A kit weakness revealed

 A customer had a problem with the Keyer Accessory kit: http://wb9kzy.com/keyacc.htm .  The LED had gone bad and he asked about adding a resistor in series.  The Keyer Accessory kit is a low power regulator board with the option of a Solid State Relay (SSR).  The SSR option wasn't purchased with this kit.  Unfortunately I told him that the regulator itself basically functioned as an automatically varied resistor and that an external resistor shouldn't be required.

What I didn't realize was that the LED had functioned as a fuse in this case.  It should have alerted me that there was a problem as all fuse / circuit breaker trips indicate.

The kit has a regulator option, either a Seiko regulator or an LM2936 regulator (see previous blog https://wb9kzy.blogspot.com/2022/02/ldo-regulator-ic-chips.html).  This customer had purchased the Seiko regulator but had built the kit as if it was the LM2936 regulator.  This connected the input voltage from the battery to the output of the Seiko regulator.

I'm not quite sure what the circuitry is inside the Seiko regulator but the result of hooking it up as an LM2936 was that it got so hot that it exploded - the LED wasn't there anymore to act as a fuse.

Three things will be done in response to this:  first, add a series Schottky diode to the kit - it wouldn't have prevented this problem but something similar could happen if the input voltage to the Seiko chip was reversed.

Second, add a piece of tape on the board to cover the holes for the "wrong" regulator.  Hopefully this will prevent incorrect insertions.

Finally, ask all customers with kit problems to photograph their kit and email it to me, that might have alerted me to the incorrect regulator insertion in this kit.

A note was put on the Keyer Accessory web page mentioning the diode along with a modifiation doc.

I hate that this happened, kits are supposed to be fun, not potentially dangerous.

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm