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What is maximum gate voltage for this scr?

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dingo27

Sun Jan 08 2012, 10:40PM

Registered Member #890 Joined: Tue Jul 10 2007, 10:06PM
Location: Slovakia
Posts: 180

I have troubles understanding this datasheet

what is a maximum gate voltage? It seems 4 volts, but i think its quite low...

i was expecting maximum gate voltage column, but there is only "Maximum required DC gate
voltage to trigger" ,

thanks in advance

Pinky's Brain

Sun Jan 08 2012, 11:03PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

It just means that if your gate voltage reaches 4 volt it is sure to trigger (at -40 celsius, 2.5 volt at room temperature).

The maximum allowable gate voltage is power limited, depends on pulse width and has to be derived from figure 8, which I have no idea how to read.

Patrick

Mon Jan 09 2012, 02:12AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

let me look may be ill remember the 90's...

to me it looks like youll need:
2.5 V, at 300-700mA to fully trigger a power pulse into a resistive load.

with the caveats that:
never let the gate drop below -10 volts, or the gate will die.
or exceed its average power/heat capacity for your waveform. thats in fig 8.

remember these SCR's are current contrlled devices, not Voltage controlled, like IGBT's or FET's.
if you include a reisistor in series with the transformers secondary (lets say 5 volts.) , the gate voltage needed will appear across the gate, with the balance across the low ohm resistor. like a zener or led circuit. these have more difficult drive condidtions then voltage controlled switches.

there was a masterful example of a scr tesla coil using a saturable inductor, in the late 90's, but i cant remember the website or name of the person. EDIT! it was Alan Sharp, but wayback doesnt have any of his work on SCRs.

teravolt

Mon Jan 09 2012, 03:18AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

2.5v@150mA at 25 deg C

Patrick

Mon Jan 09 2012, 03:45AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

teravolt wrote ...

2.5v@150mA at 25 deg C

but he needs to realise thats the lowest drive for a weak current voltage switch, with a T0-247 i assume he means to really push some power, which means he needs to drive the gate real hard.

Dr. Dark Current

Mon Jan 09 2012, 10:08AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

A SCR is a current-controlled device, much like a bipolar transistor. The gate turn-on voltage is usually somewhere around 0.7 volts, but may be higher. The trigger circuit must be able to set the gate trigger current to the required value through the trigger voltage range. Triggering a SCR from a low-impedance voltage source may damage it.

Steve Conner

Mon Jan 09 2012, 10:10AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

SCR gates are current operated, like a transistor base. The way to drive them is a 12V pulse through a resistor sized to give the required gate current.
Eg, if the datasheet says the SCR needs 100mA worst-case, and the gate voltage is 4V worst case, then the resistor is (12-4)/0.1 = 80 ohms.

A 100 ohm resistor between gate and cathode also helps to avoid spurious triggering in dimmer and inverter type circuits.

Patrick

Mon Jan 09 2012, 10:17AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

In really fast switch, high current pulses, they can kick back out of the gate too, so others ive seen use a transformer to drive the gate power.

dingo27

Tue Jan 10 2012, 12:19PM

Registered Member #890 Joined: Tue Jul 10 2007, 10:06PM
Location: Slovakia
Posts: 180

I will use it doubled in coilgun, so i need to push it to limit.

With previous scr (2x bt152-800R) i switched gate with 5V and no resistor (2200uF discharged into gate through p mosfet), but i destroyed it with dry shot

so i ordered 2 of those (with this

) and i am waiting for better results

i'm not new to electronics, but current driven devices are harder for me to understand than mosfets.

for start i will try those 4 volts and post some result and photos in electromagnetic accelerator page.

If anyone has more useful info, let me know

Thank you all one more time.

Patrick

Tue Jan 10 2012, 08:14PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

dingo27 wrote ...

(2200uF discharged into gate through p mosfet), but i destroyed it with dry shot

This statement makes me suspicious...

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