I have attached a LM1086CT-3.3 voltage regulator to a 12V 500mA unregulated wallwart. It is attached very near the wallwart. Then there is a few metres of coiled up twin core wire after the regulator. The load initially drags about 500mA then settles at about 20mA. I did not bother to add any caps as the load is quite voltage tolerant (it used to be batteries). But this one output about 8V one time I tried it. Lucky the device (gas water heater) could handle it. Every other time it output 3.3V. Actually it drops to about 3.1 volts briefly with the initial 500mA but that's OK. I was counting on any instability due to the lack of caps to be in the order of a few tenths of a volt. Is this wild instability normal? If it was a precision device it would be toast.
Many regulators need the recommended capacitors, especially on the output, to prevent oscillation. A couple of years ago, I set up a 5V regulator and had an output approaching 10V, due to oscillations, purely from forgetting a
10uF cap on the output. I would suggest that you add capacitors to the input and output as suggested in the datasheet and see if you still have problems. ... Johnny
Low drop out type regulators, like this are inherently unstable, unless they are compensated with a correct combination of output capacitance and resistance, and with input capacitance. I would place at least the recommended 10 uF capacitor at the input and a 10 uF in series with .1 to 1 ohm in series at the output, to improve the stability.
I learned that over thirty years ago, when I got my first three terminal regulator. What a great oscillator it made. I've never used one since without capacitors.
When 5 1/4" floppy disk drives first became available I built my own regulated 5V and 12V supply for one, to interface to my TRS-80. A friend of mine copied my circuit but left off the capacitors and wondered why it didn't work. As soon as I stuck a DVM probe on the 12V output it started working, it had killed the oscillation. I added capacitors across the regulator pins, which fixed it properly.
Not sure what you are getting at. Are you concerned it got up to 8 volts, or drops to 3.1?
Twin core wire? Like that cheesy 30 gauge stuff that comes with wall warts? Four meters total?
Have you looked at it with a scope? Not all DC output wall warts contain filter capacitors.
8 volts out seems impossible and 3.1 entirely likely. I'd guess the 8 volts is a mistake in measuring and 3.1 with 1/2 amp normal if measured at the load.
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Some decades ago, I stumbled on a 9" B&W monitor for about ten bucks at some surplus store. Built a nifty TV typewriter using it, which was intensely rewarding.
But the monitor needed 12V at about 1.5A. So, I took a 12V, 2A transformer, diodes, and caps, and used a 7812 plus that PNP pass transistor trick.
I must have capacitated it right (ceramics right up almost against the body of the regulator - actually, wrapped around its leads just below that discontinuity), because the pic was as clean and crisp as the frequency response of the video chain allowed. ;-)
Thanks for all that information. I just checked today, the voltage is definitely 8v. I have also used the same make and model of adapter on another unit, with the same regulator. Today I found out it has cooked the regulator, and destroyed the water heater by putting 14V or so into it. The wallwart / adapter is 12V 500mA nominal. It measures close to
16V open. Does the high open circuit reading definitively mean it is an unregulated adapter? This is what a regulated adapter would do, ie cook the regulator right? Or could a regulator with no caps added fail so badly when added to an unregulated adapter?
Yes. It also means that any linear 3.3 volt regulator has an awful lot of voltage to burn up and will produce a lot of heat doing it.
The lack of regulated adapter is not the problem. An unstable regulator combined with about 10 volts drop is the problem. You should look for a 5 or 6 volt adapter for your next attempt.
If you have an oscilloscope, you'd find that the 8V is actually an oscillating output that averages 8V. It's probably swinging from about 3V to
13V and averaging 8V. The recommended caps would have stopped this and kept the output within spec. As John Popelish mentioned, low-dropout regulators are very sensitive in this area. A 78xx series reg. is a bit more tolerant, but I don't think that they come in a 3.3V version. By the way, are you sure that the original voltage was only 3.3V? That sounds a mite low. Using an unregulated adaptor, (transformer w/ rectifier), the ideal input to this regulator is about 1.5V above the output voltage. (That's during the input troughs, not peak, average, RMS etc.) Anything else is burnt off as heat. Also, if as mentioned by an earlier poster, some adaptors don't have a filter cap, the troughs would approach 0V. No regulators will work under these conditions.
This part is particulary forgiving of a lack of load capacitors, mostly due to the PMOS pass device, which is an easier design to stabilize. Also, the internal reference is double buffered in the sense that the reference itself runs off of another internal voltage regulator. Though the datasheet indicates it needs a 10uF output cap, even 0.47uF will work.
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If the 500ma is temporary, exceeding the 350ma limit shouldn't be a problem.
Yeah, 0.47uF might work under some conditions, especially if an input cap is used. Generally, if the datasheet says 10uF, then use it! Believe me, the designers of this chip know what they're talking about. Beware of posters that suggest anything outside the ratings, as it's likely to cause premature failure, as you've discovered and can destroy more important circuit elements, as you've also discovered. Sorry to step on your toes, miso, but that's how it is. ... Johnny
Actually, I designed that part. The idiot in applications didn't believe it was stable with a small output capacitor, even though my simulations and actual silicon proved otherwise. Besides the fancy double regulated reference, the part has replica parasitic devices to give it better real life line rejection. The only negative to the part is high reference noise, but they wanted low Q current, and low noise is hard to do with low power.
In that case, I'll shut up. My apologies. I'm glad that I wasn't too rude. My suggestions would normally be relevant, or, at least, I'd like to think so). Sometimes, we all need a wake-up call. As I said earlier, "the designers of this chip know what they're talking about". (Talk about putting your foot in it). What do you think would have caused the out-of-spec output and eventual failure of the part? I suspect that there was no capacitor on the input AND that the O.C. input of 14V was actually higher, and that without smoothing, it might have exceeded the input voltage spec of the device. Again, thanks for the (very polite) correction. You've now got me even more interested in this problem.
Actually, you should always obey the datasheet, well, unless you know better. This was a case where nothing else in the company was as stable, so dammit, Apps won't skimp on that output cap. The max1658 was targeted to replace a similar Maxim LDO that used a PNP output device. That part was notorious for bad line rejection and stability problems, so the new part was supposed to be user friendly. The component count is a bit low as there are many intentional parasitic diodes in the design to balance out active devices in an attempt to get very good line regulation.
I never did a pure bipolar chip, so I can't comment on the failure mechanism. My guess would be the output device was no longer in the SOA.
It would be interesting to know which type of LDO (bipolar or the PMOS pass) is more reliable. For my own board level designs, I always buy the PMOS LDOs, generally TI since they are well stocked. Bipolar output devices can get funky as you approach saturation (i.e. approach the drop out limit).
The "anti-saturation" electronics in a bipolar design (at least mine) start to draw base current from the output BJT as they approach dropout. That is, the circuit senses the saturation level and tries to cut back the base drive. I never got a warm and fuzzy feeling about such a design. It seems like you have yet another loop to stabilize. The PMOS pass device design is much simpler. The only drawback is the output capacitance of the power fet and the load capacitance form a capacitive divider, essentially determining the ultimate line rejection at high frequency. It is predictable, but not as good as bipolar designs.
I added 2 x 33uf 16V caps to the regulator putting out 8V, and tested it. It was working OK putting out the right voltage 3.3V with an oscillation with the initial transient current. I will redo and test the other adapter, the "heater killer" soon. Thanks for all the input, the parts I could understand were interesting!
PS to the person who thought the required voltage seemed low, originally the gas water heater took 2 X 1.5V batteries which power a
At 20 mA, it should work. What is the current drawn during the start up transient, and how long does the transient last?
(snip)
That means the voltage will not instantly destroy anything, but you still have to make sure the temperature does not reach a destructive level. If you can put your knuckle against the side of the regulator while it is operating, it is doing fine, from a thermal standpoint. If it makes you jump, not so much.
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