Zener Diode Protection

You need to fit a series resistor..... or the zener will SMOKE!

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Hi,

How can I choose a right value for the resistor?

John

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Hello John,

Protection against what?

I am thinking of peak voltage/current and duration, output impedance of source, etc. How should the circuit behave under abnormal conditions?

Wim PA3DJS

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You have to decide what maxiumum voltage you are going to protect against. Lets say 100 Volts. Then find the maximum current for you diode. Ohms law will then tell you what resistor to use. You should check the power dissipation in the diode too.

George H.

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Hi,

I do not want the DC to DC input voltage to exceed 40 volts. Thats why I want to put a zener diode at the input of the DC to DC converter. If the maximum forward current is 200mA and the voltage is 40 volts than I need a resistor of 200 ohm. Am I correct? The power rating of the diode is 0.5 Watts.

John

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Hmm, You first need to decide what is the maximum possible input voltage that you are protecting against. Can someone hook up 100 Volts to this? Or is the maximum 50V? Let's try with 50V With 50 V on the input I want to limit the current through the zener to 200mA. There's ten volts across the resistor (assumming a 40 Volt zener to make the numbers easier.) So that would be a 50 ohm resistor. But now during the fault your zener has 200mA at 40 V. That's 8 Watts. (Hmm maybe you need a different solution. ) You could choose a bigger resistor, but it's aways in your voltage path wasting power.

George H.

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You probably ought not to do it this way. The current through the ballast resistor will rise rather steeply with over voltage, and will also rise with under voltage at constant output load current.

Do you want to protect the thing in the sense of a crowbar to blow a fuse when an excessive voltage is at the input?

OR

Put a manky hot linear regulator in front to drop the input voltage to be less than 40v, waste power and wreck the switching regulators performance no matter what insane voltage is presented to the black box?

In the latter case you would be better off looking for a switching part that can survive whatever voltage the input is ever likely to see.

Regards, Martin Brown

Hi,

I need to protect the inout of the following DC to DC converter

I choose the following zener diode

I put the zener between the positive input and ground of the chip but as soon as I increase the DC voltage above 40 volts it get burned. Is the zener supposed to restrict the voltage. For example, if I apply

40 volts than it should restrict to 38 volts if I use a 38 volts zener.

Thanks. Please advice!

John

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Zeners work by providing a "short" when their voltage exceeds a certain amount. They simply do not "reduce" voltage without some means.

When you put your power over 40V the zener kicks in and it is as if you simply ground the circuit. All that current flows through the zener which will burn it up. If you got a bigger zener you may end up burning up your power supply if it can't cope.

For base overvoltage protection you need a fuse and a crowbar circuit. The crowbar uses a zener like you used except it limits the current through it by using a resistor. When the zener kicks in it triggers an SCR which provides the short to ground(instead of the zener as in your case). The SCR can handle more current, usually, than a zener. When this happens a large current will flow out of your power supply into your circuit(well, up to the SCR). You need a fuse in the path that will blow due to the large current.

There are better ways that do not need fuses. A modification of the above is to use a polyswitch instead of a fuse which is like a resettable fuse. This way you don't have to replace fuses much.

Are you trying to protect continuous or transient overvoltages? A zener, TVS, or MOV can help with transients. Crowbars and other methods can help with continuous.

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There's much more to it than just that.

Your DC to DC converter is, much like a transformer, reflecting the
power consumed by the load back to its input.

That is, running at 75% efficiency with a 1.75 watt output, the input
will require 2.3 watts from the bulk supply regardless of the voltage
from the supply, as long as it stays between 6 and 40 VDC.

Assuming the DC to DC converter's output is fully loaded, 40VDC on its
input will result in its input current being:

         P     2.3W
    I = --- = ------ ~ 0.058A ~ 58mA
         E     40V

Likewise, a 6V input will result in an input current of about 383 mA.

But now I'm stuck.

You haven't specified what the range of output voltages from the bulk
supply might be, so a ready solution isn't in hand.

Can you help?

Hi,

The input voltage range is between 3 volts to 60 volts. And I need to restrict the input voltage to 40 volts. The load at the ouput of the DC to DC converter need 100mA and 3.3 volts.

Thanks

John

The first question to answer is what do you want to happen when 40v input is exceeded. Reading between the lines a bit it looks like you want the dc-dc convertor to conitniue working happily. That can be achieved by using an LDO linear regulator rather tahn a zener, so that as Vin exceeds 40v, the switcher sees 40v and the linear reg drops the excess. As already said, making your switcher handle the full 60 would likely be better, if practical.

Realise that most possible solutions will affect the minimum input voltage. An LDO reg will add a fraction of a volt to the minimum workable input voltage.

NT

Hi,

Can you recommed any voltage regulator whose output follows input linearly and does not exceed 40 volts output. Yes I do want DC to DC converter to work at 40 volts.

John

--- According to the data sheet, Vin(min) guaranteed to make the thing work is 4.2V, so if your bulk supply has a Vout(min) of 3V, you're going to be very disappointed with the performance at the low end.

Is there any way you can increase Vout(min) from your supply?

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--- OK, but if you can't solve the Vin(min) problem, you might as well hang it up.

That's not to say that there are no solutions. There are, but how much money are you willing to spend to get there from here?

-- JF

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Since a switcher is like a transformer in that it reflects the power
dissipated by the load back to the input, that won't work.

Do you need an explanation?

Hi,

The minimum Vout of the power supply is 5 volts. I miswrote the value.

Thanks John

--- Yeah, right.

I suspect that you don't know what you're doing, but you're looking to get a commercial application off on the cheap and don't want to pay for competent help.

Fuck you and the horse you rode in on.

-- JF

A 40v LDO regulator would do that.

NT

It might be simpliest to find a DC-DC converter that can handle the 60 V input.

Otherwise some active over-voltage circuit on the input.

George H.

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" Do you need an explanation?"

Sure.

With a big enough zener and a small enough current limiting resistor I think I could make it work. Hmm maybe a polyfuse too? Though I'll admit the wide voltage inputs make the problem difficult. An active 'something' on the input would be easier.

George H.