# Crowbar circuit and choice of the SCR

I am building a Crowbar circuit for one of my sensor which is power supplied through a Recom Power R-785.0-0.5 (datasheet). The crowbar SCR is controlled by this IC.

I am having troubles in selecting the right SCR because:

1. I don't understand what happen at the output of the R-785.0-0.5 when the CC happens. The datasheet says that it has short circuit protection, but what does that mean?
2. What is the maximal current which will go through the SCR when short circuited?
3. What is the output current limit of the R-785.0-0.5 of 2000mA in the datasheet?

Any suggestion for a correct SCR?

• That crowbar data sheet is awful. Nov 26, 2015 at 12:37
• I know! But I think it is the same as this: onsemi.com/pub_link/Collateral/MC3423-D.PDF (I hope) Nov 26, 2015 at 12:39
• Useless factoid: 'to crowbar' a power-supply comes from the world of electrified railways where a third rail is used for traction power. The lineman carried a crowbar/wrench on his daily inspection to tighten any loose bolts on the rails. In the event of an emergency he could throw the crowbar to bridge one of the running rails and the live rail. This would trip the circuit protection and remove power to that section. Trains could still coast a long distance - depending on gradient - but at least the driver would be alerted to a potential problem ahead. Nov 26, 2015 at 20:53

I don't understand what happen at the output of the R-785.0-0.5 when the CC happens. The datasheet says that it has short circuit protection, but what does that mean?

The clue is when it says: -

Short Circuit Input Current (Vin = 24V) All Series 60mA

This means that if you short the output the internal circuits of the device prevent a current of greater than 60mA flowing into the device and this is usually called fold-back protection. Basically when the output is shorted the output current is rapidly reduced to below (say) 100mA.

What is the maximal current which will go through the SCR when short circuited?

I estimate no more than 100mA but there is a doubt that it could be higher. It's a pity the data sheet doesn't explicitly state this.

Regards the SCR virtually any listed on page 6 of the ON semi data sheet would do and probably quite a few that are much smaller.

• Thank you for your answer! But how do you estimate the 100mA current? Nov 26, 2015 at 13:21
• It is an estimate based on it probably isn't less than 60mA and it probably isn't more than a couple of hundred mA. Basically they should really state this in the DS. Nov 26, 2015 at 14:12
• There are a few documents on line that show recom's fold back current limiting and although they don't specifically state this device uses it I think it has to because of the input current limit of 60mA. Nov 26, 2015 at 14:24
• Yes they should! But what is the 2A limit about? In any case it should be <1A right? Nov 26, 2015 at 14:25
• I can't find where they say 2A - what document and what page? Nov 26, 2015 at 14:32