Updating old sdesigns

Discussion in 'General Electronics Chat' started by macmarty15221, Jun 18, 2008.

  1. macmarty15221

    Thread Starter New Member

    Jun 18, 2008
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    I work as a "mr fixit" in a university research lab. I'm more of a problem solver, arguably an electronics novice, definitely not an EE.

    I've been asked to update the design of a vintage solenoid driver circuit that uses an SK4935-98209 opto-isolator and a 2N3053 NPN transistor to take a TTL input and drive a 24 VDC solenoid. I've found that the NTE3044 opto-isolator and NTE128 transistor work fine, but these NTE items are marketed as replacements for out-of-date parts, and I wonder if there is a way to use more contemporary parts, and any VALUE in using newer parts if replacements are still available.
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    How much current does the solenoid draw?
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    Before retirement, I was in much the same situation. We were clever enough to have cases (literally) of 2N1613 transistors, so we designed with them even is not really the correct selection.

    You need to think about future usage of the device. Do you see building numbers of these drivers, or just maintaining a repair stock? The number of devices on hand should reflect that projection.

    Obtaining specs on the old devices is reasonably easy. Finding replacements by parametric search shouldn't be too hard. The 2N3053 doesn't have a high beta, so any NPN good for 40 volts, the same or better beta, and power and current ratings will sub (you can still get 2N1613's). The optoisolator will take more time, but Panasonic, for instance, makes roughly one zillion opto devices.

    A redesign is another way out. I assume you have the means to produce your own PCB's. Look into using a FET as a sub for the 2N3053. All you will need is the opto placing 10 - 12 volts on the gate to turn it on, and a diode snubber on the solenoid coil to protect the FET.
     
  4. macmarty15221

    Thread Starter New Member

    Jun 18, 2008
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    It's 24 VCD, 4.2 watts, so 0.175 amps
     
  5. macmarty15221

    Thread Starter New Member

    Jun 18, 2008
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    0
    Future: I have a handful to maintain in house, but I anticipate building a number or these for researchers in other labs.

    Redesign: Circuit board changes are no problem, so using a FET is easy.

    And this points to the CORE of my current predicament. My boss wants me to learn WHY the FET is better, or more generally, why it is better to choose a certain device in a given situation. I have a feeling this comes down to the old "judgement comes from experience" saw.

    In case nobody ever said, it is profoundly helpful to have you experienced folks in such easy reach.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You can check out our Ebook for FET material, plus the International Rectifier site has lots of material. Frankly, handling 175 mills can't be too big a deal. Zetex makes bunches of transistors in TO-92 that will handle that current. Any ZTX6XX device will do. As will a 2N7000 FET, for that matter.
     
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