I built a circuit for my power electronics lab using part number MCR225FP. OnSemi states that the typical holding current is 20 mA. see below.
The resistors cheap wire wound resistors , capacitor and R4 is a cheap blue POT that were was bought on ebay. I make sure that I latch UP the SCR fully by making sure the gate current is about 2-3 times the rated the holding current, usually about 50-60 mA. I check the voltage drop and then reduce the current by increasing the resistance on the POT. I got an expensive pot (linear i think) . All the meters are also cheap, bought at flee markets or WALMART. Tested at different times in the day, sometimes the holding current is 18.41 mA and other times it's 19.33 mA or anything in between. What am I doing wrong that I keep getting different holding currents for the exact same component ? It's the same way with the 2 other SCRs I bought as well. What am I missing or don't understand and should do differently ? I want to know the exact holding current of this SCR.

Thank you in advance to anyone who read this post.

 

MCR225FP datasheet
https://www.onsemi.com/download/data-sheet/pdf/mcr225fp-d.pdf

Holding current varies with temperature see page 6, also holding current is anode to cathode current, not gate current.

 

Did I put the POTENTIOMETER ( R3 ) is the wrong spot ? I try to test the SCR at about 75 F 24 C. Is the interpretation of page 6 mean that the holding current can vary about 0.88-0.90 mA at 24 C ?

 

My guess is that there is noise as the potentiometer is moved, which would momentarily take the current below the holding current value and it unlatches.
Why is it so important to you? If the anode current is above 40mA it will always stay on, if it is below 40mA it might not. What else do you need to know?

 

Is there a way to stabilize the current ? Is there a way to control the noise on a POT ? How do I control noise on the circuit ? I am taking a class in active circuit design/ (independent study). I want the circuit to switch to another circuit when the current falls below 40 mA. I need to have the circuit be reliable at 40 mA. The concept is an alternate lighting system turns on when the current below 40 mA on the main circuit. The secondary circuit would be a flashing light concept to warm other drivers on a highway.

 

Is there a way to stabilize the current ? Is there a way to control the noise on a POT ? How do I control noise on the circuit ? I am taking a class in active circuit design/ (independent study). I want the circuit to switch to another circuit when the current falls below 40 mA. I need to have the circuit be reliable at 40 mA. The concept is an alternate lighting system turns on when the current below 40 mA on the main circuit. The secondary circuit would be a flashing light concept to warm other drivers on a highway.

What are you trying to achieve? If you want a circuit to compare a current to a reference current, what is the thyristor for?

 

Per the Motorola datasheet for the MCR225FP, the holding current varies approximately linearly by about 20% for a 20C temperature change; so about 1% for a 1C change. Your measurements seem consistent with this.