Misbehaving Voltage Regulators
Private19872
Posts: 61
Recently, I got the Propeller DIP Plus Kit (http://www.parallax.com/product/130-32305) and I started to assemble it on my breadboard. I didn't get too far however, when I noticed that neither of the voltage regulators were working as expected. When powered by an 8.1v source, the 5v regulator gave off 6.85v. I found a 7805 I had lying around and substituted it in for the first regulator and got a steady 5v. Then I added the 3.3v regulator. Instead of limiting the voltage to 3.3v it only went down to 4.36v. Does anyone know what may be going wrong? I found a schematic on the same page as the link, and it appears that the regulators are in the same way. I don't have anything besides the regulators on my breadboard. Any help on this would be appreciated.
Comments
Many (most?) regulators need decouplng caps, and some (like LM317) also have minimum load requirements - usually met choosing the Adj resistors.
Also, make sure you put the capacitors in the right way. They are polarized and putting in backwards with break them.
Hi Private 19872
IF you are refering to the schematic drawn on page 27 of the Propeller Education Kit Labs Fundamentals Text whose download link is available at product's page, you should verify your setup against figure 3-4.
There are two 1000uF x 6.3V electrolytic capacitors in the kit that must be correctly wired (double check them and also their polarity marks), one between LM2940-5.0 OUPUT terminal and GROUND, and the other, between LM2937-3.3 OUTPUT terminal and GROUND.
Be sure to properly assemble the 100 Ohm resistor and the green led at the LM2937-3.3 output circuit.
IF you are NOT USING a 9V battery, but an external 8.1V source, as you stated, be sure to mount the third electrolytic capacitor, 0.47uF x 25V between LM2940-5.0 INPUT terminal and GROUND.
When assembling the electrolytic capacitors, keep the connections as short as possible, and also closer as practical to the regulators pins.
The same advice do applies for the connections between the 3.3V regulator output, the Propeller and all the other circuits involved.
After you ensured proper connections between all parts, apply power to the circuit and double check all the voltages.
As a last recommendation, when you have finished assembling the parts that comes in the kit, do follow the Improve PE Kit Supply Voltage Stability topic at page 38 of the same manual.
You will need two 0.1 uF ceramic capacitors, of suitable voltage rating (between 10V and 25V is better), to accomplish it, but it realy worths the effort.
For short:
The capacitors will ensure minimal noise from the regulator circuits to reach each other, and sure, the Propeller and the other logic circuits;
The led (and current limiting resistor) provides enough current consumption to avoid unreliable regulator operation and also warns that the circuit has power connected to it.
Hope it helps a bit.
Yanomani
Regulators without the appropriate capacitors may oscillate and destroy the chips they power. Similarly
paralleling regulators can do the same thing. A regulator is a high gain amplifier combined with a feedback
network to hold the output voltage constant. Without the right frequency response of the feedback network
a high gain amplifier can become a high power rail-to-rail oscillator.
Each particular regulator will say in its datasheet the minimum (and maximum) capacitance on input and output
and may have requirements on the ESR (effective series resistance) of those capacitors.
Note: This comment has been added to assist others in future who are searching through these forums for solutions to this issue. Also note that similar requirements but with different values are present for many other linear voltage regulators.
Linear regulator problems
A. no capacitors OR the wrong value capacitors
>> The 7805 has different capacitor requirements than the LM2940-5 and so on. MUST READ the data sheet.
B. high power draw shuts down the thermal protection
>> A short circuit in your project or the lack of a heat sink will shut off the regulator until it cools down.
C. some regulators cannot tolerate electricity running backwards through them..
>> This is true of the 7805 espcially. If one puts a big filter capacitor on the Vout and a smaller filter capacitor on the Vin. When you unplug or short the Vin, you may damage the 7805. This is in the data sheet. Other regulators may survive this snag, but again one must read the data sheet.
D. unpredictable behavior in the Dropout region
>> If one insists on running their voltage regulator below the recommended Vin range, it simply doesn't regulate. It may work sometimes and not others. MUST READ the data sheet.
E. minimum load requirements
>> This is new to me. May or may not always be true with certain regulators and not others.
If I'm using a DVM I measure using DC then switch to AC. On AC I'm looking for 2mv up to 100mv. Much more than that spells oscillation. In the distant past I once measured DC of almost exactly 5 volts, called it good and went on looking for the problem. Later I looked at it with a scope and found a 2.5 volt sawtooth. The 5V DC was an average.
In your case the 4.36 volt DC might not be strictly DC.
Marty