cyw1984 Posted September 11, 2005 Author Report Share Posted September 11, 2005 Here's an idea to make the current limit programable.U3 is a differential amplifier with a gain of R4/R3, this measures the volagate across Quote Link to comment Share on other sites More sharing options...
cyw1984 Posted September 11, 2005 Author Report Share Posted September 11, 2005 Hi nearly old man Staigen,You are absolutely correct! I was way off with my selection of high-value "turn-off" resistors for the output transistors in my "fixed" circuit. I have now chosen lower value resistors based on what is inside a TIP120 darlington.I used a 1k resistor in series with the collector of the 2N3906 to limit its collector current and dissipation if the project's output is shorted. It will still provide enough drive to the output transistors to blow a fuse if the output is shorted.I think the circuit can deliver 30V at 1.5A with a 34V supply to protect the 36V max-rated opamp.It would be fun to try to stop this extremely high gain circuit from oscillating. ;DHi Alun and Cyw,I don't think a single 2N3055 or power Mosfet can dissipate more than about 60W even on a pretty big heatsink. Therefore the max current is only 2A at low output voltages. That's why I proposed using 2 output transistors in our 0V-30V modified project for 3A output, and using 3 output transistors on our modified for 5A project.Thank you for your help....Can the cirucit deliver 30V at 2A??On the other hand, I would like to use LM358 for testing, Is it OK??any op-amp is prefectly on this cirucit??As I know CA3140 is 4.5MHz, BiMOS Operational Amplifier with MOSFET Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted September 11, 2005 Report Share Posted September 11, 2005 Which U1, U2, U3, diode and P-channel MOSEFT is prefect in this circuit???ThanksThe MC34071 will be fine for U1, but U2 needs to swing down to below 0.6V and U3 needs to swing as low as possible, I was going to suggest the the CA3130 but it's only rated to 16V, :( perhaps audioguru can think of a suitable op-amp. Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 12, 2005 Report Share Posted September 12, 2005 Hi Alun,It is difficult to select U3 for your circuit.It needs inputs that operate near the positive supply, and an output to ground. Cmos opamps do this, but have limited supply voltage rating.If its 10k input resistors were increased so that they have 1.8V or more voltage drop then an MC34071 opamp could be used if you can tolerate its output going only as low as about 0.1V. ;D Quote Link to comment Share on other sites More sharing options...
cyw1984 Posted September 12, 2005 Author Report Share Posted September 12, 2005 audioguru old man, I have tied your circuit....It has some problem......It use 15K resistor series as DAC input....The Vref is not between 0-10V.voltage drop on the resistorAlso....the output is exactly the same of VinWhat is the cause???I used the LM358 to test (Vin 36V and Gnd it) Quote Link to comment Share on other sites More sharing options...
Staigen Posted September 12, 2005 Report Share Posted September 12, 2005 Hi there againMaybee the LM358 blow, its only good for 32 Volts, and you put 36 Volts on it!Hey cyw1984, cant you read? The datasheet clearly says its max voltage is 32 Volts!And Audioguru, why didn't you included some type of voltage regulation or something for the poor little IC? A zener and a resistor and a cap will do that!//Staigen Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 12, 2005 Report Share Posted September 12, 2005 Hi Staigen,Yeah, the LM358 is blown-up. It is supposed to have a gain of 3.0.I didn't regulate the supply for the opamp because the MC34071 that I recommended and most other modern opamps have a + power supply rejection ratio of about 82dB at the rectifier ripple frequency. So 1V of ripple on the supply will cause only 0.1mV of ripple at its output. ;D Quote Link to comment Share on other sites More sharing options...
Staigen Posted September 12, 2005 Report Share Posted September 12, 2005 Hi AudioguruI didn't regulate the supply for the opamp because the MC34071 that I recommendedOk, i missed that! :(//Staigen Quote Link to comment Share on other sites More sharing options...
cyw1984 Posted September 12, 2005 Author Report Share Posted September 12, 2005 thanks againI get a problem here...If my Vin 30 or 32V..can the output achieve 30V???how to change the circuit...because I want to common the VCC and GND to supply voltage....And what values of filter is suitable???thanks Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 12, 2005 Report Share Posted September 12, 2005 If my Vin 30 or 32V..can the output achieve 30V?Of course not. The max output voltage will be 3V to 4V lower than its supply voltage.how to change the circuit...because I want to common the VCC and GND to supply voltage?I don't know what you mean.And what values of filter is suitable?You'll have to try different values until its transient response isn't too slow, and that it doesn't oscillate. Quote Link to comment Share on other sites More sharing options...
cyw1984 Posted September 12, 2005 Author Report Share Posted September 12, 2005 Of course not. The max output voltage will be 3V to 4V lower than its supply voltage.I don't know what you mean.You'll have to try different values until its transient response isn't too slow, and that it doesn't oscillate.I want to common the Opamp VCC and GND to the Vin and GNDhow to do that- -sorry fot my poor english Quote Link to comment Share on other sites More sharing options...
ante Posted September 12, 2005 Report Share Posted September 12, 2005 Hi Guys,How about the MAX420 it might work here! ::) Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 12, 2005 Report Share Posted September 12, 2005 I want to common the Opamp VCC and GND to the Vin and GND. How to do that?The MC34071 opamp (or MC34072 dual or MC34074 quad) that I recommend has a max supply voltage of 44V. Its inputs can work fine down to ground. Its output can swing almost to ground.Just connect its supply pins to Vin and ground. ;DHi Ante,The MAX420 is a chopper stabilized opamp that is very slow. I have never used a thingy like that. I am amazed that its offset voltage is only a few uV. I wouldn't use it here. Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted September 12, 2005 Report Share Posted September 12, 2005 I get a problem here...If my Vin 30 or 32V..can the output achieve 30V???how to change the circuit...because I want to common the VCC and GND to supply voltage....With your circuit?No.But with my MOSFET circuit, yes but is depends on the on resistance of the MOSFET and Rsence.Vdropout = current drawn * (Rsence + RMOSFET)For example if you want 3A @ 30V and the MOSFET's on reistance is 0.4ohms and Rsence is 0.1ohms:Vdropout = 3 * (0.1 + 0.4) = 1.5VSo the input voltage must be at least 31.5V for the output to be 30V when 3A is drawn.Hi Alun,It is difficult to select U3 for your circuit.It needs inputs that operate near the positive supply, and an output to ground. Cmos opamps do this, but have limited supply voltage rating.If its 10k input resistors were increased so that they have 1.8V or more voltage drop then an MC34071 opamp could be used if you can tolerate its output going only as low as about 0.1V. ;DThen (I know it's a pain) but build a little negitive power supply for it, I don't know why I didn't think of this before. ;D Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 13, 2005 Report Share Posted September 13, 2005 Hi Alun,Now that you have a negative supply, you can use a TL071 or TL081 opamp for U3 because their inputs operate up to the positive supply. Their supply voltage is only 36V max so will probably be exceeded when the load is low.TI probably has an opamp with inputs that operate at the positive supply and a 44V max supply, but I couldn't find one. Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted September 13, 2005 Report Share Posted September 13, 2005 audioguru,You could use the MC34071, the inputs to U3 are held at the output voltage not the supply so as long as the ouput's 2.2V lower it'll be fine.You could even use the MC34072 (dual) for U1 and U2 to save space but U3 needs to be separate as it needs the extra -3.6V supply. Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 13, 2005 Report Share Posted September 13, 2005 Hi Alun,Good point! ;DThe MC34071 is rated for a 44V supply so the extra input voltage is fine. Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 13, 2005 Report Share Posted September 13, 2005 And you also save money if you use a dual MC34072 for U1 and U2. A dual costs just a few cents more than a single opamp. ;D Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted September 13, 2005 Report Share Posted September 13, 2005 We could get round the negitive supply problem and save an op-amp by putting the current sencing on the negitive rail as long as the output floating doesn't cause a problem. ;D Quote Link to comment Share on other sites More sharing options...
cyw1984 Posted September 25, 2005 Author Report Share Posted September 25, 2005 Hi nearly old man Staigen,You are absolutely correct! I was way off with my selection of high-value "turn-off" resistors for the output transistors in my "fixed" circuit. I have now chosen lower value resistors based on what is inside a TIP120 darlington.I used a 1k resistor in series with the collector of the 2N3906 to limit its collector current and dissipation if the project's output is shorted. It will still provide enough drive to the output transistors to blow a fuse if the output is shorted.I think the circuit can deliver 30V at 1.5A with a 34V supply to protect the 36V max-rated opamp.It would be fun to try to stop this extremely high gain circuit from oscillating. ;DHi Alun and Cyw,I don't think a single 2N3055 or power Mosfet can dissipate more than about 60W even on a pretty big heatsink. Therefore the max current is only 2A at low output voltages. That's why I proposed using 2 output transistors in our 0V-30V modified project for 3A output, and using 3 output transistors on our modified for 5A project.I am sorry..i have try this circuit...but i don't know it 's not work..the output is not linear and the output is disorderhow to shoot it? Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted September 25, 2005 Report Share Posted September 25, 2005 I am sorry..i have try this circuit...but i don't know it 's not work..the output is not linear and the output is disorderhow to shoot it?Revolver, air pistol, shotgun - take your pick. ;DI'm joking of course, I know English isn't your first language, I'll try to correct your post for you, I think you must've meant:I am sorry.. I have tried this circuit... but it's not working, the output isn't linear and is the voltage is wrong.How can I troubleshoot it?It might be oscillating, you should attach a 100uf capacitor followed by a 100nf ceramic disc capacitor on the output of the regulator, if this doesn't work then please answer the following questions:What are you powering this circuit from?Have you looked at the output on an oscilloscope?What output voltage are you getting at what input voltage?Have you checked the output from the DAC? Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 25, 2005 Report Share Posted September 25, 2005 You are darn right it's oscillating. It has way too much open-loop gain.An LM317 voltage regulator IC has a very tight layout and frequency compensation capacitors inside, and nowhere near as much open-loop gain. Quote Link to comment Share on other sites More sharing options...
cyw1984 Posted September 25, 2005 Author Report Share Posted September 25, 2005 Revolver, air pistol, shotgun - take your pick. ;DI'm joking of course, I know English isn't your first language, I'll try to correct your post for you, I think you must've meant:It might be oscillating, you should attach a 100uf capacitor followed by a 100nf ceramic disc capacitor on the output of the regulator, if this doesn't work then please answer the following questions:What are you powering this circuit from?Have you looked at the output on an oscilloscope?What output voltage are you getting at what input voltage?Have you checked the output from the DAC?Vin=40VI used the power supply to represent DAC Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.