I plan to design a relay board, so that i can figure out the issues when assembling the actual hardware, also learn PCB schematic design and related things. There are already lot of threads in this forum on relay interfacing after studying them I was able to reach the current level. But there are few clarifications
The requirements i choose are when 3.3V is received from micro controller port the relay to be ON / OFF.
The specifications are
Relay - SRD-05VDC-SL-C-Datasheet

Coil Voltage	3.3V
Nominal Current	37.5mA
Contactor Voltage	12V
Contactor Current	10A

I simulated the circuit in LTspice and functionally it seems to be ok.
The following are the design calculations
The coil current is 37.5mA which is the collector current i have chosen 40mA on the safer side,
Calculation of the Base Resistor
Ic = 40mA ,
Beta in saturation is 20
Hence Ib = 40mA/ 20 = 2mA
Rb = (Vcc - VBE)/ Ib
Rb = (3.3V - 0.8V)/ 2mA = 1.25kOhm
Calculation of the Collector Resistor
Vcc = Ic*Rc
Rc = Vcc / Ic = 12V / 40mA = 0.3kOhm
LED resistor
I have considered 5mA as the forward current
Rled = (12V - 0.6V)/5mA = 2.26kOhm




Schematic
I use EasyEda for schematic design i attached the schematic pdf. The input connector shall contain 3.3V, Gnd, 12V and the output connector shall contain NC, NO, Gnd for connecting the load (as per my understanding)
Few concerns are should 12V be connected externally by the user and this system works only for 12V? Is the design correct? I have seen recommendation for Mosfet, i will go without in the next step once i am confident with transistor.
Thank you in advance for your time and request to please guide for corrections.

 

Hi V123,
Are you trying to drive a 3.3V relay coil from a 12V supply?

E

 

Hi V123,
Are you trying to drive a 3.3V relay coil from a 12V supply?

E

I want to drive the 12V relay from 3.3V, i mean coil voltage is 3.3V and contact voltage will be 12V 10A.

 

i mean coil voltage is 3.3V

hi V123,
The d/s for the relay shows the required coil current at 3.3V, which relay version, standard or high sensitivity?
E

 

Initially i thought of 12V then changed to 3.3V, i will redesign the circuit for 3.3V and post it. I plan to go for standard.

 

hi V123,
A point to remember is the operation 'speed' of the relay contact open/close.
You will not get 1mSecs, more like 10mSec.

 

Calculation of the Base Resistor
Ic = 120mA,
Beta in saturation is 20
Hence Ib = 120mA/ 20 = 6mA
Rb = (Vcc - VBE)/ Ib
Rb = (3.3V - 0.8V)/ 6mA = 0.416kOhm
Calculation of the Collector Resistor
Vcc = Ic*Rc
Rc = Vcc / Ic = 12V / 120mA = 0.1kOhm

 

hi V123,
Please post the LTSpice asc file that you are using.
BTW: Check the Ic max of that BC846B, 100mA.

E

 

Please find the LTspice model i used. I will change the transistor accordingly.

 

hi V123,
The method you are you using to set the relay current to a preset value will not work as you expect.
You could use a resistor in series with the relay coil, which is wasteful in power.

Or you could use a constant current source, but I would use a 12V relay coil as the easiest solution.
E

 

You might consider using a ULN2003A (7 channel) or ULN2008A (8 channel) peripheral driver chip if you're driving more than a couple relays. They use a low voltage control signal to control relays with up to 50 volt coils and include the base resistors and the back-emf diodes for the relay coils.

 

You might consider using a ULN2003A (7 channel) or ULN2008A (8 channel) peripheral driver chip if you're driving more than a couple relays. They use a low voltage control signal to control relays with up to 50 volt coils and include the base resistors and the back-emf diodes for the relay coils.

As a first step, i plan to understand and fabricate a board for single channel and verify it.

 

hi V123,
The method you are you using to set the relay current to a preset value will not work as you expect.
You could use a resistor in series with the relay coil, which is wasteful in power.

Or you could use a constant current source, but I would use a 12V relay coil as the easiest solution.
E

Thank you for the replies. Is it possible to briefly explain the issues in the design? Also how to include the relay library, i could not find in LTspice.

 

hi V,
When using that 3.3V relay coil you have either 25R or 20R coil resistance, so you need a series resistor to limit the current when using a 12V voltage source

E
This zip has all the relay models, unzip and install, ask if you have a problem.

 

Thank you for the replies. Is it possible to briefly explain the issues in the design? Also how to include the relay library, i could not find in LTspice.

Since you are using a transistor to drive the relay coil, you have isolated the mcu drive voltage from the relay coil drive voltage. Therefore, the relay coil voltage spec does not need to be the same as the mcu drive voltage. It can be different, as long as the same ground reference is used. So, with the transistor driver, you can use a 12v volt relay, and drive it with a 3v mcu drive signal.

The thing to remember is that the mcu drive output current is usually specified to a max of 40mA before subject to damage. So, the transistor drive current should be designed to be, at most, about 70% of maximum.

 

hi V123,
A point to remember is the operation 'speed' of the relay contact open/close.
You will not get 1mSecs, more like 10mSec.


View attachment 364889

View attachment 364891

R2 is not needed. Use a 12v relay.

 

As a first step, i plan to understand and fabricate a board for single channel and verify it.

If you are eventually going to drive multiple relays, the ULN2003/ULN2008 chips are a pretty common and reliable way to do it and will greatly simplify your design when it comes to driving more than one relay. Just suggesting alternatives that I have found useful.

 

I want to design something like this
1 Channel 3.3V Relay Module with Low-Level Trigger, and Optocoupler – REES52


Am i in any way close to this design? I am not opting for optocoupler.

 

The coil current is 37.5mA which is the collector current i have chosen 40mA on the safer side,
Calculation of the Base Resistor
Ic = 40mA ,
Beta in saturation is 20
Hence Ib = 40mA/ 20 = 2mA
Rb = (Vcc - VBE)/ Ib
Rb = (3.3V - 0.8V)/ 2mA = 1.25kOhm
Calculation of the Collector Resistor
Vcc = Ic*Rc
Rc = Vcc / Ic = 12V / 40mA = 0.3kOhm
LED resistor
I have considered 5mA as the forward current
Rled = (12V - 0.6V)/5mA = 2.26kOhm

how and why did you choose that current value? did you measure it? how?
because Relay - SRD-05VDC-SL-C shows that coil resistance is 70 Ohm.
5V (coil voltage) / 70 Ohm (coil resistance) = 72mA (coil current).

so the Ib is 3.5mA, so Rb = 750 Ohm etc.

do you have higher voltage (12V or whatever)? normally that is what would be used to power relay coils. relays need same power (same magnetic field) to actuate. so relays for higher voltage need less current. at 5V, they will need more current than at 12V, but this is something you can decide. using relay driver like proposed by Jon Chandler is a no brainer unless you are limited by space only need to drive one channel.

 

I got some clarity now this is the updated requirements and design



Now i consider 5V (as it is more common than the 12V from any evaluation board) and port from any MCU evaluation board.
Ic = 71.4mA
Ib = 71.4/20 mA = 3.57mA
Rb=R1 = (3.3V - 0.8V)/3.57mA = 700 Ohm
Rc=R2 = Vc/Ic = 5V / 71.4mA = 70 Ohm
RLed = R3= (5V - 0.6V)/5mA = 880 Ohm

The current drawn from the IO port is = 3.57mA
The current drawn from the +5V supply from the evaluation board = Ic + I_led = 71.4mA + 3.57mA + 5mA = 79.97ma.

One clarification is how do i adjust the resistors if someone gives 12V in place of 5V as input? I want to test it on the actual hardware, can i go for the pcb design?