Re: [EE] Advisability of using SMD dual-diodes for relay back-EMF clamping

Hi, Dwayne:-

For general purpose (slow) applications, the ‘M4’ or ‘M7’ is the approximate equivalent of 1N4004 or 7. SMA package, so easy for hand work. 

Dual in SOT-23 there is the BAW56, BAV70 and BAV99 for common anode, cathode and series. They’re plenty good- enough. 

Dual Schottky we have the BAT54 series but note carefully that the letter after the BAT54 is A, C or S so you need to get that right, lol, or bad things will happen because that specifies the internal connections. 

SOT-23 is okay if you have a PCB, not so much for rework with fly wires. Also they more than 50% land upside down when you spill them from tape because of the shape. Irritating. 

There are a bunch of Chinese makers with 1A Schottky diodes in SOD-123 package that they call 1N5819 (same as the leaded ones in DO-41). Cheap and good, intermediate in terms of hand work. 


Best regards, Spehro Pefhany 

> On May 28, 2025, at 3:47 AM, Dwayne Reid <[email protected]> wrote:
> 
> Good day to all.
> 
> I'm working on a retrofit pair of boards for one of our long-time
> products.  Basically just a simple serial TX / RX pair to control a bunch
> of relays.  Relays are all 12 Vdc with one side of each coil connected to
> the 12V Ground.
> 
> I've pretty much decided to use 74HC595 chips driving MMBT4403
> transistors.  Because these are relay coils, I need to include clamp diodes.
> 
> Back when most of my designs were through-hole, I would simply use
> individual 1N4005 diodes.  Simple and robust.  However, this board will be
> all SMT except for the very few through-hole parts (connectors, etc).
> 
> I'm wondering about the advisability of using dual diodes in SOT-23 or
> SOT-323 packages.  The specific part I'm considering is the TBAW56 (sot-23)
> rated at 215 mA.  The relay coils take about 80 mA with a 13.0Vdc supply.
> 
> I am wary about using such tiny parts.  Opinions greatly appreciated!
> 
> As an aside, what do people like to use for inexpensive single diodes in
> SMT packages.  Something similar to the bog-standard 1N400x family.  What
> I'm looking is inexpensive and readily available.
> 
> Many thanks!
> 
> dwayne
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