# 24V H-Bridge DC motor, why voltage drop to 3v if motor connected

I want to drive a DC motor using an L298N H-Bridge Driver IC. I'm using the schematic from this website: The motor is 24 V with about 7.5 ohm impedance (thus, it draws about 3.2 A). I'm using a 24V power supply, rated 10A max.

If the motor is not connected, the voltage (measured either at Vin or motor terminal) is 24V, but once the load (motor) is connected, the voltage drops to 3V.

I've tried the logic input when load is disconnected, they work well as expected (measured as 24V or -24V at motor terminals according to logic input being fed).

Can you enlighten me about what's happening with this circuit?

P.S.: I'm using 2N3906 2N3904 for transistors, 1N5822 for flyback diodes. From schematic above I've also added a 10uF / 25V decoupling capacitor between Vin and GND.

• Are you sure the power supply isn't current-limiting? When the voltage drops to 3V, how much current is drawn from the supply? – pjc50 Aug 2 '13 at 8:58
• The L298N is a very reliable chip, the reference circuit looks ok so perhaps the fault is with your wiring. BTW the current a motor draws is determined by the load (torque). You can't simply use ohms law by measuring the resistance and dividing that into the voltage rating. The back emf generated when the motor turns will reduce the effective voltage across the resistance so I = (24 - Vback)/7.5 - a much lower value than you calculated. The 3.2A is the stall current . – JIm Dearden Aug 2 '13 at 9:10
• What logic supply are you using? – Andy aka Aug 2 '13 at 9:22
• @pjc50 : 70 mA @ 3.3V. It does current limiting at 10A. I've tried connect the motor directly to PSU, and the motor roll OK, so neither PSU nor the motor itself has problem. – excalibur Aug 2 '13 at 11:48
• @JIm Dearden : Wiring should be OK since if motor is unconnected the circuit works as expected (motor terminal J1 provides 24V on logic 10 and reversed polarity on logic 01). The motor has no load attached, so it's torque resistance should be at minimum. For logic, I'm simply connect it to ground for logic 0 and 5V for logic 1. 5V power rail come from different source (7805 regulated from 9V power supply, too much voltage burned for using 7805 from 24V supply) with both shared ground. – excalibur Aug 2 '13 at 12:12