# SCR vs saturated transistor as switch

My son's a paramedic / firefighter, studying further in fire engineering with the UK's IFE.

Seems one of his books has an article on timing devices (like in bombs and stuff.... scary), and it talks of an SCR. Knowing that I as his father am all-knowing he asked me what an SCR is and of course I have nfc. So I Googled it (which is arguably what Hugh should have done in the first place).

It seems to me that an SCR is functionally similar to a saturated transistor. Is that true? It's a diode with a switch line, and is either open or closed, just like a transistor with the base saturated or off.

If (that's true)
{why / how would one choose between the two}
else
{what did I miss?}

TIA,

Jim

EDIT: I guess a key part is that it's a rectifier, so it's a switch which converts AC to DC as well as being a switch. ...

YET MORE EDIT: And perhaps this is the good bit, it stays on even if the gate current dies?

And perhaps this is the good bit, it stays on even if the gate current dies?

Also a problem -- if you switch a DC voltage source with an SCR, there is no easy way to turn it off again. For this reason they are usually used in AC circuits, as they turn off at or near the zero crossing.

JimboZA:
EDIT: I guess a key part is that it's a rectifier, so it's a switch which converts AC to DC as well as being a switch. ...

I think the key part for a bomb-maker is the next bit.

JimboZA:
YET MORE EDIT: And perhaps this is the good bit, it stays on even if the gate current dies?

Yep. Once current starts flowing it doesn't stop.

It's more like a latching transistor.

jremington:

if you switch a DC voltage source with an SCR, there is no easy way to turn it off again.

Although if - heaven forbid- one was building a a bomb timer, that would actually be the whole point. I hasten to add, Iâ€™m not building a bomb.

I found the attached circuit for an alarm- where the lower switch sets it off and of course it stays on even if the perp shuts the window. Iâ€™ve just built exactly that circuit to show my son when he gets in from his shift.

Yes SCRs (aka thyristors) don't switch themselves off till the current drops to near zero.

They are cheaper than other high power devices as they are very simple, a sandwich of 4
layers, pnpn, no/little masking is needed to form the structures I beleve.

Triacs are bidirectional versions of SCRs, again simple, cheap, only switch off when
current falls to zero.

[ I should also have mentioned that not only are SCRs cheaper for high power, they are
more sensitive than a single BJT transistor to gate current - have higher gain - think of
them as like a latching darlington pair. You can control massive currents with them,
they are often found in large motor controllers (100kW -- 10MW).

The modern high power switching device is the IGBT which is rather like an SCR
that can be controlled via an insulated gate (switched on or off) - sort of a cross
between an SCR and a MOSFET ]

SCR's were widely used in circuits for triggering camera flashes, and this property about staying on until the current drops to near zero worked fine with old style high voltage (>200 VDC) circuits; however, when these same old circuits started getting used with modern flashes that work at much lower (digital safe) voltages the SCR's would tend to stay 'stuck' on.

Thanks for the input guys.

One closing question: the circuit I attached a few posts earlier is going to drain the battery through R1 and the NC Trigger switch. Seems to me it would be better to have an NO switch "above" the gate, so there's no power consumption under normal conditions..

So my question: if I did that would I need a pull-down (>>R1) on the gate to make sure it didn't energise from stray atmospherics? I'm asking this thinking along the lines of an Arduino input needing to be forced one way or the other: is the SCR gate susceptible to stray signals?

I think people have covered the important bits, but one detail is that SCRs have a higher "Von" (~1.5V) than a saturated transistor (0.2V), and will dissipate more power when switching a particular current. Usually that's irrelevant, since SCRs are power devices and come in big packages with high current ratings and good heat dissipation, but it does mean that they're not great in low-voltage circuits.

Does it matter where the load goes?- like the way it matters on an NPN transistor where the load is above the collector.

Yes, probably. Like an NPN transistor, the SCR is triggered when the gate is positive WRT the "cathode." If the SCR is on the "high" side of the load, that could require quite a significant voltage, so SCRs usually occupy the same spot as the NPN: between the load and "GND"...

Yes because you have to provide a +ve pulse on the gate w.r.t. the cathode. Switching
high-side means having to provide a voltage above the supply to do this, low-side
switching and its easy.

Thanks guys