I was reading a forum which showed a picture one contributor added.\u00a0 The schematic of a circuit consisted of one opamp of an LM324, its output feeding the base of a transistor and the – input connected to the emitter.\u00a0 The transistor had its emitter connected to ground through a 1 ohm resistor.\u00a0 The collector was connected to the cathode (flat spot) of the LED, and the other LED lead was connected to +5v.\u00a0 The LM324 power pins went to +5v and ground.\u00a0 The + pin of the opamp was labeled “1 mA \/ mV”.<\/p>\n
Assuming that the LED is a regular 5mm and needs 20 mA, the +pin then must be supplied with 20 mV.\u00a0 That’s not so easy to do.\u00a0 For a 5V supply, that means a two resistor V divider must be added, with values of 250k for the upper and 1k for the lower resistor.\u00a0 It would have been better if the 1 ohm resistor was 100 ohms and the input was labeled “10 mA \/ V”, so 2 volts would then be needed at the input.<\/p>\n
But if the LED current does not need to be changed, and is a constant 20 mA, then this circuit is a waste of the opamp.\u00a0 A simple two transistor current regulator would do just as well.<\/p>\n
But if the supply voltage is relatively stable, then just a fixed value resistor would be just as good.<\/p>\n","protected":false},"excerpt":{"rendered":"
I was reading a forum which showed a picture one contributor added.\u00a0 The schematic of a circuit consisted of one opamp of an LM324, its output feeding the base of a transistor and the – input connected to the emitter.\u00a0 The transistor had its emitter connected to ground through a 1 ohm resistor.\u00a0 The collector <\/p>\n (Read More…)<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[12,13],"tags":[],"class_list":["post-5230","post","type-post","status-publish","format-standard","hentry","category-led","category-lighting"],"_links":{"self":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/5230","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=5230"}],"version-history":[{"count":8,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/5230\/revisions"}],"predecessor-version":[{"id":5652,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/5230\/revisions\/5652"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5230"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5230"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5230"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}