# Thyristor (Forward Current RMS)

#### simpsonss

Joined Jul 8, 2008
173
hi,

I'm building a overvoltage protection circuit by referring to the circuits below.

From the datasheet of 2N1595, it mentioned that it is a silicon thyristor. But i'm not sure why is a TRIAC symbol there.

So to turn "on" the thyristor, current need to be applied on the gate. By refer to the datasheet, the value for FORWARD CURRENT RMS is 1.6Amp. So is it means that, at least 1.6Amp is needed to "on" the thyristor? Am i correct?

thank you.

#### ifixit

Joined Nov 20, 2008
650
1. The 2n1595 is an SCR, therefore the symbol is wrong.
2. This SCR can short out a load of not more than 15 amps for approximately 8mS. This may blow the fuse but the 1000uF cap may deliver 100's of amps for a short time.
3. The min. gate trigger current is 2mA at a gate voltage of 2V.
4. The 1n3997 5.6V zener is a huge power device and overkill for this application. Choose a half watt model.
5. Since gate voltage is 2V then the overvoltage trip point would be 7.6 volts. Is that okay?
6. If you already have the zener then just put it across the 5V output alone and get rid of the SCR. Test by shorting out the regulator to see if the fuse blows.
Although the circuit is simple, the devil is in the details.

Good Luck

#### simpsonss

Joined Jul 8, 2008
173
The min. gate trigger current is 2mA at a gate voltage of 2V.
So it means to trigger this SCR we need 2mA current at 2V. How actually they choose the spec for the zener diode and SCR for this circuit?

For what i understand, once the output voltage of the 7805 exceed 5.6V,then reverse bias will happen on 1n3997 and thus current also pass through it. And the SCR gate pin will get a voltage exceed 5.6v (am i right?) To trigger SCR we need 2mA at 2V.
For the voltage it meets the requirement which is higher than 2v but what about the current part?
A 7805 output 1A current (am i right). so 1A will pass through the zener and reach the gate pin of SCR and this meet the requirement for the SCR trigger current part. Am i correct?

thank you.

#### simpsonss

Joined Jul 8, 2008
173
Is it possible to replace the zener diode with 1N3827 (1 Watt 5.6V) and replace the SCR with a MCR100-4A (0.8A)?

thank you.

#### ifixit

Joined Nov 20, 2008
650
The zener will conduct at 5.6V and start to raise the gate voltage above ground, but the gate won't conduct and trigger the SCR until 2V is reached between gate and ground. This is why I add 2+5.6 to get the trigger voltage.

A smaller zener can be used as you suggest because the SCR is fast and will blow the fuse before alot of zener current can flow. A 1 watt 5.6V zener would be a better choice, but triggering would still be at 7.6V. Try a 3.6V zener, trigger would be at 2+3.6=5.6V.

Like diodes, SCRs have 2 current ratings: one which specifys the continuious current carrying ability (e.g. 1.6A), and one which specs a short term surge rating(e.g. 15A). You will have to determine what peak current your SCR would see in your application. The one you have may work, but one with a 5A continious rating might be a safer guess if you can't make measurements.

Good Luck

#### Ron H

Joined Apr 14, 2005
7,014
This datasheet says the gate trigger voltage is typically 0.7V, not 2V.

#### ifixit

Joined Nov 20, 2008
650
Good point from Ron H.

If this is a "one-of" design then test the device gate voltage trigger point and go with that. The actual trigger point can vary alot amoung devices. The spec I checked indicated a max of 2V, but the spec on the benz site says 3V is max

How accurate does the protection have to be? If the trigger voltage is closer to 0.7 then the zener will have to be choosen to suit.

Regards,

#### simpsonss

Joined Jul 8, 2008
173
thanks for the reply. ok 5.6V protection is just an assumption. The actual protection voltage i want is actually not to exceed 5V or 5.1V.

Ifixit:
"The zener will conduct at 5.6V and start to raise the gate voltage above ground, but the gate won't conduct and trigger the SCR until 2V is reached between gate and ground. "
You mentioned that the "start to raise the gate voltage above ground", how does it raise? do u mean the gate voltage raise depends on the voltage pass through the zener diode?

#### ifixit

Joined Nov 20, 2008
650
I'm not sure how to explain it so you can understand, but the gate looks like a small zener of 0.7 to 2 volts so it won't conduct current until at least that much voltage is present. The zener won't conduct current until at least 5.6 volts is across it. Once the zener is conducting, it tries to maintain 5.6 volts across it even if the current increases. Since the two are in series you need to add the voltages to determine the trigger point.

Experiment, and read-up on zeners, SCRs, PN junctions, ohms law etc. to be able to understand how these things work. This forum contains reference material that can help.

Good Luck

#### eblc1388

Joined Nov 28, 2008
1,542
The zener won't conduct current until at least 5.6 volts is across it. Once the zener is conducting, it tries to maintain 5.6 volts across it even if the current increases.
This is not 100% correct.

A zener diode will start to conduct current when its terminal voltage is more than 3/4 of the rated specified voltage. Therefore a 5.6V zener may conducts at say 4.5V or more, albeit it with a small current of some microamp (uA). When it is about 95% of the zener voltage, the conduction current can be in the mA range. Thus a 5.6V zener can conduct as much as a few mA at 5.2V.

The lower the zener voltage, the most pronounce is this characteristic.

In the image, the curve marked C5V6 and others, you can see that the turn ON is gradually instead of abruptly as most people would think or want it to be.

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#### simpsonss

Joined Jul 8, 2008
173
hi,

what is the purpose the line written in datasheet which under Gate Trigger Current "VAK =7.0Vdc,RL= 100ohm" ?

thank you

#### Ron H

Joined Apr 14, 2005
7,014
hi,

what is the purpose the line written in datasheet which under Gate Trigger Current "VAK =7.0Vdc,RL= 100ohm" ?
View attachment 15710

thank you
That's the test condition: 100Ω load to a 7V supply.