Author Topic: high voltage inverter circuit - LOTS of noise when powered from LDO  (Read 790 times)

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Offline ChristofferBTopic starter

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Hi, so I'm having this issue with a high voltage generator circuit I've made for a DIY geiger counter.

It's the inverter circuit from an electric fly swatter. I havent traced the circuit but it's a completely standard topolygy seen in those, a 5-pin transformer, a transistor and some resistors on the input side, a series diode and a parallel capacitor on the HV side.

I've modified the output side slightly, putting a series resistor to limit current before the HV capacitor, and then a string of zener diodes and resistors to reach around 400V. A bit crusty but it has worked great so far. Problem is, I've always operated the circuit with two power sources: a 9V battery feeding the audio amp and a 7805 regulator to feed some digital logic, and 2 AA batteries for the HV supply. This is awkward though,

and now i've replaced the 2 AA batteries with a L78L33ABZ fed from the 5V reg. Now there's a HUGE amount of noise everywhere, with 50 mv ripple between the 9V battery terminals, and almost half a volt on the 78L33 output! Issue is, if I add smoothing capacitors on the 3,3V reg output, it changes the frequency of the HV inverter, easily detected because it's in the audible region both before and after.

How do I solve this? CAN i even solve this, or is the 3,3V reg just unsuitable for the purpose?

Thanks in advance.

 
 
--Christoffer //IG:Chromatogiraffery
Check out my scientific instruments diy (GC, HPLC, NMR, etc) Channel: https://www.youtube.com/channel/UCZ8l6SdZuRuoSdze1dIpzAQ
 

Offline T3sl4co1l

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Re: high voltage inverter circuit - LOTS of noise when powered from LDO
« Reply #1 on: November 30, 2019, 08:31:21 pm »
It's drawing more than 100mA, get a bigger regulator.

Tim
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Offline ChristofferBTopic starter

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Re: high voltage inverter circuit - LOTS of noise when powered from LDO
« Reply #2 on: November 30, 2019, 08:56:42 pm »
Ah. So the oscillations is the regulator over-current'ing and then switching off repeatedly? That makes sense! I'll get the 1.5A version, thanks!
--Christoffer //IG:Chromatogiraffery
Check out my scientific instruments diy (GC, HPLC, NMR, etc) Channel: https://www.youtube.com/channel/UCZ8l6SdZuRuoSdze1dIpzAQ
 

Offline T3sl4co1l

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Re: high voltage inverter circuit - LOTS of noise when powered from LDO
« Reply #3 on: November 30, 2019, 09:20:50 pm »
Not switching off, limiting and the load browns out.

Unless you have other loads on the supply, and as long as you have enough output, it probably doesn't matter.

The linear regulator(s) and always-on HV supply won't do you any favors for battery life, of course.

Tim
Seven Transistor Labs, LLC
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Offline ChristofferBTopic starter

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Re: high voltage inverter circuit - LOTS of noise when powered from LDO
« Reply #4 on: November 30, 2019, 09:31:22 pm »
well, it matters because there's an audible screech on the AF output!

But lets say battery life isn't really an issue, is there anything wrong with putting the HV circuit directly on 5V with a dropper resistor in series to drop the voltage to 3V?
--Christoffer //IG:Chromatogiraffery
Check out my scientific instruments diy (GC, HPLC, NMR, etc) Channel: https://www.youtube.com/channel/UCZ8l6SdZuRuoSdze1dIpzAQ
 

Offline T3sl4co1l

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Re: high voltage inverter circuit - LOTS of noise when powered from LDO
« Reply #5 on: November 30, 2019, 09:47:39 pm »
It shouldn't care much, no.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 


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