Questioning the reason for this resistor?  - Page 1
Forum section: electronics / projects

author: Simon Spiers
author_rank: Regular Contributor
timestamp: June 11, 2025, 09:10:25 am
content: I have recently made a replacement CDI for a Kawasaki motorcycle thanks to someone who shared the circuit online.I understand how everything works and what it's for, except the highlighted resistor. Is this there just to ensure the 2.2uF cap discharges, or for some other reason?

author: Circlotron
author_rank: Super Contributor
timestamp: June 11, 2025, 10:38:54 am
content: Watching this thread.

author: wraper
author_rank: Unknown
timestamp: June 11, 2025, 10:44:36 am
content: IMHO ensure SCR does not stay latched.

author: jwet
author_rank: Frequent Contributor
timestamp: June 11, 2025, 08:07:59 pm
content: The function is as you say to discharge the big cap after the pulse. The SCR will come out of conduction after the main cap is discharged into the coil, but since this is based on a holding current, the cap will have some voltage left on it. There is also a phenomenon called dielectric absorption (google it). Even after the cap is discharged mostly, it will recover over some time without a DC current path. This has to do with how the charge gets out of the cap, most of it comes out fast but some is buried in the dielectric and recovers slowly. Its often modeled as a smaller cap across the main cap with a high resistance (megohms). This charge can recharge the main capacitor over time becomes a safety hazard mainly. The function of this resistor is to discharge whatever residual voltage is left on the capacitor after a cycle and maintain a small DC path to force it to zero over time. Without it, you could go to service the module and get shocked by this recovered charge.Its placement across that diode versus across the big cap is kind of clever. It never sees more that a diode drop when the diode is conducting and never sees more than the residual voltage on the cap otherwise. If it was across the cap, it would see full voltage and would need to be rated better.Here's a fair Wikipedia artlcle link.https://en.wikipedia.org/wiki/Dielectric_absorptionSome HV caps can have a lot of DA as the Wiki discusses.

author: SuzyC
author_rank: Frequent Contributor
timestamp: June 11, 2025, 11:26:13 pm
content: Wrapper: Wrongjset:Correct:The function of this resistor is to discharge whatever residual voltage is left on the capacitor.Wrong:This charge can recharge the main capacitor over time becomes a safety hazard mainly.The function of this resistor is to discharge whatever residual voltage is left on the capacitor turning off thafter a cycle and maintain a small DC path to force it to zero over time.Wrong: only at most a few volts or less will be measured over a long time across the discharge cap after discharge.Its placement across that diode versus across the big cap is kind of clever. It never sees more that a diode drop when the diode is conducting and never sees more than the residual voltage on the cap otherwise. If it was across the cap, it would see full voltage and would need to be rated better.Wrong: Not so clever, when the cap charging winding reverses, the full voltae of the capacitor appears across this shunt resistor, and for safety, a bleeder resistor across the discharge cap would ensure the cap is not left charged to a dangerous voltage. The function of this resistor is to prevent the SCR from not turning off after a discharge pulse due to holding current and also to discharge the main cap if the connectors to the coil and charging transformer wiring are left connected to the PCB.Capacitive Discharge ignitioin systems require a low ESR cap, meaning a film type is used, which will retain .2% or much less of the original charge.A bleeder resistor would not be needed to "rated better", whatever that means..perhaps cost or wattage?.

author: jwet
author_rank: Frequent Contributor
timestamp: June 11, 2025, 11:39:40 pm
content: Disagree- stand with my original response. No discussion necessary. Take care.

author: wraper
author_rank: Unknown
timestamp: June 11, 2025, 11:45:00 pm
content: Quote from: SuzyC on June 11, 2025, 11:26:13 pmTIC106M has just 5mA maximum holding current and 200uA maximum gate trigger current. So it may stay latched if there is nothing else to bleed the charge. Why anyone would care about tiny residual charge that may be left in the capacitor if there was no practical reason to get rid of it.

author: SuzyC
author_rank: Frequent Contributor
timestamp: June 11, 2025, 11:50:00 pm
content: Wrapper: The cap is fully charged to hundreds of volts before the SCR is triggered, If the capacitor is thus fully charged and no trigger occurs, the cap remains charged for a very long time at a dangerous voltage, that is, of course, without a bleeder resistor.

author: Andy Watson
author_rank: Super Contributor
timestamp: June 12, 2025, 01:33:04 am
content: I'm not buying any of the responses so far.I think the purpose of the resistor is to mitigate against the effects of discontinuous current through the diodes acting on the leakage inductance of the "exciter coil" and therefore causing false triggering and/or potential catastrophic breakdown of the diodes. Essentially the resistor is acting as a snubber.The exciter coil is shunted on its negative cycle by a diode, limiting the negative output to less than a volt - potentially 27uA through 37k.Edit: Seems the two coils of the exciter are separate entities, but : I think it will take more than that to ensure the thyristor turns off.If safety was the concern, you would put the resistor across the main capacitor and not leave the ignition coil terminal potentially floating.

author: Circlotron
author_rank: Super Contributor
timestamp: June 12, 2025, 02:41:30 am
content: After the first quarter cycle of discharge when the capacitor voltage hits zero, the energy in the leakage inductance of the ignition coil primary keeps dragging current through the capacitor for a further quarter cycle until it charges to a certain voltage in opposite polarity to what it was before the spark. This reverse voltage then resonates back the opposite direction through the diode that is anti parallel to the SCR and this turns the SCR off.Been doing stuff with car ignitions since 1977.

author: floobydust
author_rank: Super Contributor
timestamp: June 12, 2025, 04:12:58 am
content: That makes two of us that actually got off the couch and put a scope probe at the SCR lol.I have only seen across the exciter rectifier diode, just being a bleeder resistor for the big cap, like 470k.I've reverse-engineered a few of these motorcycle CDI modules. Honda, Kawasaki, Suzuki to keep old bikes running. Old KH400 CDI modules rare, going for $350-$1,000 on eBay. OMG.They are potted and difficult to take apart and figure out component values. Many posted schematics have mistakes.OP's circuit is a bit off, the semiconductors would be Japanese Hitachi, Toshiba etc.Not slow ass 1N540x (1N5407 even exists?) you need fast-recovery here, although this is a mid-1970's original module.TIC106 doesn't take enough for dV/dt compared to CDI-rated SCR's, and I'm doubtful it needs to be super sensitive gate, they are typically 5mA trigger, not 60-200uA and the pickup coil is connected right to the gate.The snubber resistor looks too high 2k5.So that's three mistakes I perceive and I'll stop there.