# DC to AC inverter, Diodes conduction

#### Mohamed Bouakoura

Joined Aug 26, 2015
6
Hello everyone

in DC to AC inverter when the load is inductive we use the fly-back diodes to conduct the current in the opposite direction, but i don't understand why the current goes to the " + "of the battery when it flows through the diodes ?! .. in my opinion it should always go from (+) to (-).

when the switches VT1 and VT4 are off, the diodes VD3 and VD2 conduct .. i don't understand this part ... check the picture bellow.

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#### Dodgydave

Joined Jun 22, 2012
9,915
The diodes are for back emf suppression, when a coil is energised by DC, the collapsed field makes a bigger DC voltage in the opposite polarity and flows through the diodes, which protects the transistor.

#### crutschow

Joined Mar 14, 2008
26,734
Current does flow from plus to minus. You just have to know which voltage is higher (plus) during the transient.

Just remember that the inductance always tries to keep the current moving in the same direction as it was when the transistor was ON, even after the transistor turns OFF (at the point of turn-off, the inductance now acts as a voltage source to keep the current moving and will generate whatever voltage is necessary to accomplish this, so you add a transient suppressor to give the current a low voltage path).

So if you look at the voltages generated from this inductor current you will see that it will increase until it forward biases the diode going to V+ and thus the current will flow through that diode into V+.

Make sense?

#### Mohamed Bouakoura

Joined Aug 26, 2015
6
Current does flow from plus to minus. You just have to know which voltage is higher (plus) during the transient.

Just remember that the inductance always tries to keep the current moving in the same direction as it was when the transistor was ON, even after the transistor turns OFF (at the point of turn-off, the inductance now acts as a voltage source to keep the current moving and will generate whatever voltage is necessary to accomplish this, so you add a transient suppressor to give the current a low voltage path).

So if you look at the voltages generated from this inductor current you will see that it will increase until it forward biases the diode going to V+ and thus the current will flow through that diode into V+.

Make sense?

Thanks crutschow, now it's clear