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MC33063A: MC33063 operation with 5V DC relay

Ahmet Inan
Intellectual 430 points
Part Number: MC33063A

Hi TI Support Team,

I am using MC33063 to step down ~13.5V battery voltage (Power Source/Vboard is 14.4V 2.2Ah Lead Acid Battery) to 5V. I am using 5V output to power 5 relay driver circuit. The MCU I am using to trigger transistors is 3.3V.  I have attached my schematic to this thread. In idle state my circuit draws around 17-20mA (MCU + 16x2 LCD +   5mm LED). 

My problem is as follows. The relay I am using is 5V relay with coil power of 0.36W. Every relay draw about 40mA in ON state when 5V is supplied from MC33063 (MC33063 supplies around 200mA for 5 of them). This is not normal operation as relay coil is expected to draw around 75mA  in ON state (0.36W/5V). I have done following to debug the issue.

1. I disconnected the relays from the circuit and leave only one of them, it again drew 40mA.

2. Wthout mosfet driver I connected relay coils between MC33063 5V out and GND, again the relay coil current was 40mA. 

3. I tested same relays with different power supplies. I connected them to my benchtop power supply 5V output, the relay coil draw around 75mA as expected. I connected relay coils between 7805 out and GND (in my another circuit), the relay coil current was again around 75mA as expected.

MC33063 does not supply requied current to relay coils. How should I modify my circuit that relay operate as expected? Could you please help me to solve the problem I mentioned? I cannot understand why the relay coil draw around half current when it is powered from MC33063?

I know that MC33063 is older IC, but hope you will help me with this issue. 

Regards,

Ahmet

  • 0
    TI__Genius 10385 points

    Hi Ahmet,

    Thank you for using E2E forums, regarding your query please allow me some time to get back to you, I will get back to you in the middle of next week.

    Maybe as another debug , you could try to have a normal DC electronic load and see if you see different behavior?

    Thank you

    BR,

    Haroon

  • 0 in reply to Haroon Wardak
    Intellectual 430 points

    Hi Haroon,

    Thank you for your reply. Unfortunately, I don't have DC electronic load, so I will not be able to test my setup with it. I will change timing capacitor value to see if it will make some improvement. 

    Best regards,

    Ahmet

  • 0 in reply to Ahmet Inan
    Intellectual 430 points

    Hi Haroon,

    I made further testing. I connected my circuit to power supply. The standby current of the circuit was around 15mA (current shown on power supply screen). When one of the relay was ON, the power supply showed around 55mA (40 mA difference was due to relay). I then measured relay coil current in ON state (current between mosfet source and gnd) with DMM and it showed around 70mA. Although the power supply screen was showing 55mA in total, the DMM coil current measurement was 70mA. I am not sure why this weird thing happens. 

    In my post, when I mentioned relay coil current was 40 mA or 75mA, I was stating the current seen on power supply screen. 

    Thank you, 

    Regards,

    Ahmet

  • +1 in reply to Ahmet Inan
    TI__Genius 10385 points

    Hi Ahmet,

    I would not trust the value shown on the power supply screen, it might not be accurate. When you measured the relay coil current using a digital multimeter you saw 70mA, which is the value it should be, correct?

    In general power supply values should be taken with a precaution as they might not be very accurate.

    Hope this helps.

    BR,

    Haroon 

  • 0 in reply to Haroon Wardak
    Intellectual 430 points

    Hi Haroon,

    Yes, DMM value was the one that coil should draw. Thank you for your help.

    Best regards,

    Ahmet