hi all, i'm teaching myself via books and online material just for the fun of electronics... i have built from spare parts a simple +5v power supply using an NTE 5v regulator... however, when i measure the voltage output, it is at 4.91v... i also have a basic stamp board... on the basic stamp board the voltage regulator is a precise 5v... is this due to the manufacturing or possibly my design?... what will the impact be on my projects if the voltage is slightly less than 5v?..
thanks!
NTE buys floor-sweepings from the semiconductor fabs, then marks them with their own numbers.
_Any_ voltage regulator will be inaccurate. The more you spend, the better accuracy you'll get.
Not knowing the voltage regulator that you used its hard to say how much your design could impact the output voltage. Any 3-terminal voltage regulator will drop out of regulation if your input voltage is too low. How much is too low depends on the regulator: a garden variety regulator will have a drop out voltage as high as 2V, "low drop-out" regulators are specifically designed to work down to a few hundreds or tens of millivolts differential.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html
thanks, tim... i can't recall the part number at the moment... do you have any recommendations for a more accurate regulator?... at the moment, i'm at the mercy of dozens of cryptic numbers in digikey et al. catalogs or a local fry's (who seem to sell only nte products)...
thanks
Most "logic level" devices are quite happy anywhere between 4.75V & 5.25V, many modern devices are even more forgiving.
I forgot to mention: The necessary accuracy of the voltage regulator depends on what you're driving. Check the data sheets (they're on the manufacturer's web sites) for the required input voltages, then make sure your regulator accuracy matches.
Generally you'll find that it's easy to get regulators that are good for
+/-5% over temperature, and that that's good enough for most digital logic.-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html
4.91 would be in tolerance in my opinion but any regulated power supply you can't adjust is a piece of crap.
nah, just cheap regulators NTE got their hands on that didn't pass the precise 5 volts out test or drifted a bit. It's close enough for what it was designed for. Most likely well with in the limits of generic specs. Now if you were talking about mill specs then, that could be an issue.
-- "I\'m never wrong, once i thought i was, but was mistaken" Real Programmers Do things like this. http://webpages.charter.net/jamie_5
A typical 7805 regular has +/- 4% accuracy over the working temperature range, that's 4.8V to 5.2V. You can get better "grade" devices with better accuracy, see the datasheet of a typical device:
Your basic stamp board is spot on 5V due to luck, nothing more. Another basic stamp board could be 5.1V or whatever.
Dave.
The regulator may be inaccurate NTE parts are not the best.
NTE doesn't manufacture parts - they buy large lots of parts (sometimes surplus and seconds) and put their brand on them They are intended for consumer repair shops - hence the large cross reference directories for NTE parts. A single NTE transistor may "replace"
100's of regular branded and specified parts. Good enough for fixing the TV or radio.LM7805 would be a garden variety 5V reg. It requires about 9 volts input to stay in regulation - and that also means you need to keep the "valley of the ripple" above 9 volts - a good rule of thumb is 1,000 microfarads of filter cap for each 1/2 amp of current, for around 10% ripple, with a full wave bridge rectifier.
You also need to allow for the .6 volt diode drop in the rectifiers - a four diode bridge will loose 1.2 volts in the rectifiers alone.
If you don't have a scope you have to guess at how much ripple is on the input - too much ripple and it would be dropping out of regulation
- 120 times a second with a FWB and 60 cycle mains. Your meter may average that and be giving you a 4.9 reading.
Some low drop out (LDO) regulators will only need .6 volts across the regulator so they might work with only 5.6 volts input.
- allowing again for transformer loss or transformer regulation (~10%), line regulation (power company accuracy in delivering 120 (or
105-125 volts), diode loss, ripple voltage, etc..Even a simple three terminal regulator takes a bit of arm waving and head scratching to apply. Read about it, then read some of the data sheets for the regulators. They spell out what is considered good and what the tolerance limits for the parts are, and how they behave under other environmental conditions.
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Incorrect. It's about 7V. The "dropout" voltage of a typical 7805 is around 2V
Dave.
National Semiconductor gives it as 7.5 volts input to maintain line regulation and yet they give the dropout voltage as 2.0 volts.
So you are correct it is 2 volts for reasonable temperatures
Fairchild just says 2 volts
ST Micro says 2 volts then runs all their specifications at 8 volts minimum . . . They also give two specifications on minimum voltage. no current minimum is 4.9, and at currents of 5ma to one amp 4.65 with a power dissipation equal or below 15 watts
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Its load and temperature dependent. I'm using a 7808 from ST. Its delivering 1A pulses, it drops out at about 10.7V.
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thank you for the info... i'm using a cheap wall wart in place of building a transformer into my project... what i currently have is very simple, nothing more than the 5v regulator, a 10mf capacitor, and an led to let me know it's powered... my goal was more to practice soldering, which certainly needs as much time as i can offer... i have been working under the assumption that with the wall wart, i didn't have to worry about a transformer or bridge rectifier?... i don't have a scope, too expensive for me as a beginning hobbyist...
i'm currently using a book i found online, "Lessons in Electric Circuits - DC"... is this a good resource?... for free, it certainly seems so..
thanks again..
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Wall warts . . . they can good or bad quality most of the run of the mill stuff is of questionable quality - they are cheaply made.
In the case of wall warts designed to recharge batteries - they sometimes incorporate a scheme where they intentionally have poor regulation to avoid overcharging and to protect them from high currents (shorted or very low batteries).
Wall warts are generic. They don't all meet some standard, or the same standard. They do usually meet some safety standard, designed to fail without starting a fire and designed to provide a high degree of isolation from the full voltage of the power mains. - but not all of them fail safe.
Wall warts are likely to be transformers (or switching regulators). The fact that you don't mention things getting hot or blowing up suggests the rectification is already inside the wall wart (they can be either AC or DC warts). Three terminal regs die with overheating on AC or reversed polarity.
10 ufd is bound to be too small for filtering. You'd want 200 or 1000 on the input to the regulator and maybe a 10 on the output. - and again some wall warts include a capacitor inside the case for filtering. What is your input voltage?Polarized Electrolytic capacitors connected to AC or reverse connected tend to "vent." Sounds like a firecracker as the case splits open or the rubber plug on the positive lead flies out. Collateral Damage is usually minimal but you wouldn't want your eyes close to it. The cap dies permanently.
I looked at the indexes and one chapter of the book - it looks very good
Problem is, chapter one won't cover the stuff you need to know about transformers and power supplies. That comes along in chapter two and three for transformers rectifiers and filters.
I started electronics in high school. They gave us meters, signal generators, power supplies and breadboards. You might have to lay out a few bucks for a "trainer" but if you plan to stay with it that might be a good way to go.
A lot of the "train at home" schools offered trainers as part of the tuition costs and there might be some on Ebay for less money.
Jameco has one for $439 (Ouch!) Seems to me I've seen them for much less $70-200 range. The jameco one comes with two variable supplies, two fixed 5 volt supplies, and a function generator and mounted pots switches leds etc as well as a breadboard space. Not a good deal in my opinion, but they aren't the only people selling them.
digital trainer for $129-159
Elenco stuff is OK - I've had one of their scopes for years more than I can say for the Tektronics Sony I had . . .
Search on phrase "analog digital trainer"
Digitals cost less because they usually only give you two or three fixed supplies.
Ebay had may still have lots of the old Hewlett Packard lab quality supplies with meters and adjustable voltage and current for $30 (and worth a whole lot more)
Stay safe, good luck
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You obviously don't know the difference between a voltage regulator, and a voltage reference.
-- Service to my country? Been there, Done that, and I\'ve got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
Excuse me ?
You obviously don't know how to read and follow the thread?
Yes, I know what a voltage reference is which wasn't part of the original post as a concern?
Should I start calling you Eeyore JR. ? That last remark resembles kinship.
I hope you don't hang out with him at that Turkish bathe house he adores so much.
-- "I\'m never wrong, once I thought I was, but was mistaken" Real Programmers Do things like this. http://webpages.charter.net/jamie_5
On Oct 24, 12:23 pm, default wrote:
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$129-159
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thanks for all of the info!... the wall wart was suggested with one of the simple schematics i downloaded... i ended up buying a coby, the cheapest of the cheap, which switches from 3v to 12v... though, as cheap as it is, my hope is that with knowing how to build a proper 5v supply, i can forget about the input (is this reasonable?)... i actually have a breadboard that i used to build the power supply, then transferred it to a small, more permanent strip board... the original actually had a 100 mf cap on the input, but my poor planning led to a disaster that forced me to remove it, leaving only the 10mf on the output... the sad part is that i understand how the capacitors work to smooth the signal, but i don't really understand how the input capacitor can help smooth since there is no load on the input and ground... though, the fact that i don't understand that may be a sign that i don't really understand this simple circuit with only a few components... thanks for the trainer info and links, but i'm not ready to shell out that much yet... i have a basic stamp, but am already losing interest simply because of the language... i'm a seasoned programmer, having started in c, and the basic language annoys me without cessation... i'm now looking at an arduino... though, i wonder if it makes better sense to spend the money on a more basic "learn electric circuits" kit?... now, i think i'll send up a flare for help in the immediate atlanta area... maybe someone will have time to take under his wing :)...
thanks, isaac
You are the one damming NTE parts for being out of spec, without checking the specs, donkey.
-- Service to my country? Been there, Done that, and I\'ve got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
snip
Here is some free info on regulators.
ON Semiconductor
You can also look here
pruned
Reasonable if you stay within the confines of the absolute maximum specifications of the chip.
Cheapest of the cheap doesn't necessarily mean it is a transformer since they do make universal switching supplies in a wall wart case these days. I make the distinction because a switcher would certainly have a built in filter cap.
From what you've said so far, I couldn't guess whether you need a filter or not . . . Cheap - suggests yes since they are usually only transformers and the manufacturer doesn't spend more money than absolutely necessary.
Coby brands a lot of DVD mp3 and other consumer electronics.
The output of a mains powered transformer/rectifier is a sine wave with every other peak missing in the case of "half wave" rectification and every peak but with the peaks all of one polarity with full wave (versus AC which has both positive and negative peaks).
Now that changing signal going into the regulator will still be on the output if there is no filter since it goes from zero volts to 1.4142 X the RMS value of the AC every 8 or 16 milliseconds (at 60 cycles mains). You need to filter it before the regulator if you expect the regulator to stay in regulation. The cap sits there and provides energy while the rectifier would be at zero volts. The bigger the cap the more energy it can supply - and the higher the "valley of the ripple" - low part of the ripple, in volts, on a scope.
If you filter after the regulator the regulator isn't really doing regulation just clipping the peaks in the rectified supply to a uniform five volts. Apply a load and the output ripple will skyrocket with only a 10 ufd on the output alone. Your meter will average that (ripple) value and show you a lower voltage. Digital circuits and analog circuits will have problems with a lot of ripple on the supply.
They usually use a small cap on the output of three terminal regulators - strictly speaking it is unnecessary. It is there to keep the regulator from oscillating as it tries to compensate for a high inductance load (AC theory)
You need to understand Inductance, Capacitance, Inductive Reactance, Capacitive Reactance, Resistance, Inductance, Impedance, and how it all ties together with RMS AC and frequency of the AC. Then this stuff makes sense - that would be chapter two? They aren't hard concepts to understand, but without it, a lot of what you are doing won't make sense.
Microprocessors and controllers can't do everything. It is an analog world and they don't groke that. There are lots of companies taking advantage of the fact to provide sensors that already do the conversions and output a serial stream of data - but that tends to be expensive, and ignores the multi channel A/D capability already built into things like the one board controllers or chips.
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