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Choosing Diode values include details

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barg

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Hi,

I need an assistance to chose the right diode values for protecting the circuite from reverse voltage and current protection.

vin 12v, max forward current /consumption of the circuit can reach 7A.

do i need to choose a 7A max forward current? can anyone specify example models?

is it better to use a schottky diode as to fast responce?

Thanks,

- - - Updated - - -

in addition, if anyone understands for voltafe spikes protecting, can i use a capacitor after the diode? can you advice for the capacitance value for the bellow?

1. 12vin, circuit load up to 1A.
2. 12vin, circuit load up to 7A.

Thanks,
 

Any 7A diode will do; you need to account for the diode drop and the heat produced on that account.

Because you are using for a specific purpose of protection from reverse voltage and current, Schottky diodes are not essential.

Yes, you can use a capacitor for voltage spike protection and the value of the capacitor and the voltage rating will depend on the typical spike energy and amplitude. Follow the standard (or recommended values for the application in mind) recommendations.
 

Thank you for your reply,

As for capacitor, i dont know how to mesure the spikes amplitude, tough it feeds from a switch walll adapter.

Can you advice where can i find this standard capacitors values?

as for the voltage, i think a 30v will be enough, i am missing only the capacitance value for 1A and 7A?
 

I agree that it is impossible to have any meaningful guess about the energy and the voltage of random spikes that arrive without notice (or any formal invitation) on the input side.

If you think that 30V is enough, please then use 63V ceramic capacitors (they have low ESR, very desirable for bypass), about 0.1uF on the power supply line. Use electrolytic capacitors for ripple filters.

As your power supply voltage is 12V, also use 22V 0.01uF ceramic capacitors liberally near (physically close) the IC power pins. They are certainly very effective in preventing unwelcome headaches.
 

A diode in series with the power supply input of a product is called: "an idiot diode". Do you know why?
 

Audioguru said:
A diode in series with the power supply input of a product is called: "an idiot diode". Do you know why?
Click to expand...

Yes. Because nothing can be foolproof because fools are so smart.
 

please enlight why an idiote diode?

i am not fully understand, please advice if a 0.1uf next to Vin is not too small for my application which use in this circuit 7A, or is that not relevant?

Audioguru, if you think that putting a diode in series with Vin for over voltage protection is idiotis, do you have alternate idea for the same solution?
 

I made a scheme with 2 capacitors and 1 schotkey, please advice if that can do the job OVP.png?
 

These are bypass capacitors; they are not meant for reducing ripple. For high-voltage, short duration spikes, they will essentially short to ground. Experience says that they do their job well and you should also put them near the power supply pins of important ICs.

It does not matter whether the main circuit is carrying 1A or 10A.

A diode in series with the power supply does not offer overvoltage protection. It ensures that the circuit is protected from reverse polarity connection (if you connect -ve to the +ve terminal of the battery, your circuit will not get damaged)
 

Thanks and well noted,

Attached is a revised scheme with Audioguru comprehensive location change. i also added a 0.1uf before each load and mosfet, please advice if this can reduce also electric spikes to protect individualy each of these mosfets? as for rest of the ic's i have already 0.1uf.

regards,

OVP.png
 

C5, C7 and C8 may be shifted across the mosfet. Yes, they can help protect the mosfets but the actual voltage and energy of each spike depends on the nature of the load. Also put a small resistance in series with the mosfet gate This will reduce the turn on and turn off spikes that may come from the loads.
 

I changed the location and added a 1k resistors, load is 3 leds, 2a each, please advice if that is right?

Thanks,

OVP.png
 

1K gate resistor is very large; please replace it with something like 10R or so. The capacitors on the three mosfets need to be re-evaluated depending on the nature of the load. Rest appears fine.
 

Mitra Thank you for your assistance.

please give me a direction for understanding the nature of the load? the load profile can't be found among the details i have noted?

regards,
 

The load profile will be:

voltage needed;
current needed;
reactance component (for non-resistive loads);
variations (changes in load current);

If your load is a heater, there is nothing much to be done.

You will need a diode-capacitor-resistor based snubber if the load is a motor (inductive).
 

Hi,

Please find the relevat details:

voltage needed:11.5-12v
current needed max 2A for each mosfet.
reactance components: not asigned (donthave any coils or similar)
variations: can vary from 0.5A to 2A max.
load is an led for each mosfet.

as it is not a heater, no need for extra paralel diode.

Thanks,
 

barg, you need lots of help. please provide part numbers for all components you have picked out.

What is the 12v power supply.

What is the output current of the micro processor per pin and the voltage. As C-mitra said your gate resistors are very high. Do you understand you should be using logic level FET's.

This is the only stocked diode i could find that would work for you. It needs a heat sink.

https://www.digikey.com/product-detail/en/sanken/FMB-G24H/FMB-G24H-ND/4289225

The voltage rating of your snubber capacitors should be 100V minimum for the few extra pennies they cost and unknown noise levels.
 

Thank you FlapJack.

I am using a wall power source ac/dc switch 12v out.

mcu output current is 25ma and 5v a pin, i will use 10r as Mitra suggested, is this value remains based on mcu details i noted now?

Diode, can i use any other shotky diode or need any specific considerations?

As for the caps voltage, i will change to 100v for all (c3, c4, c5, c7, c8), well noted your remark.

Regards,


mosfet is logic, i am aware of that, i am using FDMA430NZ.

as for the diode you suggested, i am trying just to understand, is a 10A is too high as i need to protect up to 7A and stop conducting when it reaches 7A, in that case, the diode will stop at 10A instead?

as for de
 

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