Circuit which includes a thermistor-resistor combination to compensate for temperature fluctuations by supplying input voltage to light-emitting diode, maintains constant light output. Similar circuits can be used for temperature-induced variations in photodiode applications

Burrous, C. N.

Deboo, G. J.

English

NASA Technical Reports Server

1972	 B72-10218 
NASA TECH BRIEF 
Ames Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Temperature Compensation of Light-Emitting Diodes 
The problem: 
To devise a circuit which will maintain the light 
output of a light-emitting diode constant as the 
ambient temperature changes.
LED 
Rth
E1 
The solution: 
Supply the input voltage to the light-emitting 
diode by a circuit which includes a thermistor-
resistor combination to compensate for temperature 
fluctuations. 
How it's done: 
The thermistor, Rth, shown in the diagram is a 
resistive element with a negative temperature coeffi-
cient. The other resistors in the circuit are conven-
tional, high quality components with nearly zero 
temperature coefficients. 
The operational amplifier used in the circuit is 
conventional; however, it must have high gain, low 
input current, and essentially zero offset. The light-
emitting diode, whose radiated output is to be main-
tained constant as the temperature varies, is identified 
as LED in the diagram.
When a potential is applied to the input terminal 
E 1 , the amplifier supplies power to the light-emitting 
diode; the output of light can be controlled by the 
input voltage. For example, suppose that E 1 is a 
constant, positive voltage, and that the temperature 
remains consant; because of current flow in the cir-
cuit comprised of Rth, R3 and Ri, a portion, ei, of 
the voltage applied at the input will appear at the 
positive terminal of the amplifier. The output voltage 
that is supplied by the amplifier to the series com-
bination of LED and R2 will be of such magnitude 
that the feedback voltage (e2) will reduce the net 
voltage across the input terminals of the amplifier 
to essentially zero, that is, el = e2. The current flow-
ing through LED must be the same as the current 
through R2 because the amplifier effectively draws 
no current and, therefore, the current through R2 
must be directly proportional to ei as well as e. 
However, if the temperature varies, the resistance 
of the parallel combination Rth and R3 will vary, and 
this will result in a change in the magnitude of e. 
By appropriate selection of components, the change 
in ei resulting from a change in temperature as 
sensed by the thermistor can be made to compensate 
for changes in the light output of an LED. For 
example, an experimental circuit held the radiated 
power output of an LED constant to better than 1% 
over the range of 10° to 50°C; without compensation, 
the change in light output would be more than 100%. 
Notes: 
1. Techniques are available for designing thermistor-
resistor networks which will produce the variation 
of el with temperature necessary to maintain LED 
radiated power output essentially constant over a 
wide temperature range.
(continued overleaf) 
R2 
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2. Similar circuits can be used for compensation of 
temperature-induced variations in photodiode ap-
plications. 
3. A combination of a temperature-compensated 
LED and a temperature-compensated photodiode 
provides a very stable photon-coupled isolator. 
4. Requests for additional information may be di-
rected to: 
Technology Utilization OJilcer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP.72-10218
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to: 
Patent Counsel 
Mail Code 200-1A 
Ames Research Center 
Moffett Field, California 94035 
Source: Gordon J
.
 Deboo and
Clifford N. Burrous
Ames Research Center 
(ARC-10467) 
B72-10218	 Category 01


Temperature compensation of light-emitting diodes

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