Digital Multimeter Switch Design

Discussion in 'General Electronics Chat' started by Ally Cat, Apr 28, 2016.

  1. Ally Cat

    Thread Starter Member

    Mar 28, 2016
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    I have posted a similar question on how to find a 1000 Volt switch needed to make a DMM and the end result is they are all custom jobs done by DMM manufacturer's. So it doesn't look like I can find one that competes with the ones out on the market unless I make my own. I have taken a Fluke DMM apart and looked at one. It looks like a pretty complex switch that they use. The question is, how does one go about making or designing switches? What processes and tools do I need to accomplish this task. I already know the rules of voltage and spacing from work on PCB's. I assume the same rules apply to switches. What kind of budget am I goanna need for this task.
     
  2. ifixit

    Distinguished Member

    Nov 20, 2008
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    The DMM is usually designed so that the switch doesn't see 1000V because of 9Meg resistors between it and the input. The most it will see is 100V.

    Are you using a different design than typical?

    Ifixit
     
  3. Ally Cat

    Thread Starter Member

    Mar 28, 2016
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    If they use a 9 Meg ohm resistor(I assume to create a voltage divider) before the dial then how do they use the same net to measure resistance when the dial is turned? I am confused now?
     
  4. ifixit

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    Nov 20, 2008
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    The switch doesn't have to withstand 1000v when set in ohms mode. If you applied 1000v in ohm mode you would distroy cheap meters and most likely expensive meters as well. Oddly enough, the switch would likely not be damaged directly... unless the circuit board burst into flames.:eek:

    The current measuring mode is usually protected by a fuse and or uses a separate terminal for current only.

    You should use two 4.5M resistors in series since a typical 1/2w precision resistor can only withstand 500V.

    Ifixit
     
  5. Ally Cat

    Thread Starter Member

    Mar 28, 2016
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    I understand you can't measure 1000 V when the meter is set to measure resistance. When you place the two resistors in series in front of the switch you will have those same resistors still in series so can't take a precise resistance measurement.
     
  6. ifixit

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    Nov 20, 2008
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    The area on the function switch, where the input terminal is switched from volts to ohms, is the only critical space between terminals that needs to be able to withstand 1000V. If you have a specially designed switch then that is taken care of, as it is in typical DMM with the switch integrated into the PCB design.

    If you want to use an off-the-shelf switch you can...
    1. Modify the space between the critical contacts so that the ohms circuit is not damaged by arc-over during 1000V measurements.
    2. Arrange the switching so that there is an empty position between ohms and volts function. E.G. place the OFF position between OHMS and VOLTS.
    3. Use a separate, small two position switch for switching between ohms and volts that is spec'd for 1000V.
    Ifixit
     
  7. Ally Cat

    Thread Starter Member

    Mar 28, 2016
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    Two devices that are switching can be confusing. Thanks for your suggestion though. What would you recommend on the question of how to design a switch that can handle the voltage.
     
  8. ifixit

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    Nov 20, 2008
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    You will have to now post a schematic of what you want for ranges and functions to know how to modify the switches.

    A quick Google shows there are rotary switches available with 900 VAC specs, which would be good for your 1000 VDC requirement. No modifications required, or are you looking to use some switches you already have? Do you have their specs?

    Ifixit
     
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  9. Ally Cat

    Thread Starter Member

    Mar 28, 2016
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    My google skills are not the same as yours. I have been looking for two weeks and I did not find any. I used the "900 VAC " text and I get the 19 Series from Greyhill. Both Digikey and Mouser told me they don't carry any. I hip now thanks. That will save a whole lot of time. I will need 12 positions. My schematic will not be done for a couple of weeks.
     
  10. Ally Cat

    Thread Starter Member

    Mar 28, 2016
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    A closer look of that 19 series says the voltage is only rated at 290 VAC. How are you searching for these parts?
     
  11. ifixit

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    Nov 20, 2008
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    There are 2 voltage specs, one for the voltage and current the switch contacts can handle and one for the isolation voltage withstanding between contacts. The series 19 seems to have a spec of 2500 VAC, which is way more than you need. Have a look at series 8 or 9.

    I'll check a bit more as well.
     
  12. ifixit

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    Nov 20, 2008
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    You need 12 positions. How many poles do you need?
     
  13. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Think of it this way.

    Your multimeter display is largely just a common +- 200 millivolt digital meter its what goes ahead of that 200 mV input that determines what the meter does.

    If you attached a DPDT switch to the digital meters 200 mV inputs switch position A could go to a voltage divider circuit that takes whatever input voltage you are working with and reduces it down to the needed 200 mV signal level the digital input works with.
    But when you switch it to position B then that reroutes the input of the meter IC over to a whole different circuit that could be measuring anything else and converting what ever it into a 200 mV signal that the meter driver IC can work with.

    It's not handling the high voltage and current loads. Its just choosing which additional and independent converter circuit is feeding the 200 mv input to the actual digital meter driver IC.

    That's basically all the rotary switch is doing in a multimeter just switching a bunch of 200 mV outputs and inputs around.

    Does that make more sense now?
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    Its as simple as; you could set the meter to the 10V range and put a high resistance in series to measure a much higher voltage.

    The best analogue meters have a moving coil movement, usually rated somewhere around 37 - 50uA fsd - it draws current; therefore it will drop voltage across any resistance in series - whatever voltage that resistor drops, doesn't make it to the meter.
     
  15. Ally Cat

    Thread Starter Member

    Mar 28, 2016
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    I am looking at one pole.


    I have another question the Series 9 has the following specs


    Rated: To make and break the following loads: 1/4 amp, 115 Vac resistive; 1/4 amp, 6-28 Vdc resistive; 20 mA, 115 Vdc resistive; 50 mA, 115 Vac inductive; 20 mA, 28 Vdc inductive; to carry 4 amps continuous.

    Contact Resistance: After 25,000 cycles of operation, 50 milliohms maximum

    Insulation Resistance: 1,000 megohms minimum between terminals and shaft

    Voltage Breakdown: 1,000 Vac initially

    (500 Vac or better after most environmental tests)

    Is the voltage breakdown the spec or is it the Rated value. This is probably source of much of my confusion.
     
  16. tcmtech

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    Nov 4, 2013
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    It would be helpful to us to know what you are building and why. I don't know of any standard application multimeters that switches any 1000 volt signals in its design.
     
  17. ifixit

    Distinguished Member

    Nov 20, 2008
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    The breakdown voltage is important for you on the terminals that switch the DMM input from VOLTS to OHMS mode. You don't want the 1000V potential to, 'jump' over to, or leak across the insulation to, the ohms function terminal, because it could damage that circuitry.

    The switch contact ratings apply to voltages and currents being switched, but are usually not a concern for a DMM application. You haven't said what voltage ranges you would like to have, or the current ranges. Don't forget, you not supposed to switch ranges while a voltage or current is applied to the terminals.

    What ranges and functions do you plan on having?
     
  18. Ally Cat

    Thread Starter Member

    Mar 28, 2016
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    Not their yet in my design. I just trying to see if I have to make a connector or buy one. The series 8 and 9 series switches never came up in the original post. I just bought my PCB tool yesterday so I have to figure allot of stuff out still. This is good. I don't have to custom job the switch.
     
  19. ifixit

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    Nov 20, 2008
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    Another thought. If you don't mind having separate inputs for VOLTS, OHMS, & CURRENT functions, then you don't have to worry about switch voltages so much.

    Sounds like a good project to build up your message count with. :)
    Ifixit
     
  20. tcmtech

    Well-Known Member

    Nov 4, 2013
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    SO why exactly are you building, what I can only assume is a standard off the shelf multimeter, being you can buy one for under $5 and modify it to be as fancy or as application specific customized as you want?
     
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