SV: Re: [sdiy] weird voltage regulator problem / question

[Karl Ekdahl]

Hi Harry!

> s-diy is the BEST school :^P

I'd say! 

> 1)  If your meter is reading DC without much
> ripple... that
> effectively IS RMS.
I wasn't making myself clear, sorry. What i ment was
that the AC readout before the rectifier was 14v and
that i guess that most multimeters return RMS and not
peak voltage... (right?)

> 14V (meter) is 14V.  14 - 9 = 5V (drop across the
> regulator)
> Take your current (whatever you draw, in amps) and
> multiply by the drop.
> (70mA =)  .07A * 5V = .35W
I totally get the math here, i'm just slightly
confused as why the voltage i'm dropping is the only
thing that matters, what about the remaining 9v?
Doesn't they come in to play at all, generating any
heat?

> Multiply this value by the thermal resistance of the
> regulator.
Ah! This is awesome, i'm definetly saving this e-mail
for future reference!


> If the
> ambient is 25C the
> regualtor
> will be 44C.
I guess this should be O.K. i mean, i drink tea hotter
than that!

> If this is TOO hot, add a heatsink. Usually the
> bigger, the better.
> The heatsink will have its own thermal resistance.
I threw in a random piece of heatsink that i had to
cut up to make it fit, so i've got no numbers. But as
i said - future reference!!

> 3)  "Internally regulated" means that if you get the
> junction TOO hot,
> the
> regulator will shut down, and protect itself from
> overheating. 
Aha, so that's all they ment.

> 4) Wobbling in pitch.
> The 9V should be steady. Measure the input to the
> regulator on AC
> volts...
I measured about 0.2V after rectifier by the filter
caps (1000uF is a little overkill i guess, but it
should be able to kill any possible ripple) for both
positive and negative supply so i guess i'm o.k. here
as well.

> This variation would usually be 60Hz or 120Hz...
> more an FM growl than
> a warble, IMHO
That's the thing, i don't get a steady fluctuation,
just a little bend up and down that seems to react on
RFI. Based on this i'd say that something in the VCO
isn't grounded, but still, it happends while playing
with the power cord that's far away from the VCO....

> Are you with me ???
> 
> Check this out, then reply
Awesome lecture Harry, things are becoming clearer and
clearer all the time! Thank you so much for this! I'll
see if i can find that &/#"% vco error elsewhere. oh,
bottom question; the current psu temp seems ok, right?
it doesn't hurt my fingers touching it anymore.... :)

karl

 
> H^) harry
> 
> 
> Karl Ekdahl wrote:
> > 
> > Okay, so now i have a 12v line to my regulator(s),
> 14v
> > says my multimeter so i guess 12v would be RMS?
> Anyhow
> > a lot less, but maybe still too much since the
> > heatsink i added which is very small. From what i
> > gather this would result in (14-9)*0.07=350mW
> (70mA
> > app. usage) that needs to be dissipated.
> Unfortunley
> > the datasheet only says "internally regulated" on
> > power dissipation.
> > 
> > If i understand the calculation below regarding
> the
> > voltage drop correctly it's 24v - 9v = 15v and
> then
> > what? 15v * 10v = 150ohm, yes, but why?? Anyhow, i
> > guess then that what i need for 12v would be
> something
> > in the vicinity of (14-9) * 10 = 50 ohm (@ 1W) ...
> > right?
> > 
> > sorry for asking such fundamental questions, i
> really,
> > really need to go to school again....
> > 
> > Anyhow. Why i'm asking is also because i noticed
> the
> > oscillator (powered by said psu) wobbling in
> pitch,
> > and especially if touching any powercable. With my
> > limited
> > experience this tells me there's a powersupply
> problem
> > and i can't really think of anything being wrong
> with
> > it except maybe above issue. (Yes, i have tried
> > putting filter caps everywhere; no luck so far)
> > 
> > thanks people
> > 
> > Karl
> > 
> > ----------------------------
> > 
> > The heat given out by the regulator is solely
> > dependant on the voltage
> > across the regulator and the current through it.
> > 
> > If you have 24V going in, and 9V out, that's 15V
> your
> > regulator needs to
> > drop.
> > 
> > At 60mA, you have a power dissipation of 60mA x
> 15V.
> > Which is 900mW.
> > 
> > Power dissipation = [Vin - Vout] x Iout
> > 
> > It is no surprise that your 79L09 is getting warm,
> in
> > fact, I'm
> > surprised it hasn't burned up yet. Check the
> maximum
> > heat dissipation
> > for the device on the datasheet and you will see
> that
> > you need to be a
> > lot lower than that, even if you can encourage
> some
> > air flow.
> > 
> > I would add a dropper resistor in series with the
> > input to help with
> > heat dissipation. If you try to drop around 10V
> across
> > the resistor,
> > then this should be fine. R will be roughly 150R
> or
> > so.
> 
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