i would like to trigger a capacitor to discharge using .5vdc trigger coil. As a magnet comes past a coil producing a minimum of .5v it triggers a capacitor to discharge itself to another circuit.
Capacitor: Mallory 9300uF 50vdc
I shouldn't reach max voltage of 65vdc, probably only about 25-30v
anyone have any suggestions id be thrilled
John

 

Half a volt is a bit of a problem because it is below the threshold to get any significant current to flow in a silicon-based semiconductor of any sort. Is there any chance of using a different coil? There are ways of amplifying the pulse, but they add complexity that it would probably be nice to avoid.

If you could get the voltage up to 0.7 V, you would be able to use it to turn on just about any small silicon transistor which could in turn be used to trigger a large silicon controlled rectifier (SCR) which is what is often used for capacitor discharge. An SCR, once triggered, continues to conduct until the current through it drops below some threshold. Some SCRs will trigger at about the same voltage that will turn on a small transistor, but higher power types, suitable for the sort of (presumably) large current involved in your circuit generally require a volt or more at several tens of milliamps for good "hard" turn on. (SCRs that aren't given a strong gate pulse tend to turn on over just part of the "die" area, which can cause "hot spotting" and failure it the external circuit allow fast-rising high current. A substantial gate pulse helps assure the whole area turns on at the same time, avoiding this problem.)

A transformer could be used to increase the voltage from the coil, but the sort of small transformer that used to be used in little audio amps and would be suitable has become a rarity these days. I'm assuming you hope to get the components you need easily.

An electrolytic capacitor can usually withstand voltage higher than its rating for a brief period, but be careful. 65 V on a cap rated at 50 V is certainly pushing it. Check the datasheet carefully.

Oh, and welcome to AAC!

 

Thank you!
I am a bit new to the circuit/electrical field. i will be honest i want to see if i can pulse a homemade motor using power from a small wind generator. the power collected from the coils at lower RPM to be collected in a capacitor and at a magnet mounted on a wheel passes another coil that triggers the capacitor to discharge on an electromagnet to larger magnet (opposing field) forcing the wheel to spin faster. (just a theory i want to test) im hoping it will generate a faster spin and maybe get more out of a less windy day.

 

You could use a comparator, such as an LM339/393, to readily detect a 0.5V pulse.
Just bias one of the differential comparator inputs at 200mV or so.
The comparator output can then trigger the SCR.

 

with the SCR am i limited to the trigger voltage? I'm wondering if a gust of wind makes my wheel haul arse and it spikes the voltage higher would it damage my circuit...
i didn't think about this until last night.
from the capacitor how would i wire it?

 

In my circuit I have a comparator which compares the output of a sine wave with a staircase wave, if the staircase is above the sine wave, I wish to use a Mosfet in its cut off mode which will allow the capacitor to full discharge. I have hooked up the output of the comparator to the gate of the Mosfet as shown. My problem is that the output of the comparator switches back to low almost immediately as the staircase function drops below the sine wave. I was wondering if there was a way to generate a pulse which stays put until the capacitor is 0, or a fixed period of time.

 

with the SCR am i limited to the trigger voltage? I'm wondering if a gust of wind makes my wheel haul arse and it spikes the voltage higher would it damage my circuit...
i didn't think about this until last night.
from the capacitor how would i wire it?

I don't really follow what you mean.
Are you concerned that the trigger coil might produce a voltage that is much higher than normal? If so, that is unlikely to be any significant problem. A resistor can be used to limit current. Chances are the coil voltage will be current dependent (i.e. voltage drops if current is higher than "normal") and SCRs can tolerate a pretty substantial whack of current into the gate. Usually the maximum allowable is well specified. A combination of perhaps two resistors and a zener diode should be sufficient to tame the pulse in any case.

I've been assuming that your trigger coil is completely "galvanically isolated" from the rest of the circuit. If it isn't, things become more complicated.

A comparator is a good way to effectively amplify the pulse from the coil, but an integrated circuit comparator requires its own power supply. That could probably be derived from the capacitor without too much difficulty.

 

Do you realize that according to theory, a capacitor never fully discharges. As it has less and less charge, the discharge rate becomes lower. It's exponential towards the equilibrium which is 0. It never reaches 0. (Ok, you could argue that when it has less than one electron of charge, it's discharged.) Lucky Patcher 9Apps VidMate