# Diode question

Discussion in 'The Projects Forum' started by help_mum, Sep 24, 2012.

1. ### help_mum Thread Starter New Member

Sep 13, 2012
6
0
I have a problem with what Diode to use. I need to protect the regulator and circuit from the V+ side of an AC input.
The plugs are 3 pin and reversable Yes I know get a differant plug but these are the only ones I can find that will fit. Schematic included I hope.
I'm just a beginner and still confused a lot Sorry about the faded wires on the Schematic.
I was going to use a ROHM 50Hz Signal Diode but it was only 25ma then I thought a fast Shottky 500ma would be Ok and then I got confused
The circuit is surface mount "very little space to fit it" the Diode can only be 0603 Package as largest size the Caps. are Tantalum. C and A size.

THANKS Guys!
mum

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2. ### Dodgydave AAC Fanatic!

Jun 22, 2012
7,655
1,272
How can you have a + side on an AC signal???

That's what AC does it reverses its self, so the diode will be conducting on the other cycle!

Last edited: Sep 24, 2012
3. ### cork_ie Active Member

Oct 8, 2011
400
66
Why are you using a 50HZ sq wave power supply? Surely DC would be easier? Please correct me if I am missing something or don't quite understand your circuit.

I don't understand why,you don't just use a series Si diode in the input.

You have 7.4-8.4 Volts in your supply input and your output is 3.3V. You are using an ldo regulator and should have plenty margin to drop the 0.7 Volt required.

Schottky's have a lower voltage drop but suffer from high reverse bias leakage, although this shouldn't cause a problem either in this particular application.

4. ### cork_ie Active Member

Oct 8, 2011
400
66
Yes I thought that too , the only thing I can think of is that it is that the ground of the square wave is at 0V and the signal is positive relative to that.
Wouldn't a square wave be = a pwm signal at 50% duty cycle?

5. ### help_mum Thread Starter New Member

Sep 13, 2012
6
0
That is right
The Square wave 50hz is PWM signal to a microcontroler the pulse time is 20ms The pulse can be as short as .9ms and longest is 2.1ms. It isn't like a 555 signal where a50hz signal has a multi pulse during the 20ms cycle. Every 20ms the Mc receives a pulse and this is the referance for the mc.
The Mc. converts this AC signal to a voltage this is sent to position reference Pot the voltage then goes to the OP AMP which sends out a offset voltage the 2 reference voltages go to the MC's comparator and then the Analoge control voltage to its ADC and then this is sent to control a motor via HexFets gates.
The Diode is to protect a rail to rail OP Amp. and the Mc. from putting the input plug in reverse,At first I didn't think it would cause any problems but it turns out that the pulse is strong enough to burn out the OP AMP. The schematic only shows the input to the DC power input to the voltage regulator that supplies power to the 3 volt part of the circuit, the unregulated DC powers the motor. I have left out a lot of components as they are not needed for the explanation.

Hope this Helps
I need to protect the circuit and a diode is the only thing I can think of that will do the job.