Over-voltage Protection

Ron H

Joined Apr 14, 2005
7,063
You have a max of 300mA output current ?

if so, these components are way to much heavy dutty for that!

There are SCR's with a Holding Current(IH) of less than 10ma , you will need a resistor that allows that and a Zener with a Izt equal or greater..

example : TIC 106 (5amp SCR), IH of 8mA, 1volt triger
zener BZX85c33, with Izt of 8mA
resistor : R(max) = 30v/0.008 R(max) = 3750, near EIA e24 code = 3300ohm
capacitor : more bigger the more stable versus delay it gets


this is just a cheaper option, do you agree CDRIVE ?
The zener current is determined by the voltage drop across resistor. This is not the supply voltage, it's the gate-cathode trip voltage. In your example, that's 1 volt, so the resistor should be 1v/8ma=125Ω. The nearest standard value is 120Ω.
 

edmundopt

Joined May 4, 2011
60
Hello Ron H

As you see I'm a newbie arround here, I was calculating the minimum current flowing trough the SCR to keep it at Holding current (IH) after triger, is this incorrect?

I'm still learning :)
 

CDRIVE

Joined Jul 1, 2008
2,219
edmundopt,
Here's an explanation of my circuit and associated plots:

First off, it was not intended as a parts list for Lionhart88 to replicate. Lionhart88 was having difficulty grasping how the Zener operates in this application, which is opposite of the configuration that he is familiar with. Rather, the circuit is a generic Crowbar intended only to enhance his understanding of the Zener's function. I used a 1k Zener resistor (R1) to stretch the gate voltage curve so Lionhart88 could more easily see how the Zener starts to conduct below Vz. In practice it would be much lower value.

Edit: I did further sims using the same SCR and found that the gate current properties of this SCR is such that the value of the Zener resistor (R1) has very little effect on the gate voltage curve. If this was a sensitive gate SCR R1 would play a larger role here, but it isn't. This SCR has a minimum gate trigger current of 40mA, so it is dominant over the current that R1 pulls. Removing R1 barely changes the gate plot.

Also, I realize that his power supply is only 300mA max and I'm not suggesting a fuse or breaker current of >2A, as indicated. Keep in mind though.. a shorted filter capacitor can briefly produce currents far in excess of the capacitor's charging source. See attachment.
 

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Ron H

Joined Apr 14, 2005
7,063
Hello Ron H

As you see I'm a newbie arround here, I was calculating the minimum current flowing trough the SCR to keep it at Holding current (IH) after triger, is this incorrect?

I'm still learning :)
I'm not sure what you are getting at. Holding current is the current required through the anode-cathode circuit to keep the SCR turned on. If that current drops below the holding current, the SCR will turn off. In this circuit, the fuse will blow (hopefully), and then the SCR will turn off, which is OK.
It is theoretically important to set the zener current that has to flow before the SCR turns on, because zeners are not perfect. Their voltage rises as the current through them rises. (Zener impedance specifies the magnitude of this effect.) if you set the current too low, the crowbar voltage will be too low.
Having said all that... In actuality, for high voltage zeners, the exact current is not very important, because zener tolerance is much more significant.
 

Thread Starter

leonhart88

Joined Feb 23, 2007
118
Thanks a lot CDRIVE, I really appreciate it. I think I have a better understanding of how it works now. I need to do some calculations and then I'll post my circuit up for you guys to take a look at. I'll also try to simulate it as well...but the last time I simulated anything was several years ago.

My input voltage is supposed to be 18-30VDC. The 5V converter chip I'm using can take up to 36VDC. Since the zener conducts early, I assume picking a 33V or 34V zener would be a good choice. I also hear that temperature can greatly affect the performance of this crowbar circuit. I haven't calculated the tolerance yet, but do you have an idea of how much the performance would change?

Also, what do you think about this LTC4365 chip? It seems like it would be more reliable, especially across varying temperature ranges.

http://cds.linear.com/docs/Datasheet/4365f.pdf

Thanks!
 

dollat7852

Joined Sep 22, 2010
5
All you need is to connect a resistor in series to zener diode.
The i^2r loss in the resistor reduce the output power.
Base on the new output power and voltage the rating of zener diode can be determine
 

#12

Joined Nov 30, 2010
18,224
I'm glad you did some learning while I was at work for a few days. I hope you will "sim" or build with the values I put in post 6 and let me know how that turns out. According to my beliefs, they should work, but you (I) never know the exact details until the electrons flow through the parts.

At least you have enough knowledge to zero in on what you want.
 

Thread Starter

leonhart88

Joined Feb 23, 2007
118
Hey guys,

Just to let you know...I did simulate and read up some more on the crowbar circuits and I'm pretty sure I understand how they work now. The only trickier part was to calculate the resistor values, etc. for optimizing...however, like CDRIVE mentioned, it's not THAT important.

As for my circuit...I ended up realizing that the current into my switching power supply DC-DC converter is going to be much smaller than the output. My output would be max 200mA (my electronics actually only draw 80mA which I measured this weekend). This means the current in is only around 50-70mA. Since my application is such low power, I ended up just using a TVS diode and fuse combination for protection and it should probably be sufficient. The TVS behaves like a zener, but has a sharper "knee" and better current rating. I was considering the crowbar because I couldn't find any zener diodes with enough current capacity. I ended up using a 150mA fuse and a 6.5W steady state TVS diode (1500W surge).

There are so many ways to achieve over-voltage protection so it was quite time-consuming and confusing at times. Thanks a lot for all your inputs and help!
 

CDRIVE

Joined Jul 1, 2008
2,219
Hey guys,

Just to let you know...I did simulate and read up some more on the crowbar circuits and I'm pretty sure I understand how they work now. The only trickier part was to calculate the resistor values, etc. for optimizing...however, like CDRIVE mentioned, it's not THAT important.
Uh, as I recall I didn't say exactly that. There was far more to that statement.
 
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