BCCs: For technical file. Russell said: > > ... so anything in 4001-4007 range will do at 12V. I usually buy > > 1N4007s [1000 V] as the cost difference is usually minimal and you then= usually > > don't have to ask. Isaac said: > If I'm not mistaken, apart from the reverse stand-off voltage, the only > difference between the 1N400x parts is junction capacitance. > The parts from 1N4001 up to 1N4004 have the same higher capacitance and > the other devices have a smaller one. > Even the price is the same, so I think it is better just to stock the > 1N4007. That's what I do. The one situation in which I specify the lowest voltage diode that will meet the need is in switching power supply output circuits where a Schottky diode is used in a low voltage output. For example, if a single NimH cell drives a single white LED via a boost converter Vout will be around 3V. Depending on exact application a 1N5717 diode may have 0.1 to 0.15V lower forward voltage than a 1N5819. At 3V out, an extra 0.15V in the diode represents about 0.15/3 =3D 5% loss. Given that the lower voltage rated diodes are ion this case sometimes very slightly lower in cost, using the lower voltage diode wins all round with no obvious disadvantage. I found in practise that the voltage differences were real with diodes I actually used, but whether or how much this applies may vary by manufacturer. I have pored over diode data sheets for low voltage Schottky diodes in the ~ 1A range from many manufacturers and have found that some note the lower Vf for the lower voltage diodes and some don't. Others provide broad Vreverse / Vf bands and the boundaries vary. The difference is small enough that many would not care, but in a portable lighting device (my most common area of interest in recent times) an extra 3% plus run time for free or at slightly reduced cost, is worth having. If you REALLY Care about efficiency then adding a synchronous output rectifier (usually MOSFET based) in place of the diode can yield 0.2 - 0.3 V lower Vf, which is getting very worthwhile. In production volumes a MOSFET liable to work in eg LED drivers or 12V to 19V laptop charging psus would cost about 5 cents. When used as a synchronous rectifier a high side P Channel MOSFET bust be connected with Drain & Source swapped compared to normal use. This is so that the body diode blocks when Vout > Vsupply. So Switch Drain (N Channel) goes to rectifier Drain (P Channel) , Vout is from rectifier Source. As a MOSFET is a two quadrant switch it will turn on when gate is negative relative to source and the FET will then conduct "backwards" to normal. If the driver IC is not equipped for synchronous output rectifier switching this can usually be achieved automagically by typing the gate to Vin or some reference voltage < Vout so that when the output voltage rises above the gate voltage the FET is turned on. If there are concerns re switching timing then a belts & braces solution can be achieved by placing a Schottky diode across the FET so that at worst you get Schottky performance with the MOSFET taking over when switching allows. - As Vout rises Tout decreases relative to Tin and current peaks rise accordingly. I traced the circuitry in a low cost (and dead) 12V to 19V laptop upconverter some years ago (I wanted to see why /how it had died) and noted that they were using this parallel diode + MOSFET arrangement. In such a supply Iout may be 5A+ average and much higher peak so the MOSFET is worthwhile. Some years later I glanced at a similar device and noted that there was no diode used. Whether this is based on adequacy of performance or 1 to 2 cents cost savings I know not. The body diode will conduct if the MOSFET proper does not in such cases, but the characteristics of this diode are usually so poor as to possibly cause heating problems for the MOSFET if depended on. (Maybe not as diode conduction times will be low). (The brave could uses a say 1A Schottky diode here in a 5A out supply as the diode only conducts at worst at the start and end of the output cycle. Diode I_peak ratings are usually high enough to allow a much smaller diode to ride shotgun in this manner than would be needed if the MOSFET was not used. Russell --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .