# Looking to drop 6 volts dc to 4.5 volts dc

Discussion in 'Beginner Electronics' started by .D.E, Sep 28, 2006.

1. ### .D.EGuest

I'm looking to add a mod to my pinball machine with a couple of led's and
need to drop 6 volts dc down to 4.5 volts dc. Can anyone suggest which
resistor to use?

Thanks.

--
_____________________________
Later.
Have a better one.
D.E.
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2. ### Rich Grise, Plainclothes HippieGuest

R = (Vsupply - Vled) / Iled

Good Luck!
Rich

3. ### petrus bitbyterGuest

No, due to lack of information. Suppose the LEDs do 4.5V when lighting, what
current do they use? You'd realize that LEDs are current driven devices so
the current can easily double while the voltage hardly changes.

petrus bitbyter

4. ### AlGuest

If your LED is specified at 10mA, then get two diodes whose forward
voltage is 0.75V at 10ma. Put them in series with your LED. If the
supply voltage goes up and the current goes up, the forward voltage of
the diodes will go up also thereby dropping more voltage across them.
Thus the dynamic resistance of the dropping diodes serves as a voltage
regulator. Really good diodes with a low dynamic resistance won't do.

Al

5. ### petrus bitbyterGuest

That diodes will work when you have a resistive load. But neither LEDs nor
other diodes behave like resistors. Once more: You need to regulate the
current, not the voltage. If the LED has been specified for 10mA at 4.5V
you'll need a series resistor of (6-4.5)/10=0.15k that's 150 Ohm. Most LEDs
are specified for 20mA or more. FAIK only low power LEDs require less.

petrus bitbyter

6. ### jasenGuest

yeah, if the current doubles the voltage may increase maybe 10%
how's that going to help?
it seems to me that regular resistors would work better,

7. ### AlGuest

If your current doubled for some reason, you have other big problems.

Al

8. ### AlGuest

A series resistor is not a constant current source.

And, this method is not for manufacturing millions of items. It's for
solving a special home problem. Like I said, I do it and it works for
me. No, I don't bake the circuit nor do I freeze it. And as I mentioned
above, diodes with soft knees should be used.

Al

9. ### AlGuest

The poster said his source voltage was 6 volts. Since he was using it to
drive LEDs, I presumed, perhaps falsely, that it was a steady DC. He
should have specified a range, such as 6Vdc +/- 0.5V.

Is it a true DC as derived from a dry cell? Is it pulsating DC derived
from either a half-wave or a full wave diode bridge? Does he have an LC
filter on the output of the bridge? Or is it just a big rectifier across
the bridge? Is there a linear regulator or a switcher involved? As
someone else in this thread had suggested, it might be the output of a
6.3V filament transformer that is rectified.

All of these factors, and probably many others, would have to be
considered.

The brightness of an LED is a function of the current through it.
Typically it specified to have a certain light output level at a
specified current. You may increase or decrease the current as you will.
The lifetime of the LED will depend on the current as will its light
output. Even the specified current is just a normalization of the
readings from a large sample. Your specfic LED may need more or less
current for the specified light output.

So, if the LED is specified to give a certain light output at 10mA at
4.5V and if the source voltage is a constant 6V, I would use two diodes
whose forward voltages are specified as 0.75V at 10mA to give me a 1.5V
drop.

The forward voltage drops of a typical 1N914 diode are shown as:

Vf If
volt ma

0.6 3.0
0.7 10.0
0.8 30.0

Two diodes in series would give me the approximate 1.4V drop close to
what I would need.

Al

10. ### redbellyGuest

That doesn't sound like it would work. Now you have an extra 0.1 V on
the LED. The current could be easily double (or more) what is wanted.
"Close to the right voltage", in an LED or any diode, just doesn't cut
it in terms of limiting the current.

And if the source voltage is NOT a constant, exact 6V, it's even worse.

Resistors are the standard, simple easy way to limit current through an
LED.

Mark

11. ### Jim ThompsonGuest

Has anyone mentioned to just use a current source? Even 0.5V headroom

...Jim Thompson

12. ### jasenGuest

Typically leds are specified with a voltage range for their limit current.
get the curren right and the voltage will be somewhere in that range, it
depends on the device and the environment.

If that particulasr led is 4.4V with 10mA flowing through it it could with
4.5V the current could be 30mA or more.
And when they get warm, it drops, this can lead to thermal runaway.

ordinary resistors consistently out-perform ordinary diodes as a LED
current source.

Bye.
Jasen

13. ### AlGuest

Sigh! Yes, that's true. But I routinely use one or two diodes, depending
upon current, to drop my 6Vdc battery down to a nominal 5Vdc that my
circuits use. It works. What can I say? It's a cheap and dirty way of
doing it for home projects.

For example, I have a circuit which nominally draws 50ma during
operation. Upon the needs of the project, a 200mA pulse (100mS) drives a
latching relay. If I used a 20 ohm resistor to drop the voltage to 5Vdc
during norminal operation, it would really screw up the circuit during
the pulse as the series resistor would drop the voltage by 4V!

So, tell me, how many components would you use to drop your 6Vdc to 5Vdc?

Al

14. ### ehsjrGuest

What someone else would use is irrelevant. You use
whatever you want to meet the circuit requirements.
Here's a possibility for your pulsed 200 mA.
Substitute whatever you're using in place of the switch.

/
+6 ----+-----o o--------+
| |
[20R] [5R]
| |
| |
Gnd ---+----------------+

For the LED circuit that was discussed a resistor
is fine. For something that requires regulated 5,
you can use an LDO like
http://www.national.com/ds/LP/LP38690.pdf

For something like a 4.5V toy motor, your diodes
may be fine.

Ed

15. ### AlGuest

You are correct!

I was just using the 200mA load situation just to show how a resistor
would not do the job. And, one should use what works in their particular
situation.

I've always wondered why the industry settled on 5 volts as the standard
for TTL esp. since 6 volt batteries were so common. In the 60's I
actually worked on a logic system which did use +6 and -6 volts for the
logic. I can't remember if +6 was a one and -6 was a zero, or was it
visa versa? For experimentation with this logic type, we actually used 6
volt lead/acid batteries for power. We quickly learned never to short
anything here!

Al

16. ### DanielGuest

When I did my electronics training in mid-70's, I can recall a cct that
was part positive logic (1=-5V, 0=+5V) and part negative logic (1=+5V,
0=0V).

Just a little confusing.

Daniel

17. ### Guest

If I understand your question correctly, most people makes voltage
regulators out of Zener diodes. However, I suppose you could make a
voltage regulator using a couple of LEDs although it probably wouldn't
be as accurate.

For information on how to do this look at instructions for doing it
with a Zener and then simply adapt them for using LEDs instead. For
information on making a regulator using a Zener, type in ZENER VOLTAGE
REGULATOR in Google. Here's a couple of examples: