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Updated: July 17, 2013

R/C Voltage Regulator Projects




Testing & Operation


Note: All pictures that follow are "clickable".  Clicking on them will show you an enlargement!

A lil' history ...
    You probably have heard about fail-safe switches in the past.  The purpose of these type of switches is to isolate the possibility of the mechanical switch failing (if switch fails, you get no power to your receiver, a BAD situation when in the air).  The operation of a failsafe switch is designed such that the mechanical switch (normally a DPST slide switch) turns power "OFF" to your receiver vice turning power "ON".  With this type of design, if the switch contacts fail, the receiver will still be getting it's much needed power.  So, how does this type of "fail-safe switch" work?  And how do you build one?  Ah!  Glad you asked!

For my application, not only did I want to design a fail-safe switch, I also wanted to incorporate a voltage regulator.  My need for this was due to the fact that I wanted to power my receiver and servos using 2 Lithium Polymer battery cells (nominally, 7.4 volts).  Receivers and servo do not like being powered by this excessive voltage hence you need to regulate it down to around 5 or 6 volts (my goal was 5.5v). I dug around on the internet, browsing through various voltage regulator manufacturer webpages and finally came upon a neat 5 pin regulator.  The key with this regulator is that it has a Low Drop-Out (LDO), meaning that the voltage drop (i.e. loss) across the regulator will be minimal, much less than ordinary 78xx/79xx 3-pin regulators.  But why does this LDO regulator have 5 pins you ask?  I wondered the same until I started reading the datasheet.  The other pins allow you to control the output of the regulator (i.e. enable or disable).  WOW!  Neat, just what I was looking for.  After thoroughly reading the specs (LDO, 3 amp, with adjustable voltage output, with enable/disable), I started my design.  

Design Criteria Summary:

1) Low Drop-Out (LDO) Linear Voltage Regulator (no switching/buck, etc... too much EMI noise)
2) User-Adjustable output voltage (my need was around 5.25-5.5 volts)
3) Minimal components (small PCB)
4) Deans main battery connection for low loss (as I am using a 2-cell LiPo pack with Deans mating connector)
5) Integrated heatsink/circuitboard (could also use optional aluminum heat-sink if max current is required)
6) At least 2 amp current capability (this design is based around a 3 amp LDO regulator)
7) Fail-Safe Switch Design (outboard switch used to turn regulator OFF, not on) - If switch fails, Regulator stays ON.

Parts & Tools List ...

1) IC1 = LP3966ES-ADJ (LDO 3amp Adjustable Linear Voltage Regulator by National Semiconductor)
2) R1 = 10K Resistor
3) R2 = 10K-50K Resistor (for adjusting voltage output, 33K makes output ~5.25 volts))
4) R3 = 1Meg Resistor (pull-up for switch)
5) Cf = 68pF Ceramic Capacitor
6) Cin = 100uF SMD Tantalum Capacitor (min = 68uF)
7) Cout = 47uF SMD Tantalum Capacitor (min = 33uF)
8) Ordinary R/C Switch Harness (i.e. DPDT slide switch)  or even a push on/push off button switch!)

Building Instructions...

A circuit-board makes the design very simple and makes the assembly quick and neat.


Here is a picture of the kit parts, minus any connectors/wires.

Included is the PCB, a LP3966 LDO Regulator, a 10K resistor, a 1 meg resistor, a 33K resistor, a 100uF SMD capacitor, a 47uF SMD capacitor and a 68pF disc capacitor.

The user must supply connectors such as a servo wire for powering the receiver/servos and the battery connection (the PCB is made to mount a Deans male connector).

The first step in building this neat fail-safe regulator is to solder on the LP3966 LDO regulator.  As shown in the picture to the left, use a clip to hold the regulator onto the PCB (this clip also dissipates excessive heat which could ruin the regulator) and be certain the regulator leads line up with the appropriate PCB lands.  Apply enough heat to solder the 5 regulator tabs onto the PCB.


This is what it should look like after soldering the regulator onto the PCB.  Pretty easy.  Just be sure you do not use too much heat as this will ruin the regulator (use a heat sink when ever possible).



Next, solder the main grounding tab of the regulator to the PCB.  Use plenty of heat for this connection (it might take a while to heat up the large tab,... be patient).  Again, use a clip-on heatsink to dissipate excessive heat (I moved the clip after soldering to show you what it should look like after soldering).  You could even put some conductive heatsink paste under the regulator if you want.


The next step is soldering on the all important input and output capacitors (these are SMD versions to cut down on size and weight).  BE SURE YOU ORIENT THEM PROPERLY AS THESE WILL BLOW UP IF PUT IN BACKWARDS!  The lines on each SMD capacitor indicates the "+" lead (unlike regular radial caps where the line/mark usually indicates the negative lead).  Be very careful to orient them as shown in the picture (lines point to each other).  Again, using the clip, clamp down each capacitor and solder each lead/end onto the PCB.


Next, place and solder the remaining components from the other side of the PCB.  This includes soldering the 33K resistor, the 10K resistor, the 1M resistor and the 68pf disc capacitor.  Bend the leads over and cut and solder.  Orientation is not important for either component.



This is what the finished product should look like !

(Notice that in this picture, I used different SMD capacitors... both are 68uF... I ran out of larger SMD caps for this regulator.. but it still works fine).



The final step before testing is to attach your connecting leads for both the source battery (I usually use a Deans connector soldered to the PCB as shown below) and a servo-like wire that is used to connect to your R/C gear (receiver/servos).

Please not the polarity of both the input (IN=battery) and output (OUT=to R/C receiver) PCB connections.



After connecting these leads, connect the fail-safe regulator to your battery and test the output of the regulator to be sure it is outputting roughly 5.25 volts (Test this before connecting it to your R/C gear else you could possibly ruin your gear if the regulator is not functioning properly).    

If all appears ok, test it with your R/C gear.



The last thing you will have to do, in order to use the fail-safe feature, is to connect a normally open switch to the regulator PCB (larger PCB holes just above one of the SMD capacitors.  With out this switch connected, the regulator will always be ON.  The switch (when closed) will actually turn the fail-safe regulator OFF (hence, the fail-safe feature if the switch craps out).  The Pull-up resistor (R3) is set to 1 Meg ohm, such to limit the current consumed when the switch is closed (regulator turned OFF).

Please Note:  When using this fail-safe switch feature, even in the OFF position (mechanical switch is closed), the regulator circuitry can draw a slight amount of current (roughly 4-10ma).  It is highly recommended that you remove the battery when you do not plan to use it for days at a time.


Testing and Operation Instructions...

After building this regulator, be sure to measure the output and verify it is as designed (5.25 volts, using the resistors described above) before connecting it to your R/C gear.

If you want to change the output voltage, calculate the new resistor value using the formula here:

If R1 is always held at 10K, you can calculate the value of R2 for any output voltage (Vout).

For an output of roughly 5.25 volts, R2 calculates to be roughly 33K





Additional Notes...


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