Maximum allowable SCR gate voltage

Thread Starter

RGPATTER

Joined Jun 2, 2011
16
I am trying to control the speed of a DC motor using an RC network to change the firing angle of an NTE5418 SCR. The trigger voltage gate to cathode for an NTE5418 is 1.5 volts. What is the maximum gate to cathode voltage that can be applied without damaging the SCR?
 

#12

Joined Nov 30, 2010
18,210
I don't see a "31" anywhere on that datasheet.

The gate to cathode circuit is like 2 diodes in series. What is the maximum voltage you can apply to two conducting diodes and not hurt them?

Silly question. The voltage that can be seen across those two terminals is the result of the trigger CURRENT. Apply proper current to the gate and whatever voltage shows up is a side effect.
 

Thread Starter

RGPATTER

Joined Jun 2, 2011
16
Lets assume we are dealing with someone (me) who doesn't know how to read a data sheet. Can someone tell me what maximum gate current or gate voltage is allowed if damage to the SCR is to be avoided?
 
Last edited:

#12

Joined Nov 30, 2010
18,210
Average gate power of 1/2 watt divided by max gate voltage of 1.5 = 333ma

but why don't you just feed it the 25 ma that is required to turn it on?
 

Thread Starter

RGPATTER

Joined Jun 2, 2011
16
I am varying the firing angle from zero to 80 degrees by adjusting a variable resistor in the RC circuit from zero to 1,000 ohms. With the resistor at 1,000 ohms the voltage at the gate is 1.5 volts and the gate current is 25 ma. As the resistor is decreased, the voltage applied to the gate rises; and I am concerned it will exceed the value where damage to the SCR will result.
 

#12

Joined Nov 30, 2010
18,210
The voltage supplied to the gate doesn't matter. It's the current. The voltage is just a side effect of the current flowing through the (2) forward biased junctions.

You should be thinking in terms of the voltage applied to a resistor in series with the gate and calculating so that you never exceed 333ma. This should not be a problem because you are only needing 25ma, less than a tenth of what it takes to melt the gate.

I suspect that you need a resistor in series with the variable resistor so you can never get to zero ohms as you turn it down. However, you have not said what voltage you are starting with so I can not calculate the minimum resistor that you need for safety.
 

Ron H

Joined Apr 14, 2005
7,014
The voltage supplied to the gate doesn't matter. It's the current. The voltage is just a side effect of the current flowing through the (2) forward biased junctions.

You should be thinking in terms of the voltage applied to a resistor in series with the gate and calculating so that you never exceed 333ma. This should not be a problem because you are only needing 25ma, less than a tenth of what it takes to melt the gate.

I suspect that you need a resistor in series with the variable resistor so you can never get to zero ohms as you turn it down. However, you have not said what voltage you are starting with so I can not calculate the minimum resistor that you need for safety.
I believe the gate to cathode is only one pn junction.
 

Thread Starter

RGPATTER

Joined Jun 2, 2011
16
When the resistor is 1,000 ohms I have 4.333 volts applied to a resistor of 117.3 ohms in series with the gate to cathode resistance which NTE tech support says is 60 ohms. Did #12 mean to say to say that the gate to cathode circuit is like 2 diodes in series?.
 

Thread Starter

RGPATTER

Joined Jun 2, 2011
16
I admittedly don't know how to read a data sheet, but how do we conclude that 1.5 volts or 333 ma is the max allowed on the gate?
 

#12

Joined Nov 30, 2010
18,210
1)The data sheet says that 1.5 volts is the maximum voltage required to trigger the gate.
2)I showed you the math to get 333 ma in post number 7.
3)I said there are 2 forward junctions but Ron H pointed out that there is only one forward junction.
 

Thread Starter

RGPATTER

Joined Jun 2, 2011
16
I took the data sheet to say that 1.5 volts would trigger the SCR, but I didn't think it said 1.5 was the maximum gate voltage allowed.
 

Ron H

Joined Apr 14, 2005
7,014
Here's how I read the datasheet:
The 25mA spec means the device is guaranteed to trigger if you apply 25mA gate current. IMHO, that should probably be in the minimum column.
If you apply 25mA, the gate voltage will be no higher than 1.5V. Given these two pieces of information, you can calculate a gate current limiting resistor. For example,with 5V applied to your gate current limiting resistor, the resistor should be no larger than (5V-1.5V)/25mA=140Ω.
Maximum gate current is 1A, but only for less than 3uS.
Maximum gate power is 16 watts peak (for a very short time), or 500mW average.
The minimum required gate power is 25mA*1.5V=37.5mW, so you have plenty of leeway on gate drive.
I could be wrong about this. If you other guys have a different interpretation, dump it on us.
One thing I hate about NTE is the crappy datasheets.
 

Thread Starter

RGPATTER

Joined Jun 2, 2011
16
Thanks for the interpretation. No one has commented for two days, so I am assuming there is no disagreement with your read of the data sheet. I will proceed using 5.477 volts RMS as the maximum allowable continuous voltage to be applied between gate and cathode. The 5.477 figure is arrived at using 60 ohms for the resistance from the gate to cathode, 0.5 watts for the maximum average power allowed, and the relationship E=(.5*60)^.5. Since the gate voltage at zero trigger angle will exceed this limit, I will use a second section on the rotary switch to increase the value of the protective resistor as the first section decreases the value of the phase shifting resistor in the RC network.
 

Ron H

Joined Apr 14, 2005
7,014
Thanks for the interpretation. No one has commented for two days, so I am assuming there is no disagreement with your read of the data sheet. I will proceed using 5.477 volts RMS as the maximum allowable continuous voltage to be applied between gate and cathode. The 5.477 figure is arrived at using 60 ohms for the resistance from the gate to cathode, 0.5 watts for the maximum average power allowed, and the relationship E=(.5*60)^.5. Since the gate voltage at zero trigger angle will exceed this limit, I will use a second section on the rotary switch to increase the value of the protective resistor as the first section decreases the value of the phase shifting resistor in the RC network.
That is totally wrong.
The gate to cathode does not look like a 60 ohm resistor, and nowhere in the datasheet does it say that. The gate to cathode is a diode junction. If you applied 5.477V RMS, it would DIE.

Post your schematic, and we can proceed with gate drive recommendations.
 

SgtWookie

Joined Jul 17, 2007
22,220
I don't know how NTE tech support claimed the gate to cathode resistance is 60 Ohms. In the datasheet, Vgt and Igt are specified with RL being 60 Ohms; the load resistance doesn't have anything to do with the gate to cathode "resistance".

NTE's crappy datasheet:
http://www.nteinc.com/specs/5400to5499/pdf/nte5417_19.pdf

One thing you need to understand about NTE parts is that they don't make them; they have major manufacturers produce a run of parts with NTE's markings on them, and then NTE has a cross reference from industry part numbers to NTE part numbers that only goes one way - that way you wind up getting "stuck" paying 3x to 10x as much for NTE parts as you would otherwise pay for the original part.

If you want to look at a datasheet for a more mainstream part, you might look at a Teccor E6 series S4010L SCR; 10A, 400V in a TO-220 package with an isolated tab; this has very similar specifications to the NTE5418.
Datasheet:
http://www.littelfuse.com/data/en/Data_Sheets/E6SCR.pdf
It's on page 2.
 

Thread Starter

RGPATTER

Joined Jun 2, 2011
16
I don't have a way to post the gate drive circuit, but it is a simple one so I hope it will be OK if I just describe it. It consists of a 24 volt transformer secondry feeding a variable resistor in series with a 200 microfarad capacitor. The gate is fed from a protective resistor attached to the junction of the variable resistor and capacitor. A DC motor is fed from 120 volts through an SCR whose cathode is hooked to neutral. The end of the transformer secondary where the capacitor is attached is also hooked to neutral.
 
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