Re: Voltage Regulators


Abstract Dissonance wrote:
I'm tried to do my ps but I'm having some problems. I ended up burning out
one of the regulators for some reason but the real issue is that the
regulators don't seem to be regulating.

If I setup a very basic transformer->rectifier->filter->regulator

then with no load I get the right voltage but with a 2.2k ohm + LED load the
voltage drops from 8 to 4. Its a MC78M08 regulator and surely it shoudln't
drop 4 volts at that load? I don't use any of the "extra" stuff such as
protection diodes and ripple reducing capacitors and the voltage I'm putting
in is about 35 volts.

The transformer is a 25.2VCT and I'm using the full secondary. The bridge is
4A@600V and the filter cap is a 200V @ 1mF(I know I could use a larger one
but surely it is enough).

As far as I can tell I'm not getting any.............


hmmmmmm


WTF. I just switched my DMM down to 20V cause I was measuring it on 200 and
now its reading 7.74V with variations going from +- 1V about every second.

I didn't know I had to choose the right voltage range on my DMM to get an
accurate reading ;/ Is this true? That really sucks if so ;/

i.e., turning the switch to 200V gives me a reading of 4.4V and switching it
to 20V gives a reading of 7.74V. Any reason why it would do this? Low
battery? Messed up meter? Or do I just need to make sure to select the
proper range?(which seems kinda odd cause you would have to know it ~ before
hand).

Thanks,
Jon

Hi, Jon. Take a deep breath, and look at things one at a time.

Before anything else, check your meter. It shouldn't change readings
between the 20V and 200V range. Get a known good 9V battery, and
measure 9V on the 20V and 200V ranges. If they don't agree, replace
the battery on the meter and try it again. If the meter still doesn't
agree between ranges with a fresh battery, junk it and get another one
for $10 or $15 at the hardware store. You need a known good meter to
troubleshoot.

First, you want to look at your unregulated, filtered power supply. To
see if it's working properly, try putting a load on the supply _before_
the regulator. As a rule of thumb, you should figure that, for a 60Hz
supply (120Hz ripple), 8,000uF will give you 1V of ripple per amp of
current. That means a 1,000uF cap will give you 8V of ripple per amp.
On your DVM, that 8V will look like a 4VDC reduction per amp, because
the ripple is roughly a triangle wave. So, get a power resistor, load
up the power supply, and see what's going on.

Let's assume you have a 100 ohm, 20 watt resistor handy. Put it across
the 35V peak supply. You'll have about 1/3 amp load, which means 1/3 *
8 or about 2.7V of ripple. On your DVM, that will look like your 35V
will go down half that, or down 1.4V to about 33.6V. If your DC
voltage measured across the cap doesn't go down by about 1.4V +/- 0.25V
with that 100 ohm resistor load, then something's wrong. Your bridge
rectifier may be hooked up wrong. Your cap may not be 1000uF. Your
cap may be old and bad. Your cap may actually be 1uF (which would
cause the problem you describe). Your transformer may be messed up.
But you will _know_ that there's a problem in that part of the power
supply. And if you have another value power resistor, work it out for
that load. But if that part of the power supply performs properly,
then you know that's where the problem _ain't_.

Once you get that taken care of, look at your regulator. Just get a
12VDC wall wart. Put the + at the input pin, with the - of the wall
wart goijng to the middle pin of the 7808. Measure the output voltage.
Yes, there might be some high frequency oscillations, but it _will_
give you 8V +/-10%, or it doesn't work. Then put a 220 ohm resistor
between the output pin and GND, and see if the output voltage changes.
If it changes more than several mV with a 36 mA load, it's busted.

If you look at the data ***, you'll see that the Absolute Maximum
input voltage to that device is 35V, and you've already come right up
against that limit. You need to think this one out a little better, or
lay in a good supply of spare parts. I'm figuring your 7808 is damaged
(they act that way sometimes when they've been overstressed).

Putting 35VDC at the input of an LM7808 isn't doing it right, Jon. Not
only that, but let's say you're pulling 1/2 amp from the 7808. With a
1000uF cap, you should have an average DC of 33V. Your power
dissipation will be (33V - 8V) * 0.5A = 12.5 watts. Lotta extra heat,
here.

The point of troubleshooting is to make good decisions based on
available information. Read the data***, Jon. That's always
important. And Absolute Maximum means that, if you exceed that, even
by a millivolt, the manufacturer says you will have problems. But when
you're doing something new, or even troubleshooting something that's
known to have worked, you have to be open to the available facts. Try
to make the observations that will make it apparent where the problem
is, but more importantly, where it is _not_.

Make good observations, and believe your observations, even if they
don't seem to make sense. The things you're seeing and measuring are
trying to tell you something, if you can take the time to learn their
language and are prepared to listen to them.

You also may want to take a minute and actually think out what you're
doing here, and see if you can get there with the parts you have.
Again, I'd really recommend just buying one or two open-frame linears,
combine them with an LM317 for the variable voltage, and be done with
it.

(And by the way, people stopped caling 1000uF "1 mF" about 35 years
ago. If your cap says "1mF" on it, the cap is almost certainly a
fossil, and shouldn't be used, anyway.)

Good luck
Chris

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