Those thing are horrible. Get yourself decent temperature controlled models, you can set the temperature you want and keep it fairly constant no matter how much heat your draw away from the tip. The also only draw current when needed so the overall power consumption is much less. I have a Weller temperature controlled (Curie point magnetic switch) iron here I purchased around 1975 which is still working perfectly. I also use a Chinese 'HQ-Solder/30' model with a digital readout which has been in operation for about 15 years.
Brian.