Hi All,

Description:
Please refer the attached circuit diagram, the 12 V supply driver 3 relay of 250mA and the 5V is connected to micro controller & 20x4 LCD.
Issue:
The circuit works fine if 2 relays connects but when the third relay connects, the LCD display scrambles. I believe the sudden high current drops the 5V voltage . Please let me know do i have to put another TIP2955 against 7805 Regulator.

Help is much appreciated

 

1. The 7812 should be able to drive the relays without the additional pass transistor. Why do you think Q4 it is needed?

2. The 7805 input should be connected directly to the bridge output (top of C1). In this way, the 5 V output current does not come though the 7812.

3. Are there suppression diodes across all three relay coils?

ak

 

1) When i tested without the Q4 transistor circuit, only 1 relay is connecting and that too after few seconds even that relay doesnt connect but microcontroller pass the signal correctly.

2) yes, 1n4148 used against each relay to suppress the reverse current.

With this current circuit, all 3 relay works but only the LCD display data scrambles, sometimes. Please let me know how to avoid the LCD display scrambling.

 

The regulators will supply 1amps easy, i would increase the output capacitors to 4700uF, and feed the 5V regulator from the bridge rectifier directly to avoid pulses.

 

Show us the rest of the drawing.. You don't show the relays or what they control.

 

Thanks for the response. Please find the rest of the circuit.

 

1. There are no decoupling capacitors for either the uC or the display. Add a 0.1 uF ceramic and a 10 uF electrolytic in parallel from *each* power pin to GND. Make the leads as short as possible.

Also, there is no need to squish the schematic down to a small drawing size. It will be easier to see your design if you spread out the circuit elements and eliminate all of the little steps and angled connections.

ak