Deleted
Deleted Member
Posts: 0
|
Post by Deleted on Aug 18, 2009 0:53:37 GMT
|
|
Deleted
Deleted Member
Posts: 0
|
Post by Deleted on Aug 18, 2009 1:33:15 GMT
Chris I wouldn't recommend the charts, as using higher value R1 values may cause other problems such as no load voltage out being very different to the loaded output voltages. Starting out with R1 having a value of 100 or a bit higher will normally not cause such a problem, which can be confusing. I tried their online calculator with an output voltage of 15.5V and R1 of 100 ohms, and it gave R2 as 1143 ohms. Because of the regulator's 5% tolerance, I would (and do) in that case start with R2 being the next higher preferred value of 1,200 ohms 1% Metal film, and because I have a reasonable stock of resistor values, then try MUCH higher value resistors in parallel with the 1,200 ohm to get close to the required 15.5V . A better option is to use a 2Kohm multi turn trimpot for R2 ,but this is not always feasible. Worth remembering too, is that if you have as an example, a normally stocked resistor from the normal range that goes 1,1.2,1.5,1.8,2.2,2.7,3.3,3.9,4.7,5.6,6.8,8.2,10 and so on, and parallel that with a resistor of 10 times the value, you then end up with a resistance in the other less frequently stocked range (which also includes the previous values), in between range, with values like 91,110,130,160,200,240,300,360,430,510,620,750,910 etc. They are called the E24 and E48 ranges. e.g. Parallel a 1,000 ohm resistor with 10,000 ohms, and you end up with 909 ohms, which is well within 1% of the in between value in the other range. (910) Alex
|
|
Deleted
Deleted Member
Posts: 0
|
Post by Deleted on Aug 18, 2009 11:27:48 GMT
Wow, I actually suspected there might be a problem there, without knowing why! Thanks for filling in the reason. I was playing with the calculator in the way you detailed, minimum change possible being my approach, using your modified SCpsu as the model, as I guess you realised.
|
|
Deleted
Deleted Member
Posts: 0
|
Post by Deleted on Aug 18, 2009 12:39:37 GMT
Wow, I actually suspected there might be a problem there, without knowing why! Thanks for filling in the reason. I was playing with the calculator in the way you detailed, minimum change possible being my approach, using your modified SCpsu as the model, as I guess you realised. Chris The data sheets quite often show R1 with a value of 240 ohms, however, in practice, values of 100 to 120 ohms are often used.The adjustable voltage regulators may also drop out of regulation unless there is a minimum load of several milliamps. Some sources suggest that around 8mA is optimum. With R1 in the 100-120 ohm area, I usually find that the open circuit (unloaded) voltage is quite close to that when loaded. The chosen resistor values also have a factor in noise performance of the regulator, but that is quite a specialized area best left to the experts ? Alex
|
|