NOVICE question re: regulator and prop setup

mickal

hi, i have very few parts just nows and want to run a propellor. I have a LM394 not even knowing what it was likley a regulator I tried using it with a 4.2 volt(3 rechargeable batteries)
The power has only come down to 3.6 volts. maybe i need to look at this more closely but would 3.6 volts ruin the prop ?

LoopyByteloose

It is always useful to Google the part number for a "PDF" file, a fact sheet provided to engineers.

I looked for the LM394 and it doesn't seem to be a regulator. It may be a pair of transistors and the voltage drop from 4.2volts to 3.6volts may simply be due to an internal diode effect. This is definitely NOT a good approach to voltage regulations. Get the right part.

I suspect that 3.6 volts would work if enough milliamps were available, but I also doubt that is normal operation range for any voltage regulator. They usually come in set voltages -- 3.3v, 5v, and so on or adjustible.

Undervoltage is generally not a hazard, but going 5% to 10% above the operating voltage is not considered okay.

It may be something simple - like using the wrong components with the regulator. Take a look at the facts provided and compare them to what you are exactly doing.

mickal

Thank. I did google before posting and it does seem to be the perfect regulator.The LM3940 is a 1A low dropout regulator designed to provide 3.3V from a 5V supply.

The LM3940 is ideally suited for systems which contain both 5V and 3.3V logic, with prime power provided from a 5V bus.

Because the LM3940 is a true low dropout regulator, it can hold its 3.3V output in regulation with input voltages as low as 4.5V.

The T0-220 package of the LM3940 means that in most applications the full 1A of load current can be delivered without using an additional heatsink. As it is meant to bring voltage down from 5v to 3.3v . the only thing i havent done is add capacitors. Still does seem something is incorrect getting drop from 4.2 to only 3.6v.

The full part number is LM3940IT ,that probably helps when googling!

Do the values of the capacitors affect the operation ? I am using much bigger, 1000uf, the jpeg shows 0 .47uF

I am just looking at the PE kit voltage diagrams. I have a propellor and prop plug but no crystal or much else really

localroger

With LDO regulators the output capacitors are important -- they need to be the value and type specified and they need to be as close as possible ot the regulator pins. They aren't there to smooth out ripple, they are there to prevent a kind of feedback oscillation which can occur in their absence. Larger capacitors are not always better, often they don't have enough reactance at high frequencies.

ElectricAye

mickal wrote: »

....

Because the LM3940 is a true low dropout regulator, it can hold its 3.3V output in regulation with input voltages as low as 4.5V.....

mickal,

if the data sheet says you need a minimum of 4.5 volts and you're only giving it 4.2 volts, might that be a problem?

Forrest

Your answer is in the data sheet your posted - input voltage needs to be at least 4.5V for the regulator to function properly. The solution is to add a 4th battery for your power source

Invent-O-Doc

And remember, with batter powered projects, that voltage is the 'maximum' you can expect early in the battery's charge cycle - it will go down according to a curve that depends on battery chemistry. The regulator is fine; you do need another battery or another higher voltage solution.

LoopyByteloose

Firstly, your typo, not mine. LM394 versus LM3940.

Secondly, this is a rather special regulator intended for 4.5v to 5.5 volt input. The design concept is that you are using both a 3.3v and 5.0 volt system. So the idea of adding another battery may put you over the top, while the 4.2 volt leaves you under the bottom.

I use www.futurelec.com to help me select components from time to time as they have all the PDFs and the type are grouped together. You might look for a different regulator. Or, you really need to consider having a regulated 5.0 volt supply feed this one.
Anyway, Futurelec has all the commonly available regulators listed together and you can sort out exactly which one is ideal for you. About the worst choice is a 'perfect regulator' that noone sells in small quantities.

If you want to keep your batteries to a minimum, you may have to use a boost regulator to get 5.0 volts and then come back down to 3.3 volts. That seems rather silly, but Pololu does something like that with their 3pi robot. At 4.2volt suppy, it might just be simpler to regulate with a zener diode and a few descrete components. Or insert a couple of diodes in series with the supply line to bring the voltage down to 3.3volts or less.

mickal

Loopy Byteloose wrote: »

Firstly, your typo, not mine. LM394 versus LM3940.
.

I use www.futurelec.com to help me select components from time to time as they have all the PDFs and the type are grouped together.

. Or insert a couple of diodes in series with the supply line to bring the voltage down to 3.3volts or less.

yeah sorry bout the misreading of part number. Hey I was using futurelec.com.au to read datasheets , diff country same suppler :-)

Sorry I wasted everybodys time. I added fourth battery ,things deteriorated ...meaning voltage went up . That is until I reversed the regulator!! Immediately I got my 3.3 Volts. Well who'd a thought Sorry.

Ttailspin

I don't think You wasted anybody's time... It's a good reminder.:nerd:

One of My favorite things to do is solder a resistor to the wrong place on a LM317 or a LM350T and only get 1.2 volts.

LoopyByteloose

We all make these kind of mistakes. NO reason to feel foolish about them. If you follow this Forum long enough, most of the support issues get resolved when someone calls attention to something that was overlooked, but obvious.

Aside of reversed insertion, I tend to get a lot of problems that are from overlooked solder bridges. So now, I completely check the continuity of anything I build before I power it up. Developing your own good, solid technique is rewarding.

sam_sam_sam

This site has a min power board that
I have used a few time in the past before it is a five volt regulatating board how ever you do have to change the regulator to a three.three volt regulator there are no other part that need to be changed

One note how ever it is easier to remove the I.C. Chip with a (which is a link for $19 onlinek) desoldering iron show en below because it it is a double side it board

BTW I have done this in the past and it dose work
http://www.futurlec.com/

BTW futurlec dose not carry the /LM2575-3.3G IC chip how ever Mouser dose
http://www.futurlec.com/Mini_Power.shtml

You can buy the three.three volt regulator at the page below


Here is the link to the data sheet

http://www.onsemi.com/pub_link/Collateral/LM2575-D.PDF

Available Now for only USD $6.90 for the min power board below

I hope this helps you