Basic SCR question

Discussion in 'The Projects Forum' started by Bob123456, Sep 20, 2014.

Aug 14, 2013
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2. b1u3sf4n09 Member

May 23, 2014
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Where did you get 1 to 8A? The datasheet specifies 50 mA max gate current (Table 3, electrical characteristics, row 1).

EDIT: Ah, I see. 8A is the maximum surge current the gate can survive, but that was only tested for 20 microseconds. Under normal operation, you should never bias the SCR gate above the nominal maximum value, which in this case is 50 mA.

3. Bob123456 Thread Starter New Member

Aug 14, 2013
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Apologies, I put up a slightly incorrect datasheet before (my specific model is the 800RG not the 1200N) so here is the link to the correct one, it is very similar to the previous.

This sheet still lists a similar Igt at the top of the sheet in the main features table (80mA), But I was looking at figure 4 and it shows the gate trigger current (and latching +holding currents) to be around 1A at room temperature. I'm confused as to which one should be used.

4. b1u3sf4n09 Member

May 23, 2014
115
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That is relative current. It shows the amount of variation in required current based on temperature. 25 degrees Celcius is considered ambient temperature under normal conditions, and most values printed on datasheets are obtained at this temperature (Tables 4: Electrical Characteristics (Tj= 25°C, unless otherwise specified)). This graph allows you to derate your gate current with respect to the junction temperature of your SCR. That is why the line for Igt crosses 1 at 25C.

So we know this SCR has a max nominal gate current of 80 mA. At a higher temperature, this allowable max current will go down as the junction temperature increases. As an example, let's say that your application called for running the SCR with a junction temperature of 60 C. Looking at the graph, this would derate your gate current by ~0.7, so your max nominal gate current would be 0.7 * 80 mA = 56 mA.

Clear as mud?

5. Bob123456 Thread Starter New Member

Aug 14, 2013
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Yeah that chart makes sense now. The electrical characteristics table is still confusing me though.

From that table it says that the max/min for the gate current is 80mA to 8mA respectively (I assume inside this range means the SCR is turned on). However it says that the maximum latching current is 200mA and max holding current is 150mA. This confuses me because the max latching current is well above the 80mA maximum given (maybe the 200mA is for a short time only?) . Also I thought the holding current was the minimum current that needed to flow between the anode and cathode to keep the SCR turned on, therefore the maximum would be 50A?

Also I'm not quite sure what Vd and Rl refer to in the top of that table either.

6. crutschow Expert

Mar 14, 2008
13,496
3,373
The Holding Current is the minimum anode-cathode current required to keep the SCR conducting current. Below that current the SCR may turn-off. It has nothing to do with the gate current.

The Latching Current is the minimum anode-cathode current required to insure that the SCR will turn on when triggered by the gate current. Once it is turned on then the Holding Current value is valid.

The call those values maximum, but that seems confusing.

7. Bob123456 Thread Starter New Member

Aug 14, 2013
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Yeah that's what I thought, the maximum label was confusing. The only thing I don't get now is why the max value for Igt (I assume this is current through the gate?) is only 80mA?

8. crutschow Expert

Mar 14, 2008
13,496
3,373
I would assume the small gate junction area can't tolerate a steady current any greater than that without damage. It's similar to the maximum base current rating of a BJT.

9. Bob123456 Thread Starter New Member

Aug 14, 2013
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Yeah that makes sense. 1 last question, what is the Rl in the test conditions refering to?

10. b1u3sf4n09 Member

May 23, 2014
115
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Load resistance to set current flowing through the SCR.