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I built the circuit described in a previous question/answer which I will repost here:

556 timer and solenoid circuit

It's been running quite well, actuating the solenoid at regular intervals as designed. The TIP122 transistor is barely warm, the solenoid is within normal temperature for its duty cycle, but the 556 timer IC is too hot to touch. It's been operating continuously for 24+ hours without any failures, but I am concerned because I think that this IC should not be warm at all.

Is there something I overlooked in this schematic that should be done to prevent excess heat in the IC? I haven't (yet) measured current usage by the IC alone, but the total circuit draw is 125 mA on standby and 2A when the solenoid is actuated.

share|improve this question
You are running your 666 timer from 12 V. Have you checked the datasheet whether that is OK, and if so, what current you can expect it to draw? Maybe everything is working as it should? All in all, this circuit seems like a long way to go to avoid using a PIC 10F200. – Olin Lathrop Feb 7 '13 at 19:51
@Olin Yes, the evil timer is supposed to operate on 5 to 15V. We are using a B&K Precision regulated bench power supply set to 12V. I'm more familiar with AVR than PIC, I thought about simply using an ATTiny, but I have all these timers laying around... I thought I'd use one here. :) – JYelton Feb 7 '13 at 19:52
up vote 6 down vote accepted

According to the datasheet for the NE556, the no load supply current should be a maximum of 30mA at 15V supply. Since you are reading 125mA and the unit is getting hot suggests something is wrong somewhere.

I can't see any obvious issue with your schematic at a glance that would cause this. So a few things to check:

  • Check that there are no shorts/low resistances anywhere (particularly from each output to ground)
  • Check polarised capacitors are the right way round.
  • Check resistor values are correct (in particular R4)
  • Try swapping the IC in case it's faulty
  • Make sure only the solenoid is pulling the other 1.75A when activated (e.g. put multimeter in series with it only)
  • Disconnect solenoid, test, then disconnect first timer from second, test, etc, until the current drops to a reasonable value.

If you can't find anything, trying one of the other circuits suggested might be worth a go.

share|improve this answer
R4 was indeed the fix. I used a 1K arbitrarily, and never went back to calculate a more appropriate value. Using a 10K significantly reduced the current and the heat. Thanks. – JYelton Feb 8 '13 at 0:30

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