5v Voltage Regultor

[Sentient]

I built a basic little 5v regulator which doesnt seem to be working correctly.

Basic meaning, 240VAC transformer -> 18VAC -> bridge rectifier -> 7805 with a few caps for good measure.

It is a basic fixed-voltage circuit that I pulled off the 7805 datasheet. The problem I have is that it appears to supply only 4.40VDC on the output when measured with a multimeter (10MOhm resistance).

I checked the circuit, and there is 15VDC going into the 7805, but only 4.40VDC coming out. Is it because the multimeter doesnt put enough load on the output, or is there a problem (possibly with the 7805)??

[GARTHWILSON]

The 7805 really should have at least 5mA of load, but I would expect the voltage to be too high (not too low) if you didn't have a load on it.

[schidester]

I have limited experience with the 7805, but I once hooked one up backward. After I inserted it correctly, it output nearly 5V with only the voltmeter loading it, but as soon as I loaded it with anything, the voltage dropped dramatically.

[orac]

Hi Everyone,

Also, be careful about the 15V going into the part.

This means that the regulator is dropping 10 volts.


Cheers,

Paul

[Sentient]

I have no idea what the problem was but after talking to a local electronics nut, I just rebuilt the circuit.

The schematic I had didnt advise an input filter (other than the 4700uF for the 50Hz). I have since removed the transformer and rectifier and gone for a wall-plug that outputs 7vdc.

With a input filter and output filter, 7vdc in, i get a nice clean 5vdc output - seems simpler has solved the problem.

[GARTHWILSON]

> Also, be careful about the 15V going into the part.
> This means that the regulator is dropping 10 volts.

An IC in that TO-220 package can generally dissipate a couple of watts in still air at room temperature without a heatsink, but I like to limit the amount of time they have to run at temperatures too hot to touch. 100mA times the 10V drop is only 1 watt, so you have enough margin to run a home-made computer, even with 15V input to the 7805.

Our company's products use a lot of LM317T's which are in the same TO-220 package. A 20V drop (28V from the aircraft, down to 8V) is very common. We tell the customer to just be sure the spikes stay below 50V, and in our 13 years, we've never had a unit come back for repair because of overvoltage, and I don't think we've ever had an LM317T or 78xx go bad for any reason. In some models the metal box acts as a heat sink, and in others, there is no heat sink. In every case you just have to account for the wattage dissipated and the maximum temperature of the ambient air or box metal where the heat is transferred (including if the aircraft is being started up after having been parked in the sun in the summer in Phoenix, AZ), and make sure you're inside the regulator manufacturer's recommended limits.

> and gone for a wall-plug that outputs 7vdc.

That's probably a much neater, simpler solution; but 7V is almost the minimum the regulator can use and still give a good, dependable 5V output. Any lower, and the output will start to dip, because it take a minimum of about 1.5V input-to-output difference to do its job at low currents and room temperature. It would be good to make sure that when you take the ripple into account, the troughs don't dip much lower than 7V at whatever your maximum anticipated load current is.

[Sentient]

I have an 18VA transformers here with taps for 12,15 and 18 volts.

Apart from the fact that the 12v tap isnt working for whatever reason, the thing weighs a ton and is about 1.5" x 2".

How do power packs (like my lil 7v one) convert from 240v -> 7v without a huge transformer??

I might expect a smaller pack if it had less current, but this power pack is still good for 1A, and even the 30v one I have is lightweight and supplies 750ma.

Is there a better (lighter) way to drop ac voltages?

I've heard toroidial ??sp?? is the new thing, but havent seen one implemented before.

[GARTHWILSON]

For a given frequency (like 60Hz) and core material (like laminated iron compound), the weight of the core material should be more-or-less proportional to the VA rating. You can lighten up the transformer by going to a higher frequency, so a lot of military aircraft use 400Hz. In our power grid, I suppose the frequency choice was a compromise between transformer size, skin depth, and the complications of the wavelength when you're trying to transmit power across hundreds or even thousands of miles. At higher frequencies, skin depth is less (meaning the wires need more surface area to get the current through without excessive losses from heating, since the current only travels on the outside of the conductor, not the inside), and the wavelength is shorter (meaning you'll have more problems with standing waves and reflected power). If you get shocked, 60Hz is just about the most dangerous frequency for your heart; and if you do it just right, you can kill yourself with as little as 60V. So according to the old joke, when they were choosing the powerline voltage and frequency, they picked the worst frequency, and doubled the voltage just to be sure.

In a DC power pack (like a "wall wart"), there will be rectifiers and a capacitor in there too. More capacitance will result in lower ripple but bigger size. A higher voltage rating on the capacitor will result in longer life and bigger size. If it's regulated, the regulator takes room (and produces heat) but they might have tried to get away with a smaller capacitor (which may also produce more heat), and the unit will run hotter and have a shorter life expectancy. IOW, there are a lot of factors involved in the size. Until I worked at TEAC in the early 1980's, I expected that the capacitors designed into power supply circuits were always derated enough that their life would be practically indefinite; but TEAC seemed to have a way of choosing the smallest power supply capacitors that would get them through the warranty period. I replaced a lot of them.

Torroids have been around for decades. They have the advantage on not causing interference with nearby magnetics or being disturbed by magnetic fields, but they are larger and more expensive to wind. You can get a large range of toroid sizes and materials for RF and AF applications. It would be a major project to try to wind the thousands of turns of a power transformer on a torroid by hand. Even when wound by machine, torroids are generally deemed to be too expensive for most power transformer applications. Our parent company used to make transformers of all sizes and shapes from much smaller than a shirt button for RF torroids, up to multi-ton laminated-core power transformers for malls. The largest torroidal transformers I've worked with weighed hundreds of pounds. I don't have the magnetics expertise to do any major design work with them though. I have to just work from data sheet formulas that were determined by the core designers who really know the physics of magnetics. Supposedly electrical engineers know that stuff, but it's really a field all its own in which most of us are pretty ignorant.

[bogax]

GARTHWILSON wrote:

For a given frequency (like 60Hz) and core material (like laminated iron compound), the weight of the core material should be more-or-less proportional to the VA rating. You can lighten up the transformer by going to a higher frequency, so a lot of military aircraft use 400Hz.

Quote:

In a DC power pack (like a "wall wart"), there will be rectifiers and a capacitor in there too. More capacitance will result in lower ripple but bigger size.

And the power is more or less proportional to the square root of the frequency.

So if you bump the frequency way up, the transformer and capacitors both can be a lot smaller

I can't say I know for sure that they put switchers in wall warts, but I have
seen wall warts that were 2 or 3 cubic inches that were rated for 2Amps.
(and so I surmised they must have little switch mode power supplies in them)

Quote:

Torroids have been around for decades. They have the advantage on not causing interference with nearby magnetics or being disturbed by magnetic fields, but they are larger and more expensive to wind.

Sounds like you know a lot more about it than I.
I thought, the last I'd heard, they'd decided that pot cores are better both
in terms of interference and losses (not to mention ease of winding)?

[kc5tja]

bogax wrote:

I can't say I know for sure that they put switchers in wall warts, but I have
seen wall warts that were 2 or 3 cubic inches that were rated for 2Amps.
(and so I surmised they must have little switch mode power supplies in them)

Considering the RF hash that comes off of many wall-warts, I think it's an absolute certainty that this is the case.

Quote:

Sounds like you know a lot more about it than I.
I thought, the last I'd heard, they'd decided that pot cores are better both
in terms of interference and losses (not to mention ease of winding)?

The problem with torroids is that it's more expensive (by a long shot, apparently) to make them laminated. I've never seen a laminated torroidal core before. Also, it's not feasible to wind, by hand or by machine, hundreds of turns on a torroidal core for power transformer use because it's not feasible to split the ring across its diameter either. Both lamination and split-cores are used to reduce the heating of a core, and thus, loss in power. Interestingly enough, if you were to do that with a torroidal core, you would lose some of its advantages in not being affected by stray mag-fields, and would be more "noisy" in terms of emissions.