so-hpa@stekt.oulu.fi (Heikki Paananen) (11/13/90)
tindle@ms.uky.edu (Ken Tindle) was seeking for a current
generator circuit for the LED bar graph display.
I say, there is many on-chip current generators available today
(unfortunately I can recall not even one type at this moment) But
generally speaking they all are high precision types (and you know
"high precision" means the device costs at least an arm and a leg.
When one is using the generator for driving LED(s), I think characters
like temperature stability and output impedance are less critical.
So I would like to persent my favourite circuit ( and with no doubt I
am not the only one):
Vsupply = +20V --------------------
I I
R1 R2
I I
a I
D1 I
k I
I e
--------------bT1
I c
R3 I
I * Is = Current output
GND
where, R1=1K
R3=10K
D1=1N4148
T1=BC557 (PNP transistor)
R2=1.75V/Is Where Is=required output current
1.75V is the voltage over R2
(and over R1 too!)
In some variations R1 is replaced with a diode or D1 is omitted.
(Sorry that Non-standard-ASCII -circuit diagram.) Note that the number
of LEDs connected serial the generator can drive depends on the
amount of supply voltage provided:
X = (Vsupply - Vsat)/VLED
where,
X = quantity of LEDs
Vsupply = Supply voltage (in this case 20V)
Vsat = Saturation of the generator (approx. 2V)
VLED = LED`s forward voltage drop (approx 2V).
And what is the best, the circuit above costs only 1USD or less!
(One more tip: See the data for LM3914 bar graph driver IC!)
I hope this helps!
Best regards,
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Heikki Paananen so-hpa@stekt.oulu.fi
The University of Oulu so-hsp@finou.oulu.fi
Department of electrical eng. hpa@stek1.oulu.fi
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