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Volts For Dolts

A Dummies Guide to Voltage Regulators

Bargain Hunter's 5V Switcher


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

LM705Linear 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.

LM78-5 Linear VregWith 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.

LM2940T-12Beyond 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.

PT5041N DC-DC Step Converter 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.

PT5041 circuitThe 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.

Surplus VReg Shown in Battery CaseA 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.

Here is what needs to be cut off.
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.

Here is the shrunk down VReg boardI 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.

Pactec enclosure also has a use!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.


FeedBack:

Your comments and suggestions are welcome.




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