Evaluation and modification on HV PSU

[neazoi]

This is a nice simple dual purpose HV PSU **broken link removed** I have found.
What is your opinion about the circuit, is seems very straight forward.

I also consider the next modifications and I would like your oppinion about these:
1. Adding a 220uF capacitor at the base of Q1 for less ripple.
2. Adding a 1000uF shunt capacitor at the collector of Q1 to GND. This would be good on BT ON but I do not know what will be the behaviour of this capacitor on THT ON, because the capacitor will effectively shunted between the flyback transformer pin 2 and the GND.
3. It is not clear to me what R2 does. It should limit the input voltage on the 78L12 so that it does not exceed 30V but how is it accomplished with a just a current limiting resistor?

 

[chuckey]

I would not bother with your modifications. 1. If you add your capacitor to the base if any of the out puts are accidently shorted to earth, the capacitor will discharge through the base of Q1, which may blow it up. 2. there is already a 10mF cap on the collector, so adding your extra 1mF will only make a small difference. R2 does limit the input voltage to the 7812 when its taking current, again opening its output line or stopping the oscillator could cause it to take a small currrent, leading to excessive voltage on its input pin. It could do with a zener diode at this point to limit the input voltage.
Frank

 

[neazoi]

I would not bother with your modifications. 1. If you add your capacitor to the base if any of the out puts are accidently shorted to earth, the capacitor will discharge through the base of Q1, which may blow it up. 2. there is already a 10mF cap on the collector, so adding your extra 1mF will only make a small difference. R2 does limit the input voltage to the 7812 when its taking current, again opening its output line or stopping the oscillator could cause it to take a small currrent, leading to excessive voltage on its input pin. It could do with a zener diode at this point to limit the input voltage.
Frank

Thank you Frank. I will try the zener method and remove the resistor completely. The regulator is only used at very low currents, to feed the 555, so I think a zener would be a better choice for limiting the input voltage rather than the resistor?

 

[chuckey]

You MUST have a resistor in if you fit a zener to limit the zener current.
Frank

 

[neazoi]

You MUST have a resistor in if you fit a zener to limit the zener current.
Frank

Is there any way I could monitor the output voltage with a suitable analogue meter, but without interfering with the load circuit that will be connected to the HV PSU?

The meter has some inductance I think and the laser project I am going to use this PSU for, is not tolerant to additional inductances.

Maybe if I make a loosely coupled capacitor, by make a loop of wire onto the HV wire, I could monitor some portion of the voltage out?

Or maybe making something like this http://www.kerrywong.com/2011/07/16/simple-current-transformer/ with a current meter to measure the output?

 

[chuckey]

What you need is a 0 - 50 Microamp meter with a resistor in series to drop the voltage from 30 KV. The resistor must be able to handle the very high voltage so must be at least 75mm long OR a chain of smaller resistors inside a plastic tube. Its value should be 30/50 gigaohms (600 Mohm) to give you a full scale deflection of 30 KV. If you can lay your hands on a less sensitive meter, say 100 microamps then use it with a correspondingly lower resistor. Beware of the resistors voltage rating.
Frank

 

[neazoi]

What you need is a 0 - 50 Microamp meter with a resistor in series to drop the voltage from 30 KV. The resistor must be able to handle the very high voltage so must be at least 75mm long OR a chain of smaller resistors inside a plastic tube. Its value should be 30/50 gigaohms (600 Mohm) to give you a full scale deflection of 30 KV. If you can lay your hands on a less sensitive meter, say 100 microamps then use it with a correspondingly lower resistor. Beware of the resistors voltage rating.
Frank

You mean something like this http://www.vk2zay.net/article/11
I worry if this circuit will interfere with the laser opetation (TEA N2 laser). I would consider a non-contact circuit if it exists? Because the output voltage consists of positive pulses (after the diode), maybe a one turn transformer coupler with a voltage divider will do the job?

 

[chuckey]

That is exactly the sort of circuit I was thinking about. After the diode any stray capacity will smooth the pulses out into DC.
A one turn transformer will not do anything at all for you, its for measuring current. You could put a one turn around the transformer feed it into a diode and measure the voltage from the diode. The problem is that you would have to calibrate it by measuring what the main winding was producing, for which you need the circuit you referred to!
For a "non contact" voltage measurement, use a flyingspot galvonometer. It act like a small capacitor which deflects a light spot across a screen. Its a bit old fashioned now.