firing circuit for SCR - Page 1
Forum section: electronics / projects

author: hasan081043
author_rank: Newbie
timestamp: November 26, 2013, 10:25:18 pm
content: i want to work with thyristor .please suggest some firing circuit for thyristor.

author: Jon86
author_rank: Frequent Contributor
timestamp: November 27, 2013, 12:58:38 pm
content: Have a guess, its pretty simple

author: anotherlin
author_rank: Frequent Contributor
timestamp: November 27, 2013, 10:02:38 pm
content: You may want to take a look at HP/Agilent's vintage 6200 series DC supply operation/service manual.Some of them, at least HP 6209B, uses SCR as pre-regulators.The manuals feature theory of operation along with full schematics.

author: David_AVD
author_rank: Super Contributor
timestamp: November 27, 2013, 11:38:31 pm
content: Quote from: hasan081043 on November 26, 2013, 10:25:18 pmYet another half arsed attempt at asking a technical question.How about explaining what you're trying to do for a start.

author: calexanian
author_rank: Super Contributor
timestamp: November 28, 2013, 03:39:30 am
content: They pretty much just fire when the gate reaches the threshold. Are you trying to design a circuit to vary the time delay before firing? AC or just like a DC cap discharge?

author: oldway
author_rank: Super Contributor
timestamp: November 28, 2013, 11:41:14 am
content: scr can be fired of 2 differents manners-dc firing: this ensure the scr stays always conducting even if anode current is lower than holding current.I recomand to use a 10Vdc voltage with serie resistor to limit the gate current.Current should be greater than 10 times the gate trigger current at 25°C for low temperature firing.Be aware not to exceed the max. average gate power dissipation.-pulse firing: you can fire the scr by only one pulse if anode current stay always above the holding current of by a pulse train to be sure the scr stay in conduction.Pulse firing is better then dc firing because lower average gate power dissipation.Peak gate current may also be higher what is better for firing with high di/dt .Typical pulse should be:open voltage: 10V min. (with a serie resistor to limit the current)Short circuit current: 1A peak min.duration: 1ms (to insure correct firing with highly inductive load; with resistive load, can be far more shorter)Rise time: 10µs min.These values are aplicable for medium and high current scr's.For sensitive and low current scr, gate current must be lower. See data sheet.

author: hasan081043
author_rank: Newbie
timestamp: November 28, 2013, 12:06:33 pm
content: The supply is 220v 50Hz ac.I need 60V DC output voltage .For this i need to fire the thyristor at 80degree.The main problem is to fire the thyristor at 80 degree .please sugest me the firing circuit.for half wave controlled rectifierVdc=Vm(1+cost)/2*pihere t=firing angleVm=maximum ac voltageVdc=output DC voltageSo for Vdc=60V ,Vm=1.414*220=311t=?the required firing angle would be 80 degree.so please help me how i can fire the scr at an angle 80 degree.

author: BravoV
author_rank: Super Contributor
timestamp: November 28, 2013, 12:19:37 pm
content: Quote from: Pippy on November 28, 2013, 12:12:20 pmI guess so, cause he looks so desperate that posted another new thread with the same question.

author: Zero999
author_rank: Super Contributor
timestamp: November 28, 2013, 02:12:59 pm
content: When's the project deadline?

author: ajc
author_rank: Contributor
timestamp: November 28, 2013, 02:38:13 pm
content: Quote from: hasan081043 on November 28, 2013, 12:06:33 pmIf you need 60V out of 240V, why do you have to use an SCR? Why not keep it simple and just get a transformer to step the voltage down and rectify/filter the output?If you have to use an SCR and need to fire the trigger at a precise point in time, you will need to implement a zero crossing detector on the mains supply and then use a timer to delay the trigger.I remember building an SCR trigger when I was at uni which was based on a 555 timer for generating the delay with a zero crossing detector based on a comparator sitting on the output of a small stepdown transformer. Don't remember much of the details now though... was a few years ago...

author: oldway
author_rank: Super Contributor
timestamp: November 28, 2013, 03:20:12 pm
content: You seem to be a beginner...than, better to choose a very simple solution:Use a triac (very simple to fire and a lot of schematics op internet) with an output rectifier (simple diode or bridge)NB: using 240v from powergrid without isolation is very dangerous...Dead man walking...Power electronics is NOT for beginners ....

author: calexanian
author_rank: Super Contributor
timestamp: November 29, 2013, 10:36:21 pm
content: This is a very sloppy way to implement a power supply. Traditional switching power supplies in that voltage range are already widely available. Unless we are all completely missing what your application actually is. What kind of load are we talking about here?