Circuit help

[Illum]

can someone give me tips on how to build a 5V power supply that can accomodate anywhere between 6-9V using the LM7805 IC?

My previous experiment on the LM7805 ended up with thermo runaway, so. tips appreciated :thanks:

years ago....I didnt know anything about adding a current limiting resistor
I did something like this
yeah it worked....but I had to submerge the IC in battery [deionized] water to cool it. Being the idiot that I am and the fact the 9Vs were inexpensive back then I bought 9V connectors from radioshack and wired 6 9Vs in parallel for "longivity" reasons...well, before the 9V cell died water was boiling

 

[BB]

Here is a link showing a basic setup...

Generally, the old "3-T" regulators are pretty bullet proof and usually include thermal shutdown... So, if you have thermal "run-a-way", then there may be something else that is wrong.

1. What current are you expecting. Remember the 3-T regulator is more or less acting like a variable resistor. So as an example:

P dissipated= V*I = (9V-5V) * 0.5 Amps = 2 watt

If you are using a 9 volt DC supply and expecting to supply 1/2 amp to your circuit, you will need a 2 watt heat sink to keep the LM 3-T cool. Notice, if the raw supply voltage goes up--the heat dissipated goes up too.

2. The 3-T regulators also need 1-2 volt drop across them to regulate correctly (check the spec).

3. Depending on your loads and wiring, you will probably need some capacitors. The large (10-100's of microFarad) aluminum electrolytic capacitors work well for low frequency (audio frequencies and lower) and the smaller ceramic capacitors work better at high frequencies (the smaller the cap, the higher the frequency). If you have problems with long (inductive) wiring and no-ground plane, it is best to have 1 larger electrolytic cap, 1x ~0.1 uF ceramic, and 1x 0.001 uF ceramic (on both the input and outputs if you have noisy power--or at least one electrolytic on the input, and one or two of the smaller ceramics on the output).

4. The Tab of the LM regulator is also electrically bonded to one of the leads (and is different between positive and negative regulators). Make sure that you don't accidentally short one of your other leads to the Tab.

5. Make sure of your pin numbering. IIRC the negative regulator and positive regulators have pin 1 in a different location--again, make sure you have the data sheet for your device handy.

I have used (and abused) similar parts before--and Jameco (a couple miles from my house) has a couple simple kits if you want:

Dual Adj. Regulator kit (+/-5vdc to +/-15 vdc)

<=40VDC to +1.2 to +35 VDC adj. regulator kit @ 1 amp

These kits use the LM 317 family--pretty much the same part as the LM7805 except it regulates at 1.2 volts DC instead of 5 VDC.

That is about all I can think of right now... Hope it helps.

-Bill

 

[BB]

ItN,

From your update--yes, you have found the downside of using linear regulators. Frequently, you have more energy going up as heat in the regulator than you actually have running out to the device under test.

If you need an efficient regulator and/or a larger supply current then you need to look at switch mode regulators.

You may have heard the terms before (buck, boost, buck/boost)--but these are more complex and usually need a pretty good ground plane/circuit wiring techniques. Generally, they don't work very well when built on a simple bread board or with soldering wires and components together in air because of the higher frequencies involved with switching power supplies.

Switching Power Supplies are used almost everywhere now because of their light weight and high efficiencies.

If you need more information, just ask.

-Bill

 

[Illum]

Thanks, this sounds like its going to be alot more complicated

Im interested with the set up at iguanalabs...but Im not sure how the breadboard is layed out so I have no idea how the circuit looks like

several webpages depicting the usage of LM78XX ICs recommend something about adding zener diodes with the breakdown voltage of 5 and a resistor to the Vout, so im assuming theres no open circuit protection?

 

[ckthorp]

They do sell 3-terminal drop-in replacements for the venerable 7805. They're way more expensive (~$10-15 in qty 1), but they are much more efficient. For a 1-off project where you're the end user, you'll pay for it in batteries pretty quickly.

Digikey link:
http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=75844&Row=924850&Site=US

This link is better than the first (which required 9v min Vin to run, not what you wanted). This will run down to 7v, not 6v, but close. A 1 volt in-out differential is really low, even my LDO linear regulator standards.

 

[Illum]

mmm, drop in replacements...well I suppose I'll take care of that part when I get there...I dont want to afford buying something expensive then later realizing the projects gone down the drain, I prefer to have a working prototype that charges MP3s and not light fires before going to see what imrpovements I can put on it

I saw two separate schematics on +5V power supplies
concept I with one 330uf cap

versus
concept II with 3 caps oo:


whats the difference?

 

[BB]

I am not sure of the 5V Zener and resistor on the output--sounds like it is a clamp to keep the output voltage at 5 vdc maximum--either to protect against a shorted LM 78xx (over voltage clamp) or if something like a 9 volt battery is attached to the 5 volt output (again, clamp). I would need to see a circuit to better understand--but for a normal low powered design--just the 3-T regulator and a couple of caps should be fine.

Normally, I cannot think of a reason to put an output resistor on the LM78xx--it would just cause the voltage to droop (poor regulation).

The LM78xx should work fine in open circuit/no load situations.

-Bill

 

[ckthorp]

Thanks, this sounds like its going to be alot more complicated

Im interested with the set up at iguanalabs...but Im not sure how the breadboard is layed out so I have no idea how the circuit looks like

several webpages depicting the usage of LM78XX ICs recommend something about adding zener diodes with the breakdown voltage of 5 and a resistor to the Vout, so im assuming theres no open circuit protection?

The breadboard here?: http://www.iguanalabs.com/7805kit.htm

If so, here is how it is wired: From the center grove to the side grove, the rows of holes are connected. Each set of 5 pins is connected. The pairs of 5-pin sets on the top and bottom halves of the breadboard are unconnected. The two strips of holes along both the top and the bottom of the photo (near the blue and red stripes) are connect from left to right so that there are two power "busses" along each of the two halves of the breadboard (four busses total).

The circuit consists of a power input fed directly into the 7805. The output terminal of the 7805 is connected to the capacitor. (And eventually your load, which also shares a connection with the battery input's ground).

I suspect that one of the following occured on your original 7805: it was connected backwards, the batteries were in series instead of parallel, or you were drawing an obscene amount of current. I've done it when in a hurry before. It's no biggie. When nothing is connected to the output, it should not get warm at all. As I recall, when you can read the label right-side-up (tab on top, pins on bottom), pin 1 is on the left. 1=input, 2=ground, 3=output. Connect a battery to 1 & 2, the capacitor and output in parallel to 3 & 2. That should power things up.

 

[ckthorp]

Concept I is your basic powersupply, though I'd put the cap on the output instead of the input if you're only going to have one. (This is specific to battery power. For wall power, you'd likely want more caps) Concept II is a more robust design. It is designed to handle transients (fast changes in load current) more effectively. The small output cap handles high frequencies and the bigger output cap handles lower frequencies. This has to do with how the capacitor's "equivalent series resistance" changes as a function of frequency. The input cap on "II" is to prevent sharp step changes in output current from propagating "upstream" towards the batteries/other circuits, as well as providing a large current source (for small amounts of charge) very close to the regulator which improves regulation.

 

[BB]

Typing over each other... But the extra capacitors are to keep RF noise (Radio Frequency) down and supply momentary surges in load. May be needed on your design or not--but usually a good idea to install the caps as close to the LM78xx as practical.

If you have poor bypass capacitance and/or long inductive wires with a noise (example, digital) load, you can make your circuits ring like a bell. May not cause damage put could cause noise in your audio devices or even buzzing on nearby radios.

The small caps are for RF suppression. The large cap is to supply momentary surges where the inductance of the wire would cause momentary regulated voltage drop-outs (up to around 20-50kHz).

-Bill

 

[ckthorp]

I should mention that for bypass caps, that more capacity is not always better. With these linear regulators, you're pretty safe, but can still potentially over do it (causing misc problems, though less-so on battery power). If you're charging things, just go look at LadyAda's "Minty-boost" circuit. http://www.ladyada.net/make/mintyboost/ It's powered by AA's which have much more W-h than 9v's, though the circuit is much more complicated. Maybe as future project?

 

[ckthorp]

Typing over each other...

Cheers! Lots of friendly help is what I love about CPF.

 

[Illum]

BB, i forgot to post a link where I found the zener info from...http://www.hackaday.com/2005/01/20/how-to-make-a-usb-battery/7#comments
here ya go

ckthorp, thanks that made a difference...I was reading it backwards

Typing over each other... But the extra capacitors are to keep RF noise (Radio Frequency) down and supply momentary surges in load.

The small caps are for RF suppression. The large cap is to supply momentary surges where the inductance of the wire would cause momentary regulated voltage drop-outs (up to around 20-50kHz).

First noise dampeners and now dealing with frequency....
from voltage to watts to ohms to this I've recieved an oncoming barage of info I may never find in a manual [or read correctly] :thanks: alot

If you're charging things, just go look at LadyAda's "Minty-boost" circuit. http://www.ladyada.net/make/mintyboost/ It's powered by AA's which have much more W-h than 9v's, though the circuit is much more complicated. Maybe as future project?

:wow: definitely looks alot safer and alot more complicated...."This project is suitable for beginners, some soldering tools are necessary but even if you've never soldered before it should be pretty easy." sounds promising, I think I might buy the kit rather than gather what I can from ratshack.

Im assuming this is a totally difference concept than the LM78XX so a heatsink is not required?

Hows Ladyada in terms of sales service? I've never heard of this place

the Mintyboost looks a heckalot more professional than the Aaron Dunlop model
vs

 

[ckthorp]

That hack-a-day circuit is a simple zener voltage regulator. If you have access to 7805's, I'd shy away from a zener regulator unless I had a specific reason to use it. This is because the 7805 is superior to a zener regulator in every way that I can think of -- more power capacity, more efficient, better regulation, etc.

I'm glad you figured out your 7805 issue. They're handy little chips (though grossly inefficient, they are cheap and plentiful)

I've never personally dealt with LadyAda, but she is highly regarded by "Make Magazine". I do not feel that you would have any problems, but I don't have any personal experience to base that on, just her general high regard in the community.

The MintyBoost's circuit is much more efficient than a 7805. Powered from a 9 volt battery and outputting 5 volts, the 7805 is about 50% efficient. For a 0.5A load, the 7805 dissapates about 2 watts. In comparison, the MintyBoost is likely about 90% efficient. For the same 0.5A load at 5v, the MintyBoost only dissapates about 0.25 watts. 2 watts usually needs a heatsink, 0.25 is usually fine.

I really like the MintyBoost because it uses a "boost" switching regulator. Than means that the output voltage is actually higher than the input voltage. Ounce for ounce, a pair of AA NiMh's will kick a 9v NiMh's butt. (2x2500mAh AA NiMh is roughly 3.6 watt-hours versus 1x270mAh 9v NiMh which is 2.4mAh, or 2/3rds of the capacity and far less plentiful.)

 

[TMorita]

Try this. Drop-in replacement for 7805, but is a switching regulator.

http://www.dimensionengineering.com/DE-SW050.htm

Toshi

 

[ckthorp]

It appears my previous digikey link is broken. Here is a fixed version:
http://www.digikey.com/scripts/DkSearch/dksus.dll?PName?Name=78SR105VC-ND&Site=US

Though the one posted directly above this link appears to be a much better unit.

 

[Handlobraesing]

7805 is very very very common and it is almost always used in small computer peripherals using an AC adapter such as a router, hub, switch etc. The AC adapter provides 9-12v (9v adapter is 9v under full load, but usually 12-15 under no load) and the 7805 inside the device regulates to 5.0v needed by semiconductor parts. You definitely need to use a heatsink with the regulator and don't make the input voltage unnecessarily high as higher the voltage, more loss there is.

Another way of doing this is using a 5.0v switch-mode type AC adapter and it's getting more common. If you're in need of 5v from AC line, get an Iomega Zip drive from Good Will. The AC Adapter is regulated 5v 1A.

 

[Illum]

AC adapter such as a router, hub, switch etc. The AC adapter provides 9-12v (9v adapter is 9v under full load, but usually 12-15 under no load) and the 7805 inside the device regulates to 5.0v needed by semiconductor parts.

I know exactly what your talking about, AC wall worts that are "3.6V out" measures around 9V on my meter


I found the circuit used for ladyada's minty boost


it looks distractingly complicated, but since it has better efficiency I'll go the distance.
While ordering a set I bought a couple extra caps just incase I solder one on backwards...[done that before...]

 

[billw]

The "mintyBoost", including philosophy, performance, schematic and PCB and such, is described in great detail here: http://www.instructables.com/id/EGBQJPLCB2EP287KTZ/

 

[MrAl]

Hi there,

All of the linear regulators (that includes the 7805) require some sort of
heat sinking or else they go into thermal cut off if the power dissipation
is too high. For 9v in and 5v out at 1 amp the 7805 would be trying
to dissipate four watts. That's quite a bit of power, so a decent
heat sink would be required.

Also, the 7805 regulator will only work down to about 7.5 volts.
They make a "low dropout" version which works down lower, like
down to around 6 volts.