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-MARCH NEWS LETTER

ISOTRON 20/15/10
SO SIMPLE AND SO RUGGED!
 
This article will also appear on our web site.
 
MAKING A SMALL CURRENT REGULATED POWER SUPPLY
 
Last article was about making a simple voltage regulated power supply. The device used are the LM78xx or LM79xx series device. An inexpensive component that is easy to make use of. The "xx" refers to the voltage desired.
 
This is good for a good clean constant voltage. However what if we want to regulate the current so it does not exceed a certain value?
 
This device will also do this.
 
For example, you may want to charge a battery, but do not want to exceed a current value. You may want to connect a string of LED's, all with the same current rating.
 
Below is a schematic of how to change the voltage regulator into a current regulator. Which regulator would you choose?
 
For a + current above ground, you can use any of the LM78 series. Voltage does not matter. For a - current in respect to ground, use the LM79 series.
 
Below is a LM78 device. R1 determines the value of the current limit. Using a LM7805 regulator with R1 at 250 ohms, the maximum current will be about 20 milliamps. This is ideal for a small LED. If you put more than one LED in series, the regulator will maintain 20 ma. It will adjust the voltage to deliver the 20 ma current and no more to light all the LED's.
 
For higher currents, reduce the size of R1. R1 does not need to be a high power resister, 1/4 to 1/2 watt is fine.
 
When R1 equals 10 ohms on a LM7805, the current limit is about .5 amps. You can increase it, but the LM7805 has a maximum rating of 1.5 amps.

                                                          
Input voltage should be 35V max.     

 
If the device is used as a current regulator, you can actually short the output leads together without damaging the device.
 
Some applications will require a definite voltage value. Then, the voltage regulator circuit is what you want. However, if voltage is not critical, but you would like to protect the circuit from damage, the current regulator works well.
 
73,
Ralph WD0EJA
 
01-16
 
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