TP or OC output

[Frank Birbacher]

Hi!

I want to drive a transistor. It shall in turn drive a LED. The
transistor is to be driven by a TTL chip. I can choose between
open collector or totem pole output on the chip. Does it matter
except for the pull-up resistor for OC? Do I have to use a
resistor on TP, too?

VCC
|
[R] C, LED
--------+ | B |
OC-chip |--+----[transistor]
--------+ |
E, GND

C, LED
--------+ B |
TP-chip |--[R]---[transistor]
--------+ ^ |
| E, GND
needed?

Frank

 

[Andrew Holme]

Frank Birbacher wrote:

Hi!

I want to drive a transistor. It shall in turn drive a LED. The
transistor is to be driven by a TTL chip. I can choose between
open collector or totem pole output on the chip. Does it matter
except for the pull-up resistor for OC? Do I have to use a
resistor on TP, too?

VCC
|
[R] C, LED
--------+ | B |
OC-chip |--+----[transistor]
--------+ |
E, GND

C, LED
--------+ B |
TP-chip |--[R]---[transistor]
--------+ ^ |
| E, GND
needed?

Frank

Yes, you need a series current-limiting resistor between the TP output and
the base. Your diagram is correct.

You could dispense with the transistor altogether e.g.

Logic output (TP or OC) ------[R]------[LED]------+Vcc

 

[Frank Birbacher]

Hi!

Andrew Holme wrote:

Yes, you need a series current-limiting resistor between the TP output and
the base. Your diagram is correct.

You could dispense with the transistor altogether e.g.

Logic output (TP or OC) ------[R]------[LED]------+Vcc

No, I will need a high-current transistor. I want to drive a
1Watts LED. But what resistor shall I take between TP output and
transistor B? Does it rate as usual, e.g. 10-33kOhm? That's what
I usually take to drive a transistor from a 9Volts source. Well,
in logic it will be 5V, but it shouldn't matter, does it?

Frank

 

[Andrew Holme]

Frank Birbacher wrote:

No, I will need a high-current transistor. I want to drive a
1Watts LED. But what resistor shall I take between TP output and
transistor B? Does it rate as usual, e.g. 10-33kOhm? That's what
I usually take to drive a transistor from a 9Volts source. Well,
in logic it will be 5V, but it shouldn't matter, does it?

It depends how much current the LED requires. The resistor needs to be
small enough to saturate the transistor. Calculate maximum base current by
dividing collector current by hFE (min) from the manufacturers datasheet.

For example :-

collector current = 200mA (say, I don't know if this is appropriate)
Transistor gain hFE (worst case) = 50 (get actual value from datasheet)
Required base current = 200/50 = 4mA
Drop across base resistor = 5 - 0.7 = 4.3V
Use a 1k resistor

You might be better off using a logic level drive MOSFET. The gate can be
driven directly from logic without a resistor.

 

[Joerg]

Hi Frank,

As Andrew suggested, why not use a FET? The BSS123 could be a good
candidate and around 5 Cents in quantities it isn't expensive. Then you
can drive it with a TP output and no resistor.

But there always needs to be a resistor in series with the LED to set
the correct current.

Regards, Joerg

http://www.analogconsultants.com

 

[Frank Birbacher]

Hi!

Joerg wrote:

As Andrew suggested, why not use a FET? The BSS123 could be a good
candidate and around 5 Cents in quantities it isn't expensive. Then you
can drive it with a TP output and no resistor.

I have no idea about FET whatsoever. I need to drive the LED with
around 350mA. And the logic output is generating a pwm signal at
about 10kHz. So I'd be glad if you could provide me some diagram.

But there always needs to be a resistor in series with the LED to set
the correct current.

Of course. I know about that.

Frank

 

[Joerg]

Hi Andrew, Hi Frank,

You would better off with a logic level drive MOSFET. Use it like this
(view in fixed pitch font) -

VCC
|
V
- L.E.D
|
|
.-.
| |
| | R
'-'
|
|
||-+
|\ ||<- n-Channel
-| >O----||-+ MOSFET
|/ |
|
Logic ===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


That's the way to go.

Frank, the BSS123 I suggested isn't large enough for 350mA but you can
happily parallel them. Better yet, buy one that is rated for one amp or
more and where the Rdson (the resistance of the FET when the gate is
high) is under half an Ohm so you don't need to worry much about
dissipation. This Rdson needs to happen at the logic level you are
using. The data sheet will tell you that. This FET scheme will only work
well for 5V or higher, personally I prefer to drive them with CD series
logic running at 9-12V. But there are plenty of FETs that are marketed
for a 5V drive. Make sure the logic is still a CMOS gate when using 5V,
for example the 74HC or 74 HCT series. 74LS and other bipolar chips do
not reach the full 5V on high.

Keep traces to the FET short, the source needs to go directly to a
ground plane and the trace between the logic gate output and the FET
gate should be 1/2 inch or less.

Regards, Joerg

http://www.analogconsultants.com

 

[Frank Birbacher]

Hi!

Joerg wrote:

Frank, the BSS123 I suggested isn't large enough for 350mA but you can
happily parallel them. Better yet, buy one that is rated for one amp or
more and where the Rdson (the resistance of the FET when the gate is
high) is under half an Ohm so you don't need to worry much about
dissipation. This Rdson needs to happen at the logic level you are
using. The data sheet will tell you that.

Ok. Noted.

This FET scheme will only work
well for 5V or higher, personally I prefer to drive them with CD series
logic running at 9-12V. But there are plenty of FETs that are marketed
for a 5V drive. Make sure the logic is still a CMOS gate when using 5V,
for example the 74HC or 74 HCT series. 74LS and other bipolar chips do
not reach the full 5V on high.

Well, for the LEDs I will use another power source. I don't know
the exact rating, but I guess it will be more than 5V. The logic
is essentially driven by the 5V power from a USB port. So up to
the FET the power source is the USB port, and the LEDs connected
to the FET will be powered by a second source (the USB port
cannot supply enough current for the LEDs, of course).

Keep traces to the FET short, the source needs to go directly to a
ground plane and the trace between the logic gate output and the FET
gate should be 1/2 inch or less.

Wow, what is that good for? Is it just because of the resistance
of the trace and its voltage drop?

Frank