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Tachometr / Tachometer Regulator modelarski / RC ESC PDIP multimeter version
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This multimeter was designed to measure output voltage and current in a PSU, where the current sense shunt resistor is connected in series with load at the negative voltage rail. It needs only one supply voltage that can be acquired from main PSU. An additional function of the multimeter is that it can control (switch on and off) an electric fan used to cool the main heatsink. The power threshold at which the fan switches on can be adjusted using One Touch Button Setup.

Technical Specifications - Characteristics
*Internal MULTIMETR resolution is given by equatin:
                   resolution[mA] = 1/(R[om]*3.2)
  If internal resolution is worse than 10mA, MULTIMETER displays current with internal resolution, if better - with 10mA one.
  In addition voltage drop on shunt resistor musn't exceed 2.4V, so R[om] should be less than 2.4/Imax[A]


schemat multimetru
There are elements on schematic and in the following table marked as "Do Not Assemble". That elements was needed in previous software edition. Current software version doesn't , so you just don't add them. Maybe in a future version of the multimeter there will be a simpler PCB with simpler electronic diagram too.

Part List:
Element Value/Type Case Remars
R1 100k 1206   
R2 100k 1206   
R3 10k   Potentiometer
R4 30k 1206   
R5 10k 1206   
R6 10k 1206   
R7 7k5 1206   
R8 7k5 1206   
R9 500R   Potentiometer
R10 500R   Potentiometer
R11 5k1 1206   
C1 100n 1206   
C2 100n 1206   
C3 100n 1206   
C4 100n 1206 Do Not Assemble
C5 100n 1206 Do Not Assemble
C6 100n 1206 Do Not Assemble
C7 100n 1206 Do Not Assemble
C8 100n 1206 Do Not Assemble
C9 100n 1206 Do Not Assemble
C10 22u/6V SMD A   
C11 10n 1206 Optional element - protect Q1 against voltage peek after switch off fan. Most of the computer type fans which I tested didn't produce  voltage peeks dangerous for Q1
C12 10u/50V     
L1 47u 1210  Do Not Assemble - cross PCB pads
D1 DIODE  SMD A Optional element - protect Q1 against voltage peek after switch off fan. Most of the computer type fans which I tested didn't produce  voltage peeks dangerous for Q1
D2 DIODE SMD A e.g. SK310A
U1 7805 TO-252 Voltage regulator +5V, e.g. LM7805
U2 7812 TO220 Voltage regulator +12V, e.g. LM7812
J1 GOLDPIN 1x2 FAN_CON - fan connector
J2 GOLDPIN 1x1 +12V_CON - optional +12V supply connector
J3 GOLDPIN 1x1 +35V_CON  - main supply connector
J4 GOLDPIN 1x3 ground and measured signals
S1 SWITCH     
Q1 MOSFET N SOT-23 e.g. BSS-138 (fan current lees than 200mA)

Layout - top side

Layout - bottom side

Picture of PCB is here. There are two version of PCB - normal and mirrored. I think, that anyone who makes PCBs will know, which one schould be used to produce right PCB.

After soldering all the parts on PCB:


Because µC is in TQFP package, we can program
it after soldering  all the elements on PCB. It makes programing quite easy to perform. Programing signals are delivered through LCD connector. To make the programing cable, you can use old PC HDD cable. Picture of my programing cable is shown here.
Remember, that durring programing multimetr circuit must be supplied with +5V. Depending on your programmer, supply voltage is provided either by programmer, or from separate power supply unit.

Programing cable
connection list
LCD Pin number LCD signal µC signal/Pin Prog signal

After connecting µC to prog, you should check, if µC is "visible" for prog. When everything is fine, you can upload  code to µC. The code is available here .
It is assumed here, that
µC is new and works with its internal RC clock at 1MHz. If not, set apropriate fusebits to achive above mentioned conditions.
In addition Brown-out detector should be turned on by enabling BODEN fuse. Recomended Brown-out Reset Threshold Voltage is 4V.

The next thing to do is to cross LCD soldering pads number 1 and 5. It is neccesary to provide ground for LCD RW
 After all, connect LCD module with the multimeter PCB. It is recommended to use a detachable connector for further expandability e.g. software upgrading.

Instalation in PSU and Regulation

Mount multimeter to PSU according to the diagram below:

schemat montażu multimetru w zasilaczu

Connectors and regulation elements:
S1 reset/setup connector

During normal work 
pushing S1 button cancel amount of displayed charge. In versions without charge displaying,  pushing this button doesn't make any reaction.

To enter setup push and hold S1 button, then power on multimeter. When "www.elfly.pl" appear on LCD, you are in setup mode.

First parameter to adjust is ATMega8 voltage reference. Reference voltage inaccurracy is the main measurement error factor in previous multimeter code versions, because Vref vary from chip to chip in quite wide range .
You can measure reference voltage between multimeter ground and µC pin no. 20. Measured value you should write down in setup. If you don't write anything, it will be assumed, that Vref=2.56V (due to datasheet).
After  Vref setup, button must not be pressed for about 5 seconds. The next parameter to set up is shunt  resistor value.

If the resistor value is known, repeat button pushing until correct value reached. If resistor value is unknown (e.g. self made resistor), short PSU output by ammeter, set some current by PSU current limit regulator and then, push button,  lead to equal current indication on ammeter and multimeter.
After  shunt resistor value setup, button must not be pressed for about 5 seconds.

The next parameter to set up is fan switch-on power threshold. It is the real power loosed on output transistor (transistors), because multimeter has information on voltage drop on transistor and driving current. To avoid instability switch-off threshold is automatically set to 20% less than switch-on one.
R9 Fine voltage circuit regulation potentiometer.
To reduce ADC conversion errors like un-linearity, gain factor etc. measuring range is divided into two sub-ranges 0-10V and 10-30V (switch threshold can be between 7-13V depend on sourcing current and elements tolerance).
To regulate fine sub-range connect voltmeter to PSU output, set up voltage at about 9V and turn R9 until voltmeter and multimeter indications are equal.
R10 Coarse voltage circuit regulation potentiometer.
There is over-sampling applied in multimeter software, so measuring resolution is the same in fine and coarse circuit and is 10mV. Because of the reason described above multimeter has two measuring circuits.
To regulate coarse sub-range connect voltmeter to PSU output, set up voltage at about 19V and turn R10 until voltmeter and multimeter indication are equal. (If  you posses 4.5 digit voltmeter, you could regulate at voltage 30V)
R3 LCD contrast potentiometer.
Turn that potentiometer first, if nothing is visible on LCD.
J1 Fan connector.
Pin no. 1: Fan "+"
Pin no. 2: Fan "-"
J2 +12V
If +12V DC is available in your PSU, connect it to that pin. In that case you shouldn't assemble +12V voltage regulator U2 on PCB.
That solution is convenient for multimeter, because eliminates U2 heating and permit to connect fan and LCD with higher current consumption.
If you haven't got +12V DC in your PSU, left that pin unconnected.
J3 +35V
Rectifier bridge voltage. See U2 element you used data sheet to know about maximum voltage it can work properly. On the other hand the minimum voltage on that pin mustn't drop bellow c.a. 9V, or 6.5V if low drop type U2 and U3 voltage regulators were used.
That pin should be connected even if +12V DC is connected to J2 pin. Voltage from that pin deliver information for fan switching.
J4 Measuring signal connector.
Multimeter is suitable for voltage and current measurement in PSU, where current sense shunt resistor is connected in series with load and is in negative rail.

Pin no.1: voltage measurement U - connect to "+" PSU output, best directly to output terminal;

Pin no.2: current measurement I - connect to "-" PSU output, best directly to output terminal;

Pin no.3: ground - connect to shunt resistor terminal opposite to that connected to "-" PSU output.
LCD LCD connector.
Multimeter works properly with LCD's 1x16 logical controlled as 2x8 (most of LCD's available on the market).

Because of linear voltage regulators used in multimeter, sourcing current is limited. Main current consumption elements are fan and LCD backlight, so:
- use LCD with LED backlight (typically current consumption is less than 15mA);
- use low speed, low current fan. Additional advantage of that solution will be silence.


Front panel design is HERE
Panel is suitable for Z-17 case

Remember to cut sufficient paper just after mounting and match panel with front plastic part of case.
That frame on print is only coarse outline.

Below some pictures of multimeter and PSU

PSU presented above was build acording to http://www.electronics-lab.com/projects/power/003/index.html

I prepared some multimeter code versions:

LCD 1x16 HEX
LCD 2x16 HEX
LCD 2x16
Load resistance displaying
LCD 2x16
Capacity in mAh passed to load displaying
LCD 2x16
Load resistance and capacity in mAh passed to load displaying

Caution! Wrong code (e.g. PDIP mutlimeter version) can damage µC chip after powering up multimeter

I use MULTIMETER version with measurement of the amount of electric charge sourced from PSU to charge my RC Li-Pol cells. That PSU is very convenient for that purpose, because constant-current, constant-voltage (CC-CV) charging is easy to achive. Charged capacitance indicator give possibility to estimate cells condition and level of discharge after cells use. 

This circuit itself doesn't work off the mains and there are not 220 VAC present, but PSU does.
Voltages above 50 V are DANGEROUS and could even be LETHAL.
In order to avoid accidents that could be fatal to you or members of your family please observe the following rules:
If you take all the above precautions you are reducing the risks you are taking to a minimum and this way you are protecting yourself and those around you.
A carefully built and well insulated device does not constitute any danger for its user.


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