Soldering Iron Protection

Keep those tips in tip-top shape!

There are certain cases where soldering irons have no temperature control, whether with higher power units or those types purchased by people on a budget.

They run “full bore” at their rated power which -- over the long-term -- causes tip degradation. This degradation became such a nuisance in the laboratory where I worked, that I was given the task of coming up with a solution.

The engineering team I was a part of would typically plug in their soldering irons at the beginning of the day and not unplug them until the end of the day, so they’d be ready whenever they were needed. Unplugging the soldering irons between periods of use was not really an option since waiting for them to warm up (especially if it was for just one quick solder connection) was as big of a nuisance as soldering iron tips degrading.

This led to the development of the circuit shown in the schematic.

Schematic.

S1 was the primary power switch so that the entirety of this circuit and the soldering iron could stay plugged in (no more searching for an empty power plug on a cluttered workbench!). This would cause one of the two neon lamps to come on, confirming that everything was getting power.

With S2 in the position shown, current will pass through D1, then to the box-mounted AC plug that the soldering iron is plugged into. D1 effectively cuts the voltage to the soldering iron by half.

With this reduced voltage, soldering iron tips would degrade a lot less (if at all) over time and in certain cases (i.e., a big enough soldering iron and/or a small enough solder connection), the soldering iron would be hot enough to allow a good flow of solder over a connection.

On the other hand, if a smaller (lower power) iron or a larger solder connection (like soldering a wire to a lug) was needed, S2 could get toggled causing the second neon lamp to light, bypassing D1 to provide full voltage to the soldering iron. The warm-up time of the soldering iron going from half- to full-power was much quicker than the time required when starting from cold, saving time and tips!

Parts and Assembly Requirements

Part values were intentionally not mentioned in the Parts List because they depend a lot on what size soldering iron this circuit is built for.

Designator	Qty	Value	Description
D1	1	(see text)	Diode, Rectifier
F1	1	(see text)	Fuse
LP2	2	Neon Lamp	Neon Lamp
P1	1	Power Cord	Power Cord
P2	1	Plug, AC, F	AC Power Plug, Chassis Mount
S1	1	(see text)	Switch, SPST
S2	1	(see text)	Switch, DPDT
	1	(see text)	Fuse Holder, Chassis Mount
	1	(see text)	Metal Enclosure

Parts List.

No matter what ratings the builder ultimately decides on, there are a few considerations to think about:

I chose neon lamps with a current-limiting resistor already built in. This way, there’s one component less to worry about installing.
Use a three-wire power cord and chassis-mount plug — not two wires — and make use of the ground.
Component ratings (switches, diode, power cord, and plug, etc.) should be slightly higher than the requirements of the soldering iron to allow a margin of safety.
Everything should get assembled in a metal enclosure and the enclosure should be grounded for safety (bolt the top on after checking for shorts and before plugging it in).
Although this might be over-kill, heat-shrink all connections (leave no bare wire or connections exposed).

Since we’re dealing with line power levels, safety first! For example, before plugging anything into a wall outlet, I like to check (not just visually, but with an ohmmeter as well) to make sure there are no shorts in my construction (not just from various points to ground, but between the two power leads as well).

Like the old proverb says: “Better twice safe than once sorry.” NV