Results 1 to 8 of 8

Thread: Change Coherent 210 from Analog to LCD display.

  1. #1
    Join Date
    Feb 2011
    Location
    St. Louis
    Posts
    517

    Default Change Coherent 210 from Analog to LCD display.

    I picked up this 10W power meter recently. The sensor in the head looks nice and clean and seems to work very well tested up to 6.5W against my Ophir unit but would really like a digital readout. I am hoping analog display can be pulled and use a voltage readout LCD display in its place. Wanted to get some feedback and see if anyone has been in one of these units or similar and tried to do this. Even if I have to use a separate/isolated 5V power source to power the display that is not a problem as I am guessing the analog display just has the voltage difference from the meter being fed to it and I hope I can just pull it and connect the leads to the LCD unit. If anyone has any input before I go at it let me know.


  2. #2
    Join Date
    Apr 2007
    Location
    Costa Rica
    Posts
    508

    Default

    Isn't the 210 head calibrated in mV/W? Why not use the detector with a DVM and keep the original meter intact?

  3. #3
    Join Date
    Mar 2015
    Location
    Cambridgeshire, England
    Posts
    317

    Default

    The meter will be measuring current not voltage; if there's a series resistor, that will convert it into a voltmeter and it's relatively easy to calculate the sensitivity from the meter's ratings and the series resistor value(s). Can you try and trace the circuitry?

  4. #4
    Join Date
    Apr 2007
    Location
    Costa Rica
    Posts
    508

    Default

    The schematic of the circuit.
    210sch.jpg

  5. #5
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
    Infinitus Excellentia Ion Laser Dominatus
    Join Date
    May 2007
    Location
    A lab with some dripping water on the floor.
    Posts
    8,828

    Default

    The 210 has an analog circuit based on a rare, hard to get metal can op-amp used as an amplifier. The op-amp used, at least in mine, has a different pin-out then the usual OP-7 It also has a damping circuit that is designed to speed up its reaction time, and that damping circuit is based on the mechanical and inertia characteristics of the meter coil. The meter coil is part of the calibration feedback loop. It is a rate based "lead-Lag" circuit and that means any LCD is going to show overshoot/undershoot for around a second, or two. The op-amp does drive the meter in "current" mode, meter current is a mA or so for full scale.
    ~
    To replace the meter you'll need to install a resistor in place of the meter, measure the voltage across the resistor, add a gain circuit using another op-amp as a difference amplifier to drive the LCD. You'll then need to subtract an offset with another op-amp. To do otherwise you'll loose the calibration. The resistor would have to have the same resistance as the meter coil. The problem is the damping/ feedback circuit is in the ground end of the meter. See how the meter is not directly grounded, that is the issue. Your LCD would have to electrically "float" to avoid injecting small currents into the feedback loop.
    ~

    Most people would try to tap it at the output of the op-=amp. In reality you would leave the analog meter in place and tap a weak signal at the high end of R11 with a very high impedance buffer circuit followed by a level shifter / offset adder. It would just be easier to just keep the 210 intact and make a new amplifier in its own box to drive the LCD>

    ~
    The 210 was designed for tuning laser cavities, it has a tailored time response designed to aid the users reaction time.

    ~
    If you look at the schematic, and I had one in the FTP long ago, adding a LCD looks simple, but it has subtle things that would make doing that interesting in reality, if you want to keep the absolute accuracy. For a circuit with such a low parts count, the internals do some interesting things to make it a really nice LPM.
    ~
    I love my 210.
    Steve
    Last edited by mixedgas; 08-04-2017 at 10:26.
    Qui habet Christos, habet Vitam!
    I should have rented the space under my name for advertising.
    When I still could have...

  6. #6
    Join Date
    Apr 2007
    Location
    Costa Rica
    Posts
    508

    Default

    I still use my 210 a lot. I bought it in the mid '70's and still have the original mint manual. 9V batteries are expensive here. I converted mine to run on external power. Later models had a ext PS input. Great meters. Could you not connect the detector head directly to a DVM?

  7. #7
    Join Date
    Feb 2011
    Location
    St. Louis
    Posts
    517

    Default

    Thanks for all the input. That is exactly why I wanted to get some info before digging into the unit as it sounds it is not as simple as I had hoped. From what I can decipher with my limited understanding from the diagram is you still have a dual rail like the Ophir on this with two of the 9V feeding +18V with the add from the sensor in on positive in of the op-amp and two 9V cells feeding the -18V to the negative in of the op-amp to get a difference output. After that I am kind of lost. Ok so don't mind the idea of ditching the meter and making a new box with LCD to connect to the head. The unit does work really well but really need something digital and easier to pinpoint exact power for my power tests like the the dual rail circuit with LCD I made for my Ophir. I would even consider a plug and play replacement for the 210 that is digital I can just connect to the sensor. Any options for that out there?

  8. #8
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
    Infinitus Excellentia Ion Laser Dominatus
    Join Date
    May 2007
    Location
    A lab with some dripping water on the floor.
    Posts
    8,828

    Default

    The 9v are in parallel in mine for weight. +/- 9 for the rails. I'll have something for you posted late monday, schematic wise.

    Steve
    Qui habet Christos, habet Vitam!
    I should have rented the space under my name for advertising.
    When I still could have...

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •