Need help designing/finding an oddball transformer - Page 1
Forum section: electronics / projects

author: senso
author_rank: Frequent Contributor
timestamp: January 10, 2013, 03:18:52 pm
content: Good evening folks,I'm getting my feet wet in the design of electronic ignitions for two stroke engines, and in some of my engines I dont have access to 12v or 6v AC or anything regulated, had they have racing fly wheels that only have the pick-up and the 400v high voltage coil, I would like to know if someone could help me design or find a suitable transformer that would give me about 12v after regulation and some 40mA(should be enough for an attiny an a couple transistors), the 12v is to fire the SCR(I'm using a BT151), the input voltage varies with the rotation, but it ranges between 380 to 450-460v, with the frequency going between 16hz and 266hz, I have learned a bit about how perfect transformers work, but only limited to ideal devices, and only based in 60hz 230v ac inputs, and it was a couple years ago, I can calculate the turns(I think) but the rest, ranging from what core material to choose, wire size and the like, or if there is a ready to use transformer(maybe for SMPS).Thanks in advance.

author: SeanB
author_rank: Super Contributor
timestamp: January 10, 2013, 03:37:41 pm
content: Use a small mains transformer, most will work from 50 to 400Hz with little problem, though they do become very lossy as the frequency goes up. Get 2 220V to 24V 2VA transformers, and place the primaries in series and the secondaries in parallel ( take care so you get a voltage output, if not flip one of the 4 coil connections around) to give a secondary that will deliver between 20 and 30V at around 100mA max. Then you can drive a 12V regulator and from that a 5V regulator.If you just use a 12V transformer you will get between 10 and 18V, and you can drive the thyristor direct from this with no problems, just size the gate drive resistor or use a constant current source to deliver the required current at the lowestr voltage and use a capacitor able to store charge enough to provide the pulse.

author: senso
author_rank: Frequent Contributor
timestamp: January 10, 2013, 03:48:36 pm
content: Thanks for the fast response, 100mA is more than I need and I dont want to loose to much energy from the spark, but using this method I cant have everything, I will try to find two pcb mounted transforms and will test.Going the route of designing a switching regulator for this would be a bit over the top, or what do you think?

author: SeanB
author_rank: Super Contributor
timestamp: January 10, 2013, 05:01:53 pm
content: Standard is to rectify and charge a cap to 400v then discharge it when triggered.You are looking at doing timing optimisation on the fly. You could rectify the pick up pulse and use it for power as well.

author: senso
author_rank: Frequent Contributor
timestamp: January 11, 2013, 02:51:28 am
content: Yes, thats it, I'm doing full wave rectification, because for some reason, OEM ignitions usually only do half wave retification, my fear is that I will pull to much current and the spark will lose too much energy, but only testing can show that, but the fact that I'm doing full wave, and that I'm planning to use two caps in parallel to lower the ESR I think it will work just fine.

author: SeanB
author_rank: Super Contributor
timestamp: January 11, 2013, 06:38:29 am
content: They use half wave rectification so one end of the coil is connected to the chassis for a ground lead, as the ignition coil uses the chassis as a primary current return and as a high voltage return path. If you do full wave then you will have to make sure the coils on the pickup and charge coil are not connected to the chassis and connect the ground to the negative of the rectifier. If you use the transformers it does not matter, just connect one end of the charge coil wires to the chassis ( the one normally connected there will do) and leave the secondary isolated and the output of the bridge rectifier connected to ground.

author: senso
author_rank: Frequent Contributor
timestamp: January 12, 2013, 04:42:05 am
content: Yes, you are right, all the coils have one of the ends connected to ground.I though I could do full wave rectification, but I can only do that if I have the coil floating in regard to the chassis ground?

author: SeanB
author_rank: Super Contributor
timestamp: January 12, 2013, 05:16:54 am
content: Correct, you will have to remove the rivet and solder a wire on and insulate it.