V supply.
The first step is to
remove the DC-DC VReg. Begin by cutting the four wires off the board and
then pry it from the batteries. It is held by double sided tape that will
pull apart if you work slowly at it.
Once the board is free
you will notice that it is much larger than it needs to be. No problem. We
will cut off the unused areas. This will help reduce a lot of weight too.
Please refer to the
photo on the left. The yellow lines show where the cuts need to be made.
The "optional" area can be cut away too for an even smaller board. For a
better view of the cutting areas, please click the
photo.
I used a sharp knife and
deeply scored the yellow lines, then snapped the unwanted areas off. You
could use a hobby razor saw or a hacksaw. Just be careful -- damaging a component
would mean death to your board. Some sandpaper or a file can be used to clean
up the cut edges.
All that remains is
to solder the Power In and 5V Out wires. The photo on the right shows the
pinout (click it for a larger view). The required connections are:
-
TP8: Battery
Gnd
-
C2+: Battery
Positive
-
TP6: Output
Gnd
-
TP5: +5VDC Out
It is important that
your power wires are at least 22AWG and as short as possible. The
reason for this is because switching supplies can create EMI/RFI interference
from their switched magnetic components. So, it is important to have low
impedance connections on the power leads.
By the way, despite the abuse the UPS's batteries had seen (they
were completely dead due to the lack of an on/off switch), I was able to
bring mine back to life. I just slow charged them with 150mA for 20 hours.
Then I cycled them on my R/C peak charger two times, In the end, I gained
four 4.8V 1400mAH packs that I configured for powering my video receiver.
The PacTec enclosure is a perfect case for them too!
But, if your cells
do not accept this restoration treatment then please dispose of them in an
environmentally safe way.
The Small Print:
If you need a part
then please consult the sources shown in the project. I do not work for,
nor represent, ANY supplier of the parts used in this project. Any reference
to a vendor is for your convenience and I do not endorse or profit from any
purchase that you make. You are free to use any parts source that you
wish. |
All information is
provided as-is. I do not offer any warranty on its suitability. That means
that if you build and use this device, you will do so at your own risk. If
you find documentation or software errors then please report them to me. |