# Circuit help -Need HV Pulser driven from TTL

#### inwo

Joined Nov 7, 2013
2,419
Need a HV pulse that will trigger a cheap self powered capacitive coupled hour meter from a ttl source.

I'm re-purposing an hour meter that would normally get it's signal from being near small engine spark.

This circuit works.
I'd like to know how to make it reliable.
I'm quite sure it needs more than this.

Also, I don't understand why my 400vpp pulses aren't destroying the 2n2222.

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#### #12

Joined Nov 30, 2010
18,210
Theoretically, that's impossible.
What size is the capacitor/ What is the load on the other end of the capacitor?

I would surely use a 600 volt mosfet.

Let's check the math.
The L is one.
energy = 1/2 L x I^2
about 24 ma squared is 576 e-6
and half of that is 288 e-6

At the capacitor, 288 e-6 = 1/2 CV^2
.000576 = C x V ^2
for a voltage of 400,
.000576/160,000 = C
C would be 3.6 nanofarads if the other end was connected to ground.
Am I close?

Even if I'm not close, you have a clue about how much energy is happening and what to expect out of your capacitance.

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#### inwo

Joined Nov 7, 2013
2,419
Thanks, my plan was to use the transformer. Not knowing that I would get such a big kick on the primary.
C is not needed. Works just as well without it.
There is no real load. Just a single lead going to hour meter. And of course the scope. Circuit can tolerate 1M load. Anything less damp the spike.
I notice the spike when I removed a diode from primary, and was amazed that the transistor survived.
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#### inwo

Joined Nov 7, 2013
2,419
I should also say that I'm open to any simple design that mimics a spark pulse.
Tried a strobe trigger coil, but it requires high voltage input.
Tried pulse transformer. 1 to 1 got me nowhere.
Could open the meter and connect directly, but this circuit seems to work fine.
These transformers are a little large. Only a couple bucks apiece though .

#### #12

Joined Nov 30, 2010
18,210
You are on the right track. See my blog to realize how to manipulate the inductance, current and capacitance. You can run any current through an inductor (as long as it doesn't saturate the magnetic core or melt the wires) and catch the resulting kick in a capacitor. You can tune the voltage of the kick by changing the current, the capacitor, or the inductor. I feel a need to say you don't need a whole Henry to do this job. As long as the meter is just feeling the electrostatic change, you can get by with very, very little current. That's because your load is just a few dozen picofarads.

Now, try reducing the current through the Henry and find out how much you need to kick the meter. When you find your meter's low limit, you can negotiate a smaller inductor and a higher current. Realize that LI^2 means that for every doubling in current, you can cut the inductor size by a factor of 4. The better you can minimize the capacitance of the load, the better the spike gets.

ps, change to a transistor that can survive this in the long run.

#### slackguy

Joined Feb 11, 2016
67
i would worry more about connecting loads to that nice oscilloscope your unsure about not having damaged and then follow the advice above (or use a sensitive transitor - the RLC setup might take time for a small project not worth the cost - dunno)

#### inwo

Joined Nov 7, 2013
2,419
i would worry more about connecting loads to that nice oscilloscope your unsure about not having damaged and then follow the advice above (or use a sensitive transitor - the RLC setup might take time for a small project not worth the cost - dunno)
Thanks, wouldn't have used my real scope.
I love my DSO112 scope, but for \$53 I'm not real worried. I do have a X10 probe on it.

#### inwo

Joined Nov 7, 2013
2,419
You are on the right track. See my blog to realize how to manipulate the inductance, current and capacitance. You can run any current through an inductor (as long as it doesn't saturate the magnetic core or melt the wires) and catch the resulting kick in a capacitor. You can tune the voltage of the kick by changing the current, the capacitor, or the inductor. I feel a need to say you don't need a whole Henry to do this job. As long as the meter is just feeling the electrostatic change, you can get by with very, very little current. That's because your load is just a few dozen picofarads.

Now, try reducing the current through the Henry and find out how much you need to kick the meter. When you find your meter's low limit, you can negotiate a smaller inductor and a higher current. Realize that LI^2 means that for every doubling in current, you can cut the inductor size by a factor of 4. The better you can minimize the capacitance of the load, the better the spike gets.

ps, change to a transistor that can survive this in the long run.