Basic question: 7.5V Lamp ona 9V output

[Luca]

Veery basic question:
I would like to connect a 7.5 Volt Lamp (radio shack) on a 9V outpuut
(Lego Mindstorms).
What's the best way to prevent the lamp from burning?
Is it sufficient to put a resistor in series? If so, what resistance value?
Or should I connect the lamp trough a voltage devider to bring the voltage
down to, say, 7V?
If so, which resistors values should I use? (I understand that a general
rule for voltage dividers is to use the lowest resistance I can use
compatibly witht he load).

Thank you in advance

Luca

 

[Tim Wescott]

On Wed, 17 Dec 2008 20:07:06 -0500, Luca wrote:

Veery basic question:
I would like to connect a 7.5 Volt Lamp (radio shack) on a 9V outpuut
(Lego Mindstorms).
What's the best way to prevent the lamp from burning? Is it sufficient
to put a resistor in series? If so, what resistance value? Or should I
connect the lamp trough a voltage devider to bring the voltage down to,
say, 7V?
If so, which resistors values should I use? (I understand that a general
rule for voltage dividers is to use the lowest resistance I can use
compatibly witht he load).

Thank you in advance

Luca

You need to drop 1.5V to get from 9V to 7.5V, and E = IR (voltage equals
current times resistance).

So do a bit of algebra to get R = E/I, find out the lamp's rated current
at 7.5V, and find your resistor value.

Or just use the lamp as is, enjoy the extra brightness, and plan on
replacing it every six months.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

 

[John Fields]

On Wed, 17 Dec 2008 20:07:06 -0500, "Luca" <Thot@comcast.net> wrote:

Veery basic question:
I would like to connect a 7.5 Volt Lamp (radio shack) on a 9V outpuut
(Lego Mindstorms).
What's the best way to prevent the lamp from burning?
Is it sufficient to put a resistor in series?

---
Yes.
---

If so, what resistance value?

---
Vsupply - Vlamp
Rs = -----------------
Ilamp
---

Or should I connect the lamp trough a voltage devider to bring the voltage
down to, say, 7V?

---
No.
---

If so, which resistors values should I use? (I understand that a general
rule for voltage dividers is to use the lowest resistance I can use
compatibly witht he load).

---
N/A
---

Thank you in advance

---
You're welcome, in the present.

JF

 

[Tom Biasi]

"Luca" <Thot@comcast.net> wrote in message
news:4949a23a$0$5296$9a6e19ea@unlimited.newshosting.com...

Veery basic question:
I would like to connect a 7.5 Volt Lamp (radio shack) on a 9V outpuut
(Lego Mindstorms).
What's the best way to prevent the lamp from burning?
Is it sufficient to put a resistor in series? If so, what resistance
value?
Or should I connect the lamp trough a voltage devider to bring the voltage
down to, say, 7V?
If so, which resistors values should I use? (I understand that a general
rule for voltage dividers is to use the lowest resistance I can use
compatibly witht he load).

Thank you in advance

Luca


Just use the bulb like it is.

 

[Bob Masta]

On Wed, 17 Dec 2008 20:07:06 -0500, "Luca"
<Thot@comcast.net> wrote:

Veery basic question:
I would like to connect a 7.5 Volt Lamp (radio shack) on a 9V outpuut
(Lego Mindstorms).
What's the best way to prevent the lamp from burning?
Is it sufficient to put a resistor in series? If so, what resistance value?
Or should I connect the lamp trough a voltage devider to bring the voltage
down to, say, 7V?
If so, which resistors values should I use? (I understand that a general
rule for voltage dividers is to use the lowest resistance I can use
compatibly witht he load).

Thank you in advance

Luca

If you want to use a series resistor to drop the
voltage, you need to know the normal operating
current of the lamp. If you want to drop 9V down
to 7.5V, that's a 1.5V drop. The resistance
needed will be R = V / I, so you just divide
voltage drop by the operating current (in amps).
To find the power dissipated by the resistor, use
P = I * V.

Instead of a resistor, you can use one or more
rectifiers in series. Each rectifier will drop
about 0.6 or 0.7 volts, depending on current.
Make sure the rectifiers can handle the current...
rectifiers are cheap enough that you can use a
generous safety margin.

Note that the rectifiers will dissipate just as
much heat as the equivalent resistance to drop the
same voltage. But it's usually easier to do with
rectifiers, unless you have a nice assortment of
low-ohm, high-watt resistors handy.

Best regards,


Bob Masta

DAQARTA v4.51
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
FREE Signal Generator
Science with your sound card!

 

[Jamie]

Luca wrote:

Veery basic question:
I would like to connect a 7.5 Volt Lamp (radio shack) on a 9V outpuut
(Lego Mindstorms).
What's the best way to prevent the lamp from burning?
Is it sufficient to put a resistor in series? If so, what resistance value?
Or should I connect the lamp trough a voltage devider to bring the voltage
down to, say, 7V?
If so, which resistors values should I use? (I understand that a general
rule for voltage dividers is to use the lowest resistance I can use
compatibly witht he load).

Thank you in advance

Luca


Put 2 SI diodes in series. They'' give you ~ 1.5 drop.

http://webpages.charter.net/jamie_5"