OverVoltage and OverCurrent for high current PS - Page 1
Forum section: electronics / beginners

author: AngraMelo
author_rank: Regular Contributor
timestamp: October 16, 2018, 07:02:50 pm
content: Hello friend,Im using the attached schematic for a Ham radio power supply. It will be kept around 13.8V and the max current is around 20A.Now this circuit needs some protections so I dont fry my Ham equipment in case a transistor fails or something like that.The challenge is the following: I live in Brazil, I dont have access to mouser or digikey type of parts provider. I know it is hard for you guys to know exactly what is or is not available to me but if you could help me with some ideas or concepts that I could study and get familiar with, I would then adapt it to what is available to me.Im thinking about a crowbar circuit but instead of blowing the fuse by shorting the output with a SCR it would use a relay. I would use the NC contacts for the regular functioning of the PS and then if the voltage goes above the 17V threshold a zener would switch the contact to the NO contact and cut the current flow. What do you think?Now, I have no idea what to use for over current protection besides fuses. Any ideas?

author: jeroen79
author_rank: Frequent Contributor
timestamp: October 17, 2018, 09:42:15 am
content: Why do you want to use a relay instead of an SCR+fuse?How many overvoltage incidents do you expect?I see that you are using an LM723.This IC has a built in current limiter that you could use.Your schematic already uses it.(pins 2-3 and R7-10)Or find a suitable circuitbreaker.

author: AngraMelo
author_rank: Regular Contributor
timestamp: October 17, 2018, 01:13:30 pm
content: Im thinking about a relay given that the amount of fake components (hard to find original 2n3055) is so high that a transistor popping that is working on near limit conditions wouldnt be so rare.On that note, the relay would kinda work like a resettable fuse.You are right about the current limiting on the 723, I had forgotten that, but Ive also seen those CI fail and would be nice to have another way to limit current. Maybe Im worrying too much but as I will be using a slow blow type fuse, I keep imagining it not opening and the frying the transformer.

author: macboy
author_rank: Super Contributor
timestamp: October 18, 2018, 05:03:28 pm
content: You can limit the output by robbing current from the bases of the output transistors. Very simple protection for both over voltage and over current can be added this way. This will not protect against a shorted transistor, as that will conduct no matter what the base current. In that case, you need a crowbar and/or a series shut off.The idea of using a relay is fine. Any relay coil will engage faster than it can release. So the protection relay should be normally off and be turned on by the fault condition. I would use two relays, one with normally closed contacts in series with the output. It will disconnect the output from the regulator when a fault happens. The other relay is across the output (a shunt) and uses normally open contacts. This will crowbar the output for additional protection if the first relay does not engage (or the contacts weld shut, or an arc is sustained across the contacts, etc.).

author: AngraMelo
author_rank: Regular Contributor
timestamp: October 18, 2018, 05:32:23 pm
content: macboy,That is a great idea, I already incorporated the crowbar but I have a questions so I can trust the relay circuit.Most relays I have access are either for 12V or 24V (coil), my question is, given that my rectified voltage is around 26V and my overvoltage protection should trigger at around 16V, which relay do I buy? My question is: given that I dont have enough knowledge to predict what voltage would be on the positive rail if something goes wrong and that is the rail that will trigger the relay (either directly or with the help of a zener) how do I choose the coil voltage of the relay?I hope I was clear