Freewheeling Diode Advice Needed - Page 1
Forum section: electronics / beginners

author: catlover
author_rank: Newbie
timestamp: December 07, 2021, 09:42:26 pm
content: Hello AllI'm making a small pulse circuit to protect my chickens and need some advice if a freewheeling diode is needed to protect the SCRI have a small capacitor across the pulse transformer and want to know if that would be enough protection against flyback or should a diode or MOV be fitted, I have tried a diode but it is having an effect on the output voltage ( seems to be wasting energy)

author: bob91343
author_rank: Super Contributor
timestamp: December 08, 2021, 01:24:22 am
content: Since you have a resonant circuit, you are expecting the diode to suppress the oscillation within one cycle. Perhaps your diode is breaking down. You need a high voltage rating for that spot.

author: catlover
author_rank: Newbie
timestamp: December 08, 2021, 01:38:30 am
content: HiThanks for the replyThe diode seems ok as its rated at 1000V but it seems that its discharging the capacitor the end of every pulse300v difference in the output voltage when the diode is not connected, seems like must wasted energy

author: Circlotron
author_rank: Super Contributor
timestamp: December 08, 2021, 03:02:06 am
content: Follow this general idea.Leave out D5.Leave off everything in the drawing below the Ground line.D3 is useful because it will allow the transformer to partially recharge the dump capacitor as it resonates back the opposite polarity after the pulse.

author: catlover
author_rank: Newbie
timestamp: December 08, 2021, 11:08:59 am
content: Hi CirclotronThanks for the diagram, apologies mine is not as professionalI have attached a sketch with the diode changed, is this correct what I have done?

author: Circlotron
author_rank: Super Contributor
timestamp: December 08, 2021, 12:13:53 pm
content: I can't really follow your diagram. Here's the essential part below. You need to disconnect the HV supply before you trigger the SCR otherwise the SCR will lock on and short the supply.

author: catlover
author_rank: Newbie
timestamp: December 09, 2021, 12:33:17 am
content: Hi CirclotronI've added some more information to the diagram

author: Circlotron
author_rank: Super Contributor
timestamp: December 09, 2021, 01:21:48 am
content: With all due respect, your circuit is not going to work very well. C1 and C2 are effectively in series across the SCR and when it switches on it will short out those two caps with a HUGE current that will damage the SCR as well as wasting lots of energy. Get rid of C2 for a start. Having done that, when the SCR is triggered, C1 voltage will resonate down to zero, at which point the HV pulse transformer primary current will be at a maximum. The transformer inductance will will then keep pushing the cathode end of D1 below ground, causing current to flow in the secondary of T1. This is not good.If you could somehow disconnect D1 from C1 during the discharge period this would be better. Maybe use another SCR instead of a diode. This SCR would easily switch off every half cycle of T1 output voltage. Just make sure you trigger the main SCR in time with the AC supply voltage to T1.Edit -> I just realised T1 is a HF tranny fed from a mosfet. I was thinking it was a mains transformer. In that case, just shut off the mosfet drive before you trigger the SCR. You will still need to isolate D1 from C1 because of the below ground resonant voltage swing. And use a flyback converter for charging the cap because they are a current source.

author: catlover
author_rank: Newbie
timestamp: December 10, 2021, 09:42:22 am
content: Hi CirclotronI think I will have to do with the loss in voltage. I wouldn't be sure how to make such changes as not familiar with flyback converter