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

Hi Dwayne,

TBAW56 seems fine. Clamp diode only conducts as the relay turns off, 80mA 
current seems well within the diodes 215mA average and 500mA peak specs, 
power dissipation should be almost trivial.

For general purpose cheap and common 1 Amp SMD diode equivalent to 1N400x 
through-hole parts look at the S1x family (x = voltage: A=50, B=100, D=200, G=400, 
J=600, K=800, M=1000). Many many manufacturers. Typically these come in an 
SMA footprint. For a 1A diode it's not the smallest you can get these days, but size 
helps when power dissipation is a factor.

Brent

On 27 May 2025 at 13:46, Dwayne Reid 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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