Relay SDT-S-112LMR Controlled by Arduino

Old boards with relays that are controlled by transistor switches are an excellent source of ready-made (already calculated) relay modules for Arduino. Below is a photo of the board from the old CRT monitor SAMTRON 56E which is designated as "AN15V(M)1.1".

Arduino

This board has one relay 12V/5A250V (SDT-S-112LMR) in the kinescope demagnetization circuit (degaussing system) and I found a datasheet for this monitor without much effort, so I did not have to redraw it directly from the board - I just sketched it from there as is.

Arduino

The relay is controlled by a simple transistor switch with a minimal set of elements (plus a resistor in the collector wire at 47 ohms): a NPN transistor KSC945Y (marked as "C945Y"), a protective diode 1N4148 and a resistor in the base wire for 10 kilohm.

// Relay testing (blink)

  int relayPin = 4;  // Pin D4

void setup () {

  pinMode (relayPin, OUTPUT);

}

void loop () {

  digitalWrite (relayPin, HIGH);
  delay (1000);
  digitalWrite (relayPin, LOW);
  delay (1000);

}  // Arduino IDE 1.8.4
Arduino

To test this relay module, I used a standard sketch for blinking LED through the 13th pin with a time delay of 1 second, after changing the variable name for this pin and the number of this pin. Everything is very simple here and it does not need a special detailed explanation.

Now, purely for the sake of interest, I will calculate (approximately) the saturation coefficient of the transistor that the engineer chose when designing this section of the scheme.

► 1. The load of the transistor is a relay (SDT-S-112LMR) with a coil resistance of 580 ohms (Rc1) and a resistor with a resistance of 47 ohms (Rc2). Calculate the collector current of the transistor (Ic):

Ic = Vcc/(Rc1+Rc2) = 13/(580+47) = 0.021A = 21mA (Vce(sat) => 0)

► 2. Calculate the base current (for checking - the measured value Ib = 0.4mA):

Ib = Vr/Rb = (Vin-Vbe)/Rb = (5-0.7)/10000 = 0.00043A = 0.43mA

► 3. The DC current gain (β) of the transistor (KSC945Y) is 120...240, so I take the minimum value β = 120 and calculate the saturation coefficient of the transistor (S):

Ib = S*(Ic/β) => S = (β*Ib)/Ic = (120*0.00043)/0.021 = 2.5

(!) Usually for such transistor switches it is recommended to take this value within 3...5.

For visual control of the current state of the relay module (ON/OFF), I soldered parallel to relay coil (12V) a LED with a resistor of 1 kilohm and this is not shown in the diagrams.
Since 2005, Payoneer's cross-border payments platform has empowered millions of entrepreneurs across the world to expand beyond their borders and grow successful and sustainable businesses. Thousands of leading companies and marketplaces including Airbnb, Amazon, Google, Upwork and others rely on Payoneer's services to send funds to their merchants worldwide. Register NOW and get your Payoneer prepaid MasterCard!