Fight Posted May 26, 2010 Report Share Posted May 26, 2010 In lm338 data sheet askingFeatures:Guaranteed 7A peak output currentGuaranteed 5A output currentBut problem is that in Electrical Characteristics:Vin-Vout = 30V Than max current 1Awhat is meaning that lets Vin = 38V and Vout = 8V38-8 = 30and Vin-Vout = 10V or less than 5A can handlemean 38-28 = 10mean 8 to 28V 1A handle and 28 to above 5A handleAttatch Diagram2. in Application Hints diagram all diagram is designed that can handle 5A Attatch Diagram Quote Link to comment Share on other sites More sharing options...
hotwaterwizard Posted May 28, 2010 Report Share Posted May 28, 2010 Exactly what are you wanting to do here?You Must Heatsink the components for 5 amps each if not they will only handle 1 amp each.There is another way to make a high current power supply. Quote Link to comment Share on other sites More sharing options...
hotwaterwizard Posted May 28, 2010 Report Share Posted May 28, 2010 http://www.national.com/ds/LM/LM138.pdfhttp://www.national.com/mpf/LM/LM338.html#Overview Quote Link to comment Share on other sites More sharing options...
Hero999 Posted May 28, 2010 Report Share Posted May 28, 2010 Exactly what are you wanting to do here?You Must Heatsink the components for 5 amps each if not they will only handle 1 amp each.There is another way to make a high current power supply.That circuit will have very poor regulation because the base-emitter saturation voltage of the 2N3055 will vary depending on the current drawn and the 0.47R resistors will have a significant voltage accross them.Suppose the current draw is 0A and you set the voltage to 12V.Then you increase the current to 5A.@0A the VBE of the transistors will be near 0.6V.Each transistor will have 5/6 = 833mA through it, the same for the emitter resistor.The voltage will drop by the 0.47R resistor will be 5/6 Quote Link to comment Share on other sites More sharing options...
Fight Posted May 28, 2010 Author Report Share Posted May 28, 2010 The LM338 will only be able to output 5A if the difference between the input and output voltage is under 10V and a good heat sink is used (1 Quote Link to comment Share on other sites More sharing options...
Hero999 Posted May 28, 2010 Report Share Posted May 28, 2010 Quote Link to comment Share on other sites More sharing options...
Fight Posted May 28, 2010 Author Report Share Posted May 28, 2010 Very Very Thanks Hero Quote Link to comment Share on other sites More sharing options...
Fight Posted May 28, 2010 Author Report Share Posted May 28, 2010 Thats good answer Quote Link to comment Share on other sites More sharing options...
Fight Posted June 5, 2010 Author Report Share Posted June 5, 2010 Attatch diagramwhat is the role of emittr resistance 0.1 ohmwhy using this resistor Quote Link to comment Share on other sites More sharing options...
Hero999 Posted June 5, 2010 Report Share Posted June 5, 2010 They help to ensure the current is shared equally between the transistors. The value isn't critical, I chose 0.1R because I originally designed it for high currents, for 5A you can use 1R which will be easier to get hold of. Quote Link to comment Share on other sites More sharing options...
Fight Posted June 6, 2010 Author Report Share Posted June 6, 2010 if i change the value of 1R instead of 0.1R than what change in circuit currentthe resistor wattage the voltage across R = ?the current across R = ? Quote Link to comment Share on other sites More sharing options...
Hero999 Posted June 6, 2010 Report Share Posted June 6, 2010 Changing the value will not change the current, the circuit does not provide current regulation, it's just that if the voltage drop across the resistors is very low, the current sharing won't be very good. Quote Link to comment Share on other sites More sharing options...
Fight Posted June 6, 2010 Author Report Share Posted June 6, 2010 it's just that if the voltage drop across the resistors is very low, the current sharing won't be very good. please explain it using examplesif 10R resistor use than Quote Link to comment Share on other sites More sharing options...
Fight Posted June 6, 2010 Author Report Share Posted June 6, 2010 what is the efficiency of these circuitif this method using with lm317 and lm338 than thses ic better current and voltage or not Quote Link to comment Share on other sites More sharing options...
Hero999 Posted June 6, 2010 Report Share Posted June 6, 2010 please explain it using examplesif 10R resistor use thanThere's no hard and fast rule, I just use current balancing resistors which have a voltage drop of a couple of hundred mV or so at full current. The voltage drop can be calaculated using Ohm's law.what is the efficiency of these circuitIt's as efficient as an linear regulator.if this method using with lm317 and lm338 than thses ic better current and voltage or not? Quote Link to comment Share on other sites More sharing options...
Fight Posted June 6, 2010 Author Report Share Posted June 6, 2010 There's no hard and fast rule, I just use current balancing resistors which have a voltage drop of a couple of hundred mV or so at full current. The voltage drop can be calaculated using Ohm's law.ok Quoteif this method using with lm317 and lm338 than thses ic better current and voltage or not?my mean that if using transistor current increasing method for lm317 or lm338 regulator ic than this method ok or not because volatge vary from 1.2 to 30 v7805 or 7812 is fixed regulator Quote Link to comment Share on other sites More sharing options...
Hero999 Posted June 6, 2010 Report Share Posted June 6, 2010 The variable voltage requirement doesn't make any difference, why would it? Quote Link to comment Share on other sites More sharing options...
Fight Posted June 6, 2010 Author Report Share Posted June 6, 2010 Attatch diagramfirst circuit is use for current limitingtransiter is use for current increasing transistor current provide 5A at all voltage range 1.2-30Vthis circuit is goodbetterbest or not Quote Link to comment Share on other sites More sharing options...
Hero999 Posted June 6, 2010 Report Share Posted June 6, 2010 It's not a very good idea because the extra regulator will increase the drop out voltage by 4.2V. Quote Link to comment Share on other sites More sharing options...
Fight Posted June 7, 2010 Author Report Share Posted June 7, 2010 than any idea for increasing current and tell me that this circuit provide good current limiting and 5A current provide at all voltage range 1.2 to 30V andgood voltage regulation from 1.2 to 30Vif 0-30V instead 1.2-30V than modification on circuit Quote Link to comment Share on other sites More sharing options...
Fight Posted June 7, 2010 Author Report Share Posted June 7, 2010 It's not a very good idea because the extra regulator will increase the drop out voltage by 4.2V.where 4.2V drop 2V drop across IC 1 that for current limiting/regulating0.7V drop across BE resistor2V drop across IC 2 that for voltage regulatingi am right or notif input voltage 38V than 4.7V drop acrosss these the output will vary easily 1.2 to 38-4.7 = 33.3Vyes or not Quote Link to comment Share on other sites More sharing options...
Hero999 Posted June 8, 2010 Report Share Posted June 8, 2010 The LM338 has a drop out voltage of 3V at full current and when configured as a current regulator, the voltage reference of 1.25V across the current sense resistor, adds to it making 4.25V. Quote Link to comment Share on other sites More sharing options...
Fight Posted June 9, 2010 Author Report Share Posted June 9, 2010 This circuit provide good current limiting and 5A current provide at all voltage range 1.2 to 30V andgood voltage regulation from 1.2 to 30Vif 0-30V instead 1.2-30V than modification on circuitThe LM338 has a drop out voltage of 3V at full current and when configured as a current regulatorif lm338 configured as voltage reglator than voltage drop across it = ? Quote Link to comment Share on other sites More sharing options...
audioguru Posted June 9, 2010 Report Share Posted June 9, 2010 It looks like you did not read the datasheet for the LM338:1) It shows a graph of the typical dropout voltage at various currents but does not list a max dropout voltage.2) It lists a max output current of 5A only when the Vin - Vout is 10V or less (then your minimum output voltage is 23V. It lists its max output current to be only 1A when Vin - Vout is 30V. Quote Link to comment Share on other sites More sharing options...
Fight Posted June 12, 2010 Author Report Share Posted June 12, 2010 hello hero where are you your site silicontronics does not open whyand you are angry with me whyplease answer my question Quote Link to comment Share on other sites More sharing options...
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