The first thing that comes to mind when the experimenter wants to find out how much current is flowing through the LED is to measure it with a DMM set to read milliamps.\u00a0 This works okay if the LED is connected in series with a current limiting resistor.\u00a0 The DMM will add maybe 10 ohms to a resistance of several hundred ohms, so it is hardly noticeable.\u00a0 But when the 10 ohms is added to a circuit that is driving the LED with pulses, then 10 ohms may affect the circuit and skew the readings.\u00a0 For example, if the circuit is driving the LED with 100 milliamp pulses, then the voltage drop across 10 ohms is…\u00a0 1 VOLT.\u00a0 Instead of 3 volts across the LED we have 4 volts across the LED and internal resistor.\u00a0 The result is a current reading which is too low and far from what it would be without the DMM.<\/p>\n
What to do?\u00a0 Well obviously lowering the DMM’s internal resistance would help. If we look at what is going on inside the DMM when it is reading milliamps, we see a voltmeter across a known resistance, known as a shunt.\u00a0 Well, we can move that known resistance outside of the DMM, and use the DMM as a voltmeter to measure the voltage drop across it and calculate the current.\u00a0 Instead of 10 ohms internal resistance, we use a 1 ohm resistor, and measure the voltage across it.\u00a0 Since 1 amp through 1 ohm gives 1 volt, then 100 milliamps through 1 ohm will read 100 millivolts.\u00a0 Likewise 100 millivolts means there is 100 milliamps flowing in the resistor.\u00a0 And if the resistor is in series with the LED, there is 20 milliamps flowing through the LED and resistor when there is 20 millivolts across the resistor.<\/p>\n
Since 1 ohm is 1\/10 of the internal 10 ohm resistor, the circuit is disturbed much less, and the reading is much closer to what it would be without the resistor.\u00a0 This 1 ohm resistor is known as a current sensing resistor, even though it would be called a shunt if it were inside of the DMM.\u00a0 We can temporarily put this resistor in series with the LED, or even leave it there permanently if it doesn’t affect the LED too much – 20 milliamps times 20 millivolts is 0.4 milliwatt, which is a very small loss of power in this resistor<\/p>\n
Back to experimenting….<\/p>\n","protected":false},"excerpt":{"rendered":"
The first thing that comes to mind when the experimenter wants to find out how much current is flowing through the LED is to measure it with a DMM set to read milliamps.\u00a0 This works okay if the LED is connected in series with a current limiting resistor.\u00a0 The DMM will add maybe 10 ohms <\/p>\n (Read More…)<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[6,4,12],"tags":[],"class_list":["post-682","post","type-post","status-publish","format-standard","hentry","category-electronics","category-joule-thief-smps-dc-dc","category-led"],"_links":{"self":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/682","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=682"}],"version-history":[{"count":7,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/682\/revisions"}],"predecessor-version":[{"id":1554,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/682\/revisions\/1554"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=682"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=682"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=682"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}