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 Post subject: Is full wave rectification with ICL practical?
PostPosted: Jan Mon 09, 2017 4:08 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Toronto, Ontario, Canada
I would like to achieve some kind of 'slow turn on' effect on a 1000V B+ supply.
Is it practical to use an inrush current limiter such as Keystone CL-90 (about 500mA at 7.8 ohm) on the ground rail as shown in the following diagram?


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fullwave with ICL.png
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Kevin
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 Post subject: Re: Is full wave rectification with ICL practical?
PostPosted: Jan Mon 09, 2017 4:33 pm 
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Location: Hudson, MA
How slow a turn on are you looking for?
If you want to reduce inrush put the CL-90 on the 120V primary side. The current limit would be 120V/120 ohm = 1 Amp
The CL-90 is not a good choice on the 1000V B+ since the current limit on the B+ would be 1000/120 = 8.3 Amps
If you want a time delay, a time delay relay may be a better choice (with 1kV DC contact rating).
The CL-90 on the DC B+ will reduce the inrush current, however the time delay due to this will only be about a millisecond. For example consider the cold resistance of 120 ohm and an output cap of 10uF, this yields a time constant of 120 x 10^-6 = 1.2mS.

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 Post subject: Re: Is full wave rectification with ICL practical?
PostPosted: Jan Mon 09, 2017 5:15 pm 
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Location: Toronto, Ontario, Canada
Thanks Eddy, your response has helped me greatly in icl application.
My power transformer has several secondary windings, I want the B+ to have a 'ramp up' turn on.
Where can I find a 1000Vdc of 200mA contact rating?

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 Post subject: Re: Is full wave rectification with ICL practical?
PostPosted: Jan Mon 09, 2017 5:25 pm 
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If D1 & D2 were vacuum tube rectifiers , there would be a delay as the heaters warm up. Can you explain in detail exactly what you are trying to achieve, and is the delay relative to another circuit? What equipment are you looking to modify?

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 Post subject: Re: Is full wave rectification with ICL practical?
PostPosted: Jan Mon 09, 2017 5:42 pm 
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Location: Toronto, Ontario, Canada
I am planning to build a 845 single-ended amplifier. I have got the power transformer custom made already. I prefer not to use 866 or 866 Junior rectifiers because I want to keep the amplifier foot print small. I was thinking of using a 'braking resistor' in the ground rail and shot out the braking resistor by means of a time delay relay to achieve some kind of 'step up' B+ turn on but I am concern that the relay contact will soon burn due to arcing. Suggestions are most welcome.

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 Post subject: Re: Is full wave rectification with ICL practical?
PostPosted: Jan Mon 09, 2017 6:02 pm 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
High voltage power supplies for big RF amps usually have a resistor in series with the primary of the plate transformer. After a short delay, the resistor is shunted by a relay.

Image

Having all the windings on one core presents a problem, as you have found out. Delaying the B+ also delays the filament voltages.

Actually, since you are using a choke input filter, the in-rush current should be rather limited.

How much time delay are you seeking? When I built my linear amp, I was concerned only about false tripping of the circuit breakers when the amp first turned on. Just a few milliseconds delay was enough.

Time delay relays are usually rated for 120 or 240 VAC and would not be safe at 1000 V DC. http://www.newark.com/amperite/115c30/t ... dp/24F2348

Rich


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 Post subject: Re: Is full wave rectification with ICL practical?
PostPosted: Jan Mon 09, 2017 6:25 pm 
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Location: Toronto, Ontario, Canada
Thanks Rich. I plan to use three 1N4007 (1000V) in series on each side of the 1600VCT winding, will copy your diode string schematic. Will three 1N4007 in series be safe enough for a 1000Vdc output?

Will use a CLC filter, C1 = 4uF 1500V, L = 10H, C2 = 36uF 3800V

The time delay I am seeking is to let the 845 self-bias around -150V to come on first before the B+ is applied to the 845, protecting the 845 from plate current 'flooding'. Will the ICL on the negative rail provide enough time delay in this case?

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 Post subject: Re: Is full wave rectification with ICL practical?
PostPosted: Jan Mon 09, 2017 6:50 pm 
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MV rectifiers wouldn't work anyways since they shouldn't ever warm up with B+ on them. You could, however, use a pair of 6DE4s to replace your diodes and you'd get your slow turn-on B+ but you'd need to run a stand-alone transformer or winding for them and elevate the heaters to a minimum of 100V(rated to 900VDC H-K).

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 Post subject: Re: Is full wave rectification with ICL practical?
PostPosted: Jan Mon 09, 2017 7:09 pm 
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Location: Lexington, KY USA
The CL-90 cold resistance is too low to do much to slow the HV turn-on.

You should consult with your transformer vendor to be sure the CT is insulated well enough to be safe if not close to ground.

Since your rectifier is solid state already, you might consider a solid state circuit to delay, and even slow the HV rise. For the switch, an IGBT is inexpensive and should do the job reliably.

If you want to use a relay, use a power contactor to do the switching, not the contacts in a time delay relay. Many open frame contactors and relays offer the possibility of increasing the the contact spacing to work at your high voltage. The venerable PRD series relays come to mind.

Ted


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 Post subject: Re: Is full wave rectification with ICL practical?
PostPosted: Jan Mon 09, 2017 9:48 pm 
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Location: Santa Rosa, CA
It is not my schematic... I just searched for something close on the Internet.

Quote:
Thanks Rich. I plan to use three 1N4007 (1000V) in series on each side of the 1600VCT winding, will copy your diode string schematic. Will three 1N4007 in series be safe enough for a 1000Vdc output?


In a full-wave CT rectifier, each leg has a PRV of about 2x DC peak volts. So in your case, about 2000V. 3 should be fine, but 1N4007s are so cheap, why not use 4 just to have some added margin.

http://www.hammondmfg.com/pdf/5c007.pdf

Rich


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 Post subject: Re: Is full wave rectification with ICL practical?
PostPosted: Jan Tue 10, 2017 1:13 am 
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Joined: Oct Thu 11, 2012 2:45 am
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Location: Westmoreland, Kansas
Years ago I built a linear amp with a pair of 4-400a. They need 3000v dc on the
plates. The transformer is and old power line transformer with a 2700 vac primary
and a 120vac secondary. The time delay I used was a power contactor in the
now 120v primary controlled by a time delay relay.
Has worked fine for years. Filament voltage is from a separate transformer.


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 Post subject: Re: Is full wave rectification with ICL practical?
PostPosted: Jan Wed 11, 2017 3:56 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4813
Location: Rochester NY USA
A choke input filter with solid-state rectifiers will need an input cap of perhaps 0.1 uF, 2KV or more to protect the rectifiers from voltage transients on turn-off. An alternate would be an MOV across the choke to limit its reverse kick. I have seen 1400V rectifiers that failed at 208V when not protected in his manner.


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 Post subject: Re: Is full wave rectification with ICL practical?
PostPosted: Jan Wed 11, 2017 5:17 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 248
Location: Toronto, Ontario, Canada
Thanks to everyone for your valuable inputs.
I think I will use 4 x 1N4007 on each leg, mov on the choke and a contactor for B+ delay-on.

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