电流表电压表+液晶面板电路,Voltmeter + Amme

 ELEMENT ACTION
S1 Setup button
When pushing this button the shunt resistor value appears. 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  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 not 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, s
- 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.

Miscellaneous
 

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I this picture you can see the multimeter module installed along with 0-30 VDC STABILIZED POWER SUPPLY WITH CURRENT CONTROL 0.002-3A, presented under projects section.

Original design page located here: http://elfly.pl/multimetr/multimetr_en.htm

Warnings
 

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CAUTION
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:

DO NOT work if you are tired or in a hurry, double check every thing before connecting your circuit to the mains and be ready to disconnect it if something looks wrong.
DO NOT touch any part of the circuit when it is under power.
DO NOT leave mains leads exposed. All mains leads should be well insulated.
DO NOT change the fuses with others of higher rating or replace them with wire or aluminium foil.
DO NOT work with wet hands.
If you are wearing a chain, necklace or anything that may be hanging and touch an exposed part of the circuit BE CAREFUL.
ALWAYS use a proper mains lead with the correct plug and earth your circuit properly.
If the case of your project is made of metal make sure that it is properly earthen.
If it is possible use a mains transformer with a 1:1 ratio to isolate your circuit from the mains.
When you are testing a circuit that works off the mains wear shoes with rubber soles, stand on dry non conductive floor and keep one hand in your pocket or behind your back.
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.

BEWARE: ELECTRICITY CAN KILL IF YOU ARE NOT CAREFUL