Low cost Voltage References
              Malone Electronics                     
418 W. Main St. #1542
Battle Ground, WA 98604 USA
email: doug@voltagestandard.com
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A new website, www.dmmcheckplus.com, is now up and running to take care of sales and service for DMMCheck Plus units.  A DMMCheck Plus production run is in process now and available for purchase via the new website.  Thank you for your interest and your patience.

The DMMCheck Plus is a battery powered instrument that checks seven important functions on a DMM: AC and DC Voltage, AC and DC current, Resistance, Frequency, and Duty Cycle.  It you have a DVM or DMM that has questionable accuracy, the DMMCheck Plus can give you the confidence you need before relying on your test equipment. Recent improvements include the use of turret terminals on the PCB so that you connect your meter to these sturdy terminals instead of component leads or wire loops.  Mounting holes are now in the board corners to allow mounting of the PCB assembly in your own enclosure or with the optional enclosure, described below.

Provided on the small 3" x 3" circuit board are the following circuits:
Resistance : Four 0.1%, 10ppm precision resistors are included: 100Ω, 1.00KΩ, 10.00KΩ, and 100.0KΩ
DC voltage : 5V, ± 0.007% voltage reference
DC current : 1mA, ±0.1% current source
AC voltage : 5V RMS, bipolar (+5V, -5V) square wave, ±0.1% voltage source
AC current : 1mA RMS, ±0.2% current source
Frequency : 100Hz, ±0.02% signal source
Duty Cycle : 100Hz, 5V RMS, signal source has 50% ± 0.05% duty cycle

An external power supply is not needed; The included 9V battery makes the unit self-contained and easily transported.  An on-board on/off switch allows you to maximize battery life when not in use.  Reverse polarity protection is included, so even if the battery is placed in the holder backwards no damage will result.  When the  AC/DC slide switch is in the AC position, AC voltage and AC current reference signals are present at the “V” and “I” terminals.  When in the DC position, DC voltage and DC current reference signals are produced.  A high brightness, low current LED clearly indicates when the DMMCheck Plus is powered-up.


The 5V reference voltage is based on the Linear Technology LT1021BCN8-5 buried Zener reference IC. After the DMMCheck Plus is assembled, it is powered up with a 9V power supply for a minimum of 250 hours to "age" the reference and improve its long term stability.  After aging, the trim potentiometer is carefully adjusted for a reference output of 5.0000V as measured by our calibrated 8.5 digit Keysight 3458A DMM that has an accuracy of 0.0008%.  (Keysight Technologies, Loveland Standards Laboratory, Certificate of Calibration 1-10871332528-1, valid through February 27, 2020)Accidental movement of the trimpot adjustment is prevented by the application of a small amount of acrylic enamel.  The ambient temperature is also recorded (typically 68F).  The circuit board was designed to minimize the effects of mechanical stress on the voltage reference IC.  Specifically, the reference is placed on the edge of the circuit board and 2 adjacent board slots prevent stresses in the board from reaching the reference IC which could otherwise cause the output voltage to shift.

Recalibration of the voltage reference is free for the first two years after purchase - just send it back to us and for USPS Priority return shipping please include $7.00 in stamps or US currency or PayPal payment to doug@voltagestandard.com.  After two years the charge is $15.00 plus shipping. Please contact us for shipping charges to destinations outside the USA.


100Ω, 1KΩ, 10KΩ, and 100KΩ 0.1%, 10ppm/C resistors (Vishay type PTF56), connected in series are available to check the resistance function of your DMM. Depending on how you connect your meter leads many different resistance values are available.  The resistors are guaranteed accurate within 0.1% of their stated values, but the actual value obtained with a 4-wire (Kelvin) measurement using our HP 3458A (.001% accuracy on OHMS) is provided on the as-measured record shown below.


A 0.1% accurate active 1mA DC current source is provided to check the DC current measurement accuracy of your DMM.  The compliance voltage is approximately 500mV which accommodates load resistances from 0 Ohms to 500Ohms. The only DMM's that we are aware of that are NOT compatible with DMMCheck Plus is the Fluke 116 which has a burden resistance of 3KOhms when measuring DC current and the Uni-Trend UT61E which has a burden resistance of about 1KOhms.  To use this feature, set your DMM to measure DC current and connect the test leads to the provided current source terminals.  The DC current sourced by DMMCheck is guaranteed to be within 0.999mA and 1.001mA, with the actual value, as measured by our 3458A, indicated on the included as-measured record.


A 0.1% accurate, 5.00VAC RMS, square wave 100Hz voltage source is used to check the AC voltage accuracy of your DMM.  The signal is true AC (bi-polar), not pulsed DC.  It can also be used to check the compensation of oscilloscope probes.  Note that true RMS-reading DMM's such as Fluke 87, 287, 289 or Extech 540, 570 will agree with the AC voltage shown on the "Measured Values" sheet shown below.  Less expensive DMM's are average responding and are calibrated for sine waves not square waves.  The result is that average responding meters read 1.11 times higher than true RMS meters when measuring a square wave such as the one produced by DMMCheck Plus.  The bottom line is that an average responding meter, when measuring a 5.00V RMS square wave, will display 5.55V and a RMS responding meter will display 5.00V.


A 0.2% accurate, 1.00mA AC RMS, 100Hz current source is used to check the AC current accuracy of your DMM. 


The 100Hz, 5VAC voltage reference is generated by microcontroller firmware running off a 4MHz quartz crystal controlled clock.    The firmware can be modified to generate other frequencies and duty cycles – please inquire with your specific needs.The frequency and duty cycle are measured by our calibrated Agilent 53220A frequency counter.

You can have two frequencies programmed into your DMMCheck Plus instead of just the standard 100Hz signal.  Specify any two frequencies between 25Hz and 20KHz when you place your order.  The list, below, shows how close the generated frequencies will be to what you specify, with the actual frequencies and duty cycles recorded on the included "Measured Values" table as measured by our calibrated Agilent 53220A frequency counter.  This option costs an additional $10.00

25Hz - 999Hz 0.1%
1KHz - 9.99KHz 1%
10KHz - 20KHz 2%

Here is how you switch between the two frequencies:  If the AC/DC switch is in the AC position on power-up or after switching from DC to AC with the power on, the lower of the two frequencies is generated.  While keeping the unit powered-on, slide the AC/DC switch to the DC position and then back to AC- the higher of the two frequencies is then generated.  Subsequent switching from AC to DC and back to AC will select the other frequency.  On October 17, 2014 the firmware was updated to rev 2 to provide this functionality- this is an improvement over the old rev 1 firmware where the power had to be cycled to repetitively switch between the two frequencies.  New units and those returned for calibration will receive the new firmware at no charge (as long as the dual-frequency option has been purchased).

The DMMCheck Plus optional enclosure (shown below)  provides the following:
• Same circuitry and electrical specifications as the standard DMMCheck Plus

• Black plastic enclosure surrounds 9V battery mounted underneath circuit board
• Small rubber feet in bottom corners of enclosure keep unit from sliding on your work surface
• Clear 1/8" thick acrylic cover plate prevents damage if an object is accidentally dropped onto your DMMCheck Plus.  Since the cover is clear, the power-on LED and connection terminals are easily seen.
• Robust PCB turret terminals on voltage, current, and resistor connections. 

AC/DC voltage output, output resistance : 25Ohms, typical*
Current drain: 5mA typical
Dimensions (no enclosure) : 2.5" W x 2.2" L
Dimensions (with enclosure option): 3" W x 3" L x 2 1/8" H
Powered by included Alkaline 9V battery

*The DMMCheck Plus is calibrated with a 10MΩ load connected to the voltage output terminals when measuring DC; 10MΩ is the typical input resistance of most DMM’s when measuring DC Volts.  A 1M load is used for AC Volts.  Please specify if you require a different load resistance.

Below, is an example of the Measured-Values sheet that is included with every DMMCheck Plus.  In this example, the customer specified the dual-frequency option with oscillator values of 100Hz and 10KHz.

Shown below is our 3458A in 4-wire (Kelvin) resistance mode measuring a DMMCheck Plus 100K precision resistor.

Shown below is our 53220A Universal Frequency Counter measuring the 100Hz output from a DMMCheck Plus.  Our Spectracom 10MHz GPS disciplined oscillator is used as the timebase for the 53220A and is indicated by "ExtRef" in the upper-right of the counter display.