Tuesday, June 24, 2014

Energy Saver Solder Iron Station Circuit


To design a circuit for solder iron which would not only save the energy but also avoid the over-heating of solder iron tip.


a) Switch ON and warm-up the solder iron for about 1 minute.

b) Check if the solder iron is present in stand or not.

c) If not present, the solder iron gets 100% power, directly from AC mains.

d) If present, the solder iron gets 20% power thru regulated circuit.

e) Go to procedure (b).


a) A 555 timer is configured to delay power on for about minute. During this period the solder iron is connected to AC mains thru the "NC" contacts of the relay. The red LED would indicate the initial warm-up of 1 minute after which it goes off and the green LED would light up to indicate that the solder iron is ready to use.

b) IC LM358-A is configured as voltage comparator to check the presence of the solder iron in its stand using a thermistor. The (-)ve input of the comparator is provided with a reference voltage of 6V using R4/R5 potential divider. The (+)ve input is also connected to a potential divider formed with R6 and the thermistor TH1.

If the solder iron is not present in its stand the thermistor would acquire the room temperature. At ambient temperature the resistance of the thermistor would be roughly 10k thus the potential divider R6/TH1 would provide 2.8V at the (+)ve input, which is less than 6V at the (-)ve input. Thus the output of LM358-A remains low and there is no change in the operation; the solder iron continues to get power thru the "NC" contacts of the relay.

c) If the solder iron is present in its stand, the increase in temperature will increase the resistance of the thermistor. As soon as it crosses 33k, the potential divider R6/TH1 provides more than 6V at the (+)ve input hence, the output of LM358-A goes HIGH. This energizes the coil of the relay via NPN transistor T1 and therefore the solder iron is disconnected from the AC mains.

The HIGH output of LM358-A also powers ON the LM358-B network, which is configured as an astable oscillator with a duty cycle of about 20%. The duty cycle is controlled thru the potential divider R8/R10. The output is connected to the gate of triac BT136, which conducts and switches on the solder iron for 20% of a cycle, thus 80% of power is saved while the solder iron is at rest.


1) Since the triac (operating AC mains) is directly connected to the rest of the circuit via R12, care should be taken and the circuit should not be touched when powered on. For protection, opto-isolator like MOC3020 can be incorporated.

2) Any value of thermistor may be used but, the value of the R6 should be selected accordingly such that R6/TH1 should provide about 3V at normal temperature. Moreover, the increase in temperature of the spiral steel wire sleeve due to the presence of solder iron should also be taken into account.

3) The triac cannot be replaced with a relay because of two main disadvantages:

a. Continuous rattling sound of the relay contacts could be annoying.

b. The continuous and swift switching of the relay contacts will cause high voltage sparks.

4) The thermistor legs should be covered with heat resistant insulation sleeves and then installed suitably on the iron stand.

5) The 12V DC supply (not shown) may be obtained from AC mains using a step-down 12V transformer, 4 x 1N4007 diodes and a filter capacitor. For details, read this article http://homemadecircuitsandschematics.blogspot.com/2012/03/how-to-design-power-supply-simplest-to.html

Written and Submitted By: Abu-Hafss

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