How do you work out the required base current from the ULN2803 Datasheet??
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Hey
I want to use the ULN2803 to interface to high current devices. I
have read the data sheet but I have been unsuccessful in working out
how much base/input current will be drawn at the maximum ouptut
current of 600ma Peak. Can anyone help me out and tell me how much
it will be. It would also be a great help if you could teach me how
to work it out my self also.
Many thanks
Arridh
I want to use the ULN2803 to interface to high current devices. I
have read the data sheet but I have been unsuccessful in working out
how much base/input current will be drawn at the maximum ouptut
current of 600ma Peak. Can anyone help me out and tell me how much
it will be. It would also be a great help if you could teach me how
to work it out my self also.
Many thanks
Arridh
Comments
I'm looking at the STMicro data sheet at
http://us.st.com/stonline/books/pdf/docs/1536.pdf
Note on the upper right corner of page 6 the graph titled "Figure 15: Input
Current as a Function of Input Voltage (for ULN2803A)." At the nearly 5
volt output from the Stamp I/O pin, you can see that the current is expected
to be in the 1.3 mA to 1.9 mA range. I just measured all the input currents
on one ULN2803A, and they averaged 1.5 mA -- right on target.
This device has another characteristic that should be kept in mind... The
Darlington transistor configuration guarantees a significant voltage drop in
the device. In other words, when it is switched on the voltage drop between
the connection to the load device and ground is substantial -- not 0.0 volts
as desired. Any switching device will have some voltage drop, since it
isn't a perfect conductor, but Darlingtons have quite a bit more than some
other common alternatives.
At 500+ mA, you will generally see a 1.3+ volt drop. So if your power
supply voltage to the load is 6.0 volts, only 4.7 volts will be applied to
the load. The other 1.3 volts (or more) will be wasted in the ULN2803A
(along with the associated heat dissipation). For some applications this is
acceptable, but for others it isn't. A power MOSFET, for example, could be
used to dramatically reduce the unwanted voltage drop in the switching
device.
Randy
www.glitchbuster.com
Original Message
From: "arridh_shashank" <arridh_shashank@y...>
To: <basicstamps@yahoogroups.com>
Sent: Saturday, January 10, 2004 11:07 PM
Subject: [noparse][[/noparse]basicstamps] How do you work out the required base current from
the ULN2803 Datasheet??
> Hey
>
> I want to use the ULN2803 to interface to high current devices. I
> have read the data sheet but I have been unsuccessful in working out
> how much base/input current will be drawn at the maximum ouptut
> current of 600ma Peak. Can anyone help me out and tell me how much
> it will be. It would also be a great help if you could teach me how
> to work it out my self also.
>
> Many thanks
> Arridh
>
>
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<arridh_shashank@y...> wrote:
> Hey
>
> I want to use the ULN2803 to interface to high current devices. I
> have read the data sheet but I have been unsuccessful in working
out
> how much base/input current will be drawn at the maximum ouptut
> current of 600ma Peak. Can anyone help me out and tell me how much
> it will be. It would also be a great help if you could teach me how
> to work it out my self also.
>
> Many thanks
> Arridh
The word devices caught my eye here.
600mA is peak, 500mA is continuous, per channel.
Also the package total heat dissapation is 2.25 watts so you cannot
have all 8 channels at 500mA continous.
Interestingly enough, you can get TO92 package mosfet's that will
carry an amp, but TO220 package mosfets will cost less and carry more
amps. I didn't check the specs on the parts, but a 60V 27amp TO220
package cost less than a TO92, 1amp package.
Check http://www.glitchbuster.com
a set of eight PN2222 that can each carry 500ma is only slightly more
than one 2803.
A IRF510 TO220 MOSFET 100V 5.6A is a single channel device that still
costs under a buck.
Dave