Multimeter Modification to achieve Giga-Ohm input impedance for Transducer Preload measurement

[@klave8511]

Those DT830.. (yellow one) multimeters are readily available, they look the same but have been dramatically changed internally. New tiny PCB, very cheap feel about them. They did always use the Intersil ICL7106 dual slope ADC ( 1/2 digits) and fortunately those are available online as kits (using the ICL7107 with LED display) for very little. I just looked at mine, you remove 1 resistor and a pot across the input and you have the high input impedance again. They do draw some current so the voltage climbs a bit when you have a capacitor across the input but the rate is quite low. I would suggest to add some protection to the high impedance input, maybe some diodes from the input to the supply or a JFET across the input since it doesn't need more than 200mV. I enjoy the new series so far. I'm hoping to see a charge amplifier join the ranks of clamping force measurement methods some time or maybe a charge transfer measurement using the arduino, they can be made using just the port lines and a reference capacitor. Measuring and discharging the charge is done by switching the ports from input (Hi Z) to output, setting it high or low as needed. Your online calculator is a good idea. With automatic torque wrenches you can control the torque wrench (with a microcontroller) to turn off when a clamping force is reached, a definite plus for quality manufacturing.

[@ultrasonicadvisors]

I appreciate your detailed comment! Yep, the cheap multimeters come in a whole variety of levels of quality. - nice point about protection of the ADC - a charge amp makes sense, however, I'd still need to deal with the drift from the bias current. I hope, however, to demonstrate it soon. - it's definitely an awesome thing to connect a torque wrench to the output of the voltage. You could even do it with a comparator instead of a microcontroller :)

[@ricksampson6780]

Greetings Husain, did you consider using the "Peak Hold" function now commonly available on most DMM's? Bypassing the input circuitry and connecting directly to the ADC is not standard engineering practice, cheers.

[@ultrasonicadvisors]

Thanks for the suggestion regarding the peak hold function. It’s really useful! However, the downside is that you would be unaware of anomalies that cause the preload voltage to reduce. Also, you’d be unable to track voltage in specific cases where load is put and removed a few times to reduce hysteresis. I agree that bypassing input circuitry meant to stabilize signals is not standard. I have seen that it can have consequences like drift. However, I hope the method is useful for those people who want to have access to an entry level high impedance measurement from standard equipment.

[@ricksampson6780]

Excellent detailed reply, cheers Husain.@@ultrasonicadvisors

[@inzamamashraf1813]

high bro, i am a student and i want to know what is the driving voltage of 60w Ultrasonic cleaning transducer 40Khz

[@ultrasonicadvisors]

They use a few hundred voltage driving at the antiresonance.

[@inzamamashraf1813]

@@ultrasonicadvisors yes but how much,i tride 400vpp or 800vpp both at resonance freq bhut both are high transducer drawing so much current then calculated and circuit get very hot