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How to change a manual knob to BNC connector

  1. Jul 27, 2009 #1
    Hello everybody,

    Would you please help me with this problem:

    - I have a pulse gating system that has manual knob to adjust applied high voltage:
    http://img156.imageshack.us/img156/908/75377199.jpg [Broken]
    Now I want to control it through a BNC connector (I have signal generator).

    Please give me a piece of advice for what I should do. I really appreciate your help. It will be great if you can show me a detailed circuit (if it's possible). If not, please tell me where I can find the solution or which book I should read to solve it.

    Thank you very much!
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jul 28, 2009 #2


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    I don't know that what you want to do is even possible; certainly it would not be trivial. And without knowing the schematic of the pulse gating system--at least the voltage control portion, it's not possible to tell you how to proceed, even generically. As it stands, we're going to need a LOT more details to even begin because what you're asking for doesn't make much sense on the face of it: the adjustment knob is most likely some flavor of potentiometer and a signal generator, well, generates signals; these two things don't naturally interface.
  4. Jul 28, 2009 #3
    I like how the picture of the nob is supposed to help :)
    You'll need to put in a lot more work. First you want to check what is behind the knob. If its a resistor then search the web for "electrically adjustable resistor", if it's a capacitor then look for "electrically adjustable capacitor" and just maybe you could get something to work. But this is not the way how these problems are generally solved. You first find out how your machine works to see what you can replace, but since it seems that you haven't even opened it, I doubt that you will be able do do that by yourself.

    The other question is why you need to use BNC. If you frequencies are that high then I share negitron's fear that what you are trying to do is not really possible.

    Why don't you try to use a little motor. That way you won't kill yourself, and there are ways to control the knob with a computer. Maybe LEGO mind storms can so the trick.
  5. Jul 28, 2009 #4
    Dear negitron and 0xDEADBEEF,

    Thank you for your replies, honestly I do not have much experience on electronic circuit. Actually, I opened the cover and check that the HV adjustment is just a variable resistor. So I thought that I will measure the voltage difference at minimum and maximum position and use one voltage amplifier to apply my external signal to match the original settings. The reason why I use BNC connector is that it's output of PID controller (SRS product, this PID controller is connected to different parts of my experiment). The speed is not so high, it's around kHz.

    These are details of my system:
    - I want to change the properties of a crystal by the modifying the applied high voltage. The applied high voltage is modified by that manual knob (as I described before, a variable resistor). Now I want to modify the applied voltage by the output of PID controller. The input of PID controller is supplied by different sources.
    - Since the variable resistor is isolated from the high voltage part of electronic circuit so I think I dont need to make an isolation part for the input (before going to high voltage part).
    - The reason why I dont want to use a motor (do you mean a stepper motor) is because it can not offer the high speed (~kHz). Stepper motor offers the high solution, but not high speed.

    I would like to hear from you!
  6. Jul 28, 2009 #5


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    It sounds like you have a varicap diode being controlled by a variable voltage and you want to frequency modulate the crystal frequency by applying an external AC signal?
    Is that right?

    If so, you would still need to trace out that part of the circuit or find it in a manual somewhere.

    It is possible that you might be able to leave the variable DC signal alone and just apply the AC to the variable DC by means of a capacitor.

    If you could listen on the crystal frequency in a receiver, you may be able to apply a small level signal to the DC line via a capacitor and see if you can hear it. Then increase it until it becomes distorted.
  7. Jul 29, 2009 #6
    I am very curious about the response to this...
  8. Jul 30, 2009 #7
    Dear vk6kro and 0xDEADBEEF,

    These are my pictures:

    http://img90.imageshack.us/img90/9943/insidee.jpg [Broken]

    I made some explanations, as you can see from this picture.

    I magnified it
    http://img238.imageshack.us/img238/7231/0907300001.jpg [Broken]

    This is the front panel:
    http://img238.imageshack.us/img238/1985/frontfdh.jpg [Broken]

    The manual knob is the one I described.

    In principle, we want to change the refractive index of our crystal so we change the applied voltage. However, as you can see at the front panel's image, that's high voltage. The adjustment is done through a variable resistor (as I checked). I found its information from its serial in this website:


    In experiment's operations, we usually set this knob at a fixed position. However, in order to fulfill our next purpose, we need to change it continuously by the output of a PID controller. Its output can be described like this: DC signal, amplitude can be changed from 300mV to 310mV (for example).

    Please tell me what information do you need more, I will try to get them.

    Thanks for helping me!
    Last edited by a moderator: May 4, 2017
  9. Jul 30, 2009 #8


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    You may be interested in a motorized potentiometer (you find them on higher-end audio/visual equipment where your remote control causes the knobs to spin around when adjusting volume or channel, or some such). They're neat, but you'll probably need to find somebody that can do electronics to build you a little interface. For instance:
    http://www.micronor.com/products.php?category=Motorized Potentiometers&offset=0

    I'm confused when you mention that a motor wouldn't be appropriate as it wouldn't be fast enough, however. As vk6kro asks, are you trying to continuously modulate the high voltage (I assume that's what the knob does), or are you trying to deliver a series of short-duration but constant-peak voltage pulses?

    If it's the latter, you might be able to accomplish that using a pulsed DC power supply (we bought one a while back which took input from a regular high-voltage power supply, and pulsed it, but you can buy stand-alone units as well). For instance:

    EDIT: Could you also tell us what it is that you have? I went to the Lasermetrics website, but have no idea what unit (or even class of unit) you have.
    Last edited: Jul 30, 2009
  10. Jul 30, 2009 #9
    Hi all,

    I am sorry that I did not tell you clearly the devices I am working on just because I dont want you to be confused by too many things and just focus on my problem. Here is it:

    Pulse Gating System: its model is 5046ER, as you can see in http://www.fastpulse.com/pulse.php" [Broken]

    We are working on pulsed laser systems so we need this device to "gate" our pulses. In principle, we pass the laser beam through the crystal in this device. Once we turn on the device's function, the crystal's property will change and it will modify the beam's direction to desired direction. Furthermore, by modifying the applied high voltage, we can control the beam intensity (beam intensity is refractive index dependent). So we use PID controller to keep the beam intensity stays the same. As you can see, supposed that the output of PID controller is DC signal, around 300mV, it will change continuously but still in the range of 300mV +/- 10mV (for instance). By this way, we can control the output power of our laser, for instance, in the range of 3W +/- 0.3W. So basically, I am trying to deliver a DC signal, but its magnitude might be changed just a little.

    Dear MATLABdude, your idea of using a motorized potentiometer is useful, I will think about it and try to get it into practical use.

    In my opinion, this potentiometer does not change the applied high voltage directly, I think that it acts as a voltage divider and the divided voltage will come to high voltage part to get final output voltage. So can I apply my signal (PID output) directly to replace this potentiometer (and use one voltage amplifier to make the same voltage levels)?

    I am sorry that due to one reason, I could not operate this device right now to measure the voltage's difference between two ends of this potentiometer.

    If you think that I need to read something, please tell me. (or to learn something). It's great to hear from you.
    Last edited by a moderator: May 4, 2017
  11. Jul 30, 2009 #10


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    That is very different to your original question.

    You want to pulse a high voltage power supply so that you get pulsed high voltage DC on a laser.

    This seems like an attractive idea, but in practice it will be difficult to do.
    It would at least depend on the frequency response of the circuitry that controls the high voltage. This may not be able to respond quickly to the changes of control voltage you are trying to use.

    Can you contact the manufacturer of the power supply and ask one of their technicians to evaluate your request?
  12. Jul 30, 2009 #11
    Ok I read your stuff like this:
    1) You have a system that "gates" the output of a laser, producing pulses. You have a knob that can adjust the lasers brightness by adjusting a resistor.

    2) You have a PID. This means you have a way to measure the lasers power to create a feedback loop. There is no signal generator anymore.

    3) Your PID has a large I part so it doesn't notice the pulses

    My recipe for you:
    0) I don't know your machine so don't come complaining if you break it or your multimeters
    1) Get "The Art of Electronics" and keep it under your pillow
    2) I don't think there will be high voltage or high current on the resistor. Measure the voltage and current going through the resistor when the box is running. (For the current you will have to get the ampmeter in series obviously).
    3) Also measure the resistance of your knob, and see, if the resistance is smaller or larger for higher settings
    4) If everything looks harmless (DC, tenth of mAmps, a few volts) get an optical insulator and replace your knob with it

    You have to think about a few things like possibly inverting the signal, getting the input and output currents right

    But even if you did everything alright, your box might simply not work at the frequencies that you want.

    Just buy an electrical laser attenuator and save yourself the trouble.
  13. Jul 30, 2009 #12


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    I don't think he is wanting to pulse the HV. He simply wants to modulate it with a controller that provides a dc output to take the place of the resistor/knob. If I understand correctly, he's using http://en.wikipedia.org/wiki/Kerr_effect#DC_Kerr_effect" to adjust the lasers intensity.

    TChi: I think what is confusing is saying you want to use a http://en.wikipedia.org/wiki/PID_controller" [Broken]. These types generally use pulsed outputs to do the controlling. Aren't you just wanting a variable dc output from you controller?
    Last edited by a moderator: May 4, 2017
  14. Jul 30, 2009 #13
    Hello everyone,

    Dear vk6kro, actually I contacted the manufacturer's technicians but they regretted that they could not help me.

    To dlgoff: you are completely right, I am using Kerr effect to adjust the lasers intensity and we want variable DC output from our controller. Could you give me any idea?

    Dear 0xDEADBEEF:
    1. We already have pulsed laser, we use this system to "gate" direction and intensity.
    2. You are right
    3. You are right, too.

    Let me explain a little bit more: the output beam comes to detector and pass through the low pass filter (SRS SIM965m) then come to PID Controller. By this way, the signal coming to PID Controller is almost DC signal. We use the large I part (as 0xDEADBEEF said) so the output is also almost DC.

    To 0xDEADBEEF, I'll remember your recipe (0->4), however, could you please explain a bit more about your 4th line? I am not sure how I will do with optical insulator and why do you think that an electrical laser attenuator will solve the expected problem?
  15. Jul 31, 2009 #14


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    1. We already have pulsed laser, we use this system to "gate" direction and intensity
    So, how fast do you want to change the high voltage? Would it be like 10 times a second from minimum to maximum and back again? Or, much faster than that?

    Why is it that you can't measure the voltages across the potentiometer and at the tap point for maximum output and minimum output? Your query will disappear off the bottom of this page quite quickly, so now is your chance.

    There are some optocoupler circuits that might be worth trying, but a lot depends on the existing setup. Do you have someone who could help you with the electronics if necessary?
    Modifying commercial equipment is really expert territory.
  16. Aug 1, 2009 #15
    Hi vk6kro,

    Maybe it's my fault to did not tell you clearly my requirement:
    Assume that my crystal is controlled with the high voltage of around 6kV and that voltage will be kept the same for a long time. I can change that voltage by adjusting the knob (in my opinion, that knob does not change the high voltage directly, I think there is already a isolator between the knob and the high voltage part but I did not measure it to make it for sure, I'll try next Monday). So ideally, I just want to change the voltage from around 5kV to 7kV continuously for several times per second. However, as I thought, I will not change the HV directly, I just change my PID controller output from ~300mV to 400mV (for instance) and apply it to the potentiometer.

    I could not measure the voltage difference because of personal reason: someone is playing with my accessories so as soon as he finish his experiment I can start mine.

    There is no one can help me out of this problem in my department so that's why your help is really valuable for me.

    I am learning necessary things to play with this problem, so please tell me what should I need to do.
  17. Aug 2, 2009 #16
    Not really. I am not talking about an optical isolator but about an opto-isolator or opto-coupler. Basically an LED coupled with a photo transistor or probably better a photo resistor. With these you don't really need to understand the grounding scheme of the circuit. I will not give much more advice on this, because for dimensioning you will have to read data sheets and understand what voltages and currents you are controlling, and for that you have to understand yourself what you are doing.

    Your phrases are a bit confusing. Expected problem?

    You want to control the lasers brightness. So an electrically controlled attenuator should work. Or a rotating disc blackened such that its transparency varies with angle. This is much simpler than modifying unknown electronics.
  18. Aug 3, 2009 #17
    Dear 0xDEADBEEF,

    Thanks for your comments, you gave me several good ideas. Certainly I should put more effort on this problem. I am working on this one and whenever I solve it, I will let you know.

    To vk6kro, negitron and MATLABdude, thank you for your helps!
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