Should I add a resistor the the base...?

K

Kasterborus

Guest
I'm working with an RGB LED and an Arduino MPU. I wired the LED into 3
of the PWM outputs and with the correct resistor on each leg it gives
me a nice range of colors.

Now I want to drive 20 LEDS and so I added 3 heavy duty NPN
transistors and will be driving the LEDS through these.

My question is - should I wire the PWM output of the Arduino straight
to the base of the transistor, or would something like a 10K resistor
be a good idea to limit the current?

Dave
 
Kasterborus <kasterborus@yahoo.com> writes:
My question is - should I wire the PWM output of the Arduino straight
to the base of the transistor, or would something like a 10K resistor
be a good idea to limit the current?
Click to expand...
NPN transistors are current multipliers, so you can do the math
and figure it out. Example:

Need: 20 mA
hFE: 100 (this is the current multiplier number)
base current = 0.02 / 100 = 0.0002 (0.2mA)

Vbe: 0.7 V
Voh: 3.2 V
Vres = 3.2 - 0.7 = 2.5 V

Res = Vres / Ibase = 2.5 / 0.0002 = 12.5k (or less, for wiggle room)

You'll have to fill in your own numbers, of course. Note that you can
buy transistors with the resistors built-in, comes in handy. Also, if
you replace the NPN with an N-MOSFET you don't need the resistors.
For your usage, they're pretty much interchangeable.

Also, keep in mind that more transistors means more base current (if
you wire them all to the same output pin), so either use one resistor
per transistor (best) or one smaller resistor (not as good, in case
the transistors aren't matched closely).
 
On Wed, 2 Apr 2008 18:21:26 -0700 (PDT), Kasterborus
<kasterborus@yahoo.com> wrote:

I'm working with an RGB LED and an Arduino MPU. I wired the LED into 3
of the PWM outputs and with the correct resistor on each leg it gives
me a nice range of colors.

Now I want to drive 20 LEDS and so I added 3 heavy duty NPN
transistors and will be driving the LEDS through these.

My question is - should I wire the PWM output of the Arduino straight
to the base of the transistor, or would something like a 10K resistor
be a good idea to limit the current?

Dave
Click to expand...
Consider using a logic-level compatible n-channel mosfet. Then you
don't need a resistor, and there's no load on the port pin.

John
 
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:loh8v395sn5sl3j7jgl0c48cuabncbvdpi@4ax.com...
On Wed, 2 Apr 2008 18:21:26 -0700 (PDT), Kasterborus
kasterborus@yahoo.com> wrote:

I'm working with an RGB LED and an Arduino MPU. I wired the LED into 3
of the PWM outputs and with the correct resistor on each leg it gives
me a nice range of colors.

Now I want to drive 20 LEDS and so I added 3 heavy duty NPN
transistors and will be driving the LEDS through these.

My question is - should I wire the PWM output of the Arduino straight
to the base of the transistor, or would something like a 10K resistor
be a good idea to limit the current?

Dave

Consider using a logic-level compatible n-channel mosfet. Then you
don't need a resistor, and there's no load on the port pin.

John
Click to expand...
John,

Have you ever come across a so-called logic-level fet that has a Vgs(max) of
1V -- even if Rds(on) is fairly big (several ohms would be okay).

I've used fets that meet TTL levels, but none that will work with FPGA
outputs in banks that have their Vcco set to 1.2V (or lower).

Thanks.
Bob
 
"BobW" <nimby_NEEDSPAM@roadrunner.com> wrote in message
news:RsCdnenQyvWT0GnanZ2dnUVZ_gCdnZ2d@giganews.com...
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message news:loh8v395sn5sl3j7jgl0c48cuabncbvdpi@4ax.com...
On Wed, 2 Apr 2008 18:21:26 -0700 (PDT), Kasterborus
kasterborus@yahoo.com> wrote:

I'm working with an RGB LED and an Arduino MPU. I wired the LED into 3
of the PWM outputs and with the correct resistor on each leg it gives
me a nice range of colors.

Now I want to drive 20 LEDS and so I added 3 heavy duty NPN
transistors and will be driving the LEDS through these.

My question is - should I wire the PWM output of the Arduino straight
to the base of the transistor, or would something like a 10K resistor
be a good idea to limit the current?

Dave

Consider using a logic-level compatible n-channel mosfet. Then you
don't need a resistor, and there's no load on the port pin.

John


John,

Have you ever come across a so-called logic-level fet that has a Vgs(max)
of 1V -- even if Rds(on) is fairly big (several ohms would be okay).

I've used fets that meet TTL levels, but none that will work with FPGA
outputs in banks that have their Vcco set to 1.2V (or lower).

Thanks.
Bob


There is the FDC604P, with a typical threshold of 0.75 volts and maximum
Click to expand...
1.5. It's a P-channel, 20V, 5.5 amps, in an SSOT-6 package. For N-channel,
try FDC637AN, 0.82 V typical Vg, 20V, 6.2A. It has an LTSpice model in the
library. Or FDR6580, or FDR844, or FDS6064. The last part has a typical
threshold of 0.6V.

http://www.fairchildsemi.com/ds/FD%2FFDS6064N3.pdf

Check the product selector guide for 1.8V characterized parts:

http://www.fairchildsemi.com/collateral/psg04_discrete.pdf#page=18

Another possibility is to use a small pullup resistor on the FPGA output to
a slightly higher voltage (maybe about one diode drop, or 1.2 + 0.6 =
1.8V). It should not do any damage, and the output might be soft enough to
be pulled a little above the supply rail. Otherwise, you could use a diode
and a pullup on the gate to get a level shift that will drive a more usual
gate.

Paul
 
"Paul E. Schoen" <pstech@smart.net> wrote in message
news:47f47d83$0$19838$ecde5a14@news.coretel.net...
"BobW" <nimby_NEEDSPAM@roadrunner.com> wrote in message
news:RsCdnenQyvWT0GnanZ2dnUVZ_gCdnZ2d@giganews.com...

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message news:loh8v395sn5sl3j7jgl0c48cuabncbvdpi@4ax.com...
On Wed, 2 Apr 2008 18:21:26 -0700 (PDT), Kasterborus
kasterborus@yahoo.com> wrote:

I'm working with an RGB LED and an Arduino MPU. I wired the LED into 3
of the PWM outputs and with the correct resistor on each leg it gives
me a nice range of colors.

Now I want to drive 20 LEDS and so I added 3 heavy duty NPN
transistors and will be driving the LEDS through these.

My question is - should I wire the PWM output of the Arduino straight
to the base of the transistor, or would something like a 10K resistor
be a good idea to limit the current?

Dave

Consider using a logic-level compatible n-channel mosfet. Then you
don't need a resistor, and there's no load on the port pin.

John


John,

Have you ever come across a so-called logic-level fet that has a Vgs(max)
of 1V -- even if Rds(on) is fairly big (several ohms would be okay).

I've used fets that meet TTL levels, but none that will work with FPGA
outputs in banks that have their Vcco set to 1.2V (or lower).

Thanks.
Bob


There is the FDC604P, with a typical threshold of 0.75 volts and maximum
1.5. It's a P-channel, 20V, 5.5 amps, in an SSOT-6 package. For N-channel,
try FDC637AN, 0.82 V typical Vg, 20V, 6.2A. It has an LTSpice model in the
library. Or FDR6580, or FDR844, or FDS6064. The last part has a typical
threshold of 0.6V.

http://www.fairchildsemi.com/ds/FD%2FFDS6064N3.pdf

Check the product selector guide for 1.8V characterized parts:

http://www.fairchildsemi.com/collateral/psg04_discrete.pdf#page=18

Another possibility is to use a small pullup resistor on the FPGA output
to a slightly higher voltage (maybe about one diode drop, or 1.2 + 0.6 =
1.8V). It should not do any damage, and the output might be soft enough to
be pulled a little above the supply rail. Otherwise, you could use a diode
and a pullup on the gate to get a level shift that will drive a more usual
gate.

Paul
Click to expand...
Thanks, Paul. I'll take a look at those Fairchild parts.

Also, I like your idea about pulling the output higher than its Vcco
(assuming the higher supply is available). I believe that Xilinx rates each
output's protection diodes to +- 10mA without direct damage or any chance of
inducing scr latchup.

Bob
 
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