PCB Assembly Services - Screaming Circuits: Parts Substitution


Parts Substitution

Schottky_bottom Take a look at the two parts in these pictures and see if you can guess what the difference is. It's the same two parts in both photos, just a top and a bottom view. Click on the images for a close-up view.

I'll give you some hints. They are both Schottky diodes. The photos aren't quite clear Schottky_topenough to read the part numbers so I'll give you that one. They're both from International Rectifier. You can kind of see the "IR" on one of them in the photo, but not the other. The foot print looks pretty much identical - both SMA packages.

Enough with the suspense. I'll answer the question from three different perspectives.

One is an IR 10MQ100N, 100V, 2.1A with a Vf of 0.68volts. The other is an IR15MQ040N, 40V, 3A with Vf of 0.43V. From the perspective of Screaming Circuits process technicians, these are two completely different parts. If the BOM called for one and we saw the other, we would have to stop the build and try and call you. Doing so might very well delay your job, but we would have to do it.

So, is there really a difference? Are we right or wrong to stop the build?

Design A is a triple voltage (12, 5 & 3) DC to DC converter. It takes 15 to 30 volts input and converts it to the three outputs at a combined maximum current of 1.0 Amps. The diode, in this case, is used for reverse voltage protection at the input. Either diode will work fine in this application. The 40V, 3A would provide a little more current head room and cause less of a voltage drop for a bit more margin on the low-voltage end. The other one would give more voltage overage head room. For the purposes of a prototype, though, they are effectively equal in this application.

Design B is a free-wheeling diode application in an H-bridge motor driver. The maximum free wheeling current is expected to be under an amp at approximately 48 volts. Elsewhere, the circuit is spike protected with 72 Volt TVS diodes. In this case, the 100V part would fit the requirements and would give adequate headroom. The 40V part might very well smoke at first turn on.

For designer A, the parts are completely interchangeable in a prototype. For designer B, putting the wrong one in could smoke the part, and potentially also damage some mosfets and an expensive mosfet driver chip. We can't tell which is the case, so we have to assume the worst case and stop the build. It's the same with close but not exact value caps and resistors. We have to assume the worst and stop the build.

If you do need to sub, change the BOM before you zip it up and send it to us or at least tell us in the special instructions section of the online order form. If you make the substitution after you've placed the order and sent us the files, give our customer service folks a phone call (866)784-5887. We don't want to delay your project, but we REALLY don't want to smoke it.

Duane Benson
Where there's smoke, there's bile


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Experience has taught me that when many electronic parts fail smoke is released. One could deduce that the smoke is critical to the proper functioning of the part. The logical assumption is that electronic failures are due to the escape of smoke from critical components. Does anyone stock electronic component smoke and the necessary tools for reinserting it into the part? This could save a lot of expensive repairs and part replacements.

An older but not quite senile Ham.

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