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

P.S. if you’re not adverse to MELF parts, the LL4148 is the equivalent (like BAxx) to the 1N4148 and is plenty good enough as well for an 80mA coil. Having the part close to the coil is a bit better because it minimizes the loop area, but that probably not too important for a relay coil. 

I keep at least one reel of each of the aforementioned parts around. They’re jellybeans. 

Best regards, SP

> On May 28, 2025, at 11:20 AM, Spehro Pefhany <[email protected]> wrote:
> 
> 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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