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 Post subject: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Aug Sun 18, 2019 6:21 pm 
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I'm working on a power supply for an ART-13 transmitter (AM).
Both the plate(1250vdc) and screen(~450v) supplies are solid-state, choke input, and the B+ will be engaged by PTT (by switching the AC primary voltages). The primary for the plate supply comes from a variac.
One ART-13 PS design I saw online had a typical soft start circuit with an AC resistor, relay and RC combo.

I do not have the diagnostic equipment to measure how this would benefit, and don't want to blindly build something.

What would be the risks of Not having a soft start system, if any? I've read online there could be a "thunk" noise in my transmission when PTT.
Also, if I'm designing in a vacuum, I'm not sure what time constant range would be effective.
73!

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 Post subject: Re: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Aug Sun 18, 2019 10:50 pm 
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The main reason for an inrush current limiter is to take the stress off the power transformer, rectifier, and filter capacitors. There must be a better way to add PTT to that rig.

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 Post subject: Re: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Aug Mon 19, 2019 2:45 am 
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I use NTC inrush limiters for this purpose, simpler to implement than a step/start relay/resistor system and they are perfectly adequate for what you are doing. You will want to choose an inrush limiter where the expected steady state current flow is around 75-85% of the device rating. I use these in my Johnson Desk KW, Viking 500, and several other medium and high power transmitters here.

The only real drawback of these devices is they have a slight thermal time lag to "recycle" so if you press and hold the PTT, release for an instant, and then immediately go back into transmit the inrush limiter will still be in its hot state and will do little to limit the current.

IF you decide to go with a step/start relay and resistor combination then you should seriously consider a fuse in series with the resistor if for some reason the relay doesn't close and short the resistor because otherwise it will quickly overheat and send resistor fragments flying. Several commercial "linear" amps that use these have had failures causing collateral damage from the resistor and there is a VERY easy way to avoid this problem with the typical linear amp. I did design a step start circuit into my homebrew high power linear to avoid inrush when power is first applied however I use an additional set of contacts on the step/start relay in series with the control line so that the amplifier cannot go into transmit until the relay closes. This simple approach isn't suitable for what you are doing so if you decide to go with a relay instead of the NTC inrush limiters consider a slow blow fuse to protect if the relay doesn't close.

The NTC devices are available from Mouser and Digikey. Jim W8ZR provides an excellent explanation of how to properly use these devices in the July 1997 issue of Electric Radio in which he describes how he added them to his Johnson Viking 500.

Rodger WQ9E


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 Post subject: Re: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Aug Mon 19, 2019 3:52 am 
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Thanks Gents.

Rodger - I had discounted the NTC units because of that time required for cool-down before they're effective again (which you mentioned) as well as some of the literature indicates their use is for capacitor input supplies.

I'm a subscriber of ER; I'll order that back issue (July 1997). As always, thanks for the suggestions.
73

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 Post subject: Re: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Aug Mon 19, 2019 4:18 am 
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In looking at the data sheets they recommend a configuration post-rectifier, and the current is relatively low. So I would put these on the AC side.

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 Post subject: Re: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Aug Mon 19, 2019 4:33 am 
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I am preparing to build a power supply for my ART-13. Are you working from a schematic? I could use a good design.

73,
Mike

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 Post subject: Re: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Aug Mon 19, 2019 12:36 pm 
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SmoothOscillator wrote:
In looking at the data sheets they recommend a configuration post-rectifier, and the current is relatively low. So I would put these on the AC side.


I haven't run across that recommendation for post-rectifier operation in the data sheets for the Amtherm and TDK components I have used. That is probably an application note focused upon line operated switching mode power supplies. You definitely wouldn't want them in the DC side of the supplies you are building because you would exceed the recommended voltage rating.

These are simply heat variable resistors and they will work fine with either choke or capacitor input power supplies, incandescent lamps, etc. They are especially recommended for capacitor input supplies to tame the initial charging current surge but are also very useful with choke input supplies which still have a very high initial inrush current. Nominal time in free air from full temp/resistance back to starting temp/resistance is around 30 seconds, considerably less if in moving air. Resistance will start to increase immediately after power is removed and it will still provide some inrush dampening but it won't meet full inrush limiting capability until it has several seconds to cool. In normal vintage transmitter operation this isn't an issue.

It sheds heat via its leads so the longer the leads, the less time to cold start resistance. When mounting, keep in mind the device does get hot since it is effectively a small resistor in series with the power supply load so heat generated during sustained usage equals voltage drop across device times current. Don't mount them too close to the filter caps or any other highly heat sensitive device.

They are very reliable and much simpler than a step start relay circuit. The limiters in my Desk KW (plate, screen, 4-400A filament transformer, 810 filament transformer) are over 10 years old and have several thousand heat/cool cycles by now. I installed the first ones I used in receivers over 20 years ago and none have failed.

Rodger WQ9E


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 Post subject: Re: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Aug Mon 19, 2019 1:43 pm 
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Rodger - thank you again; I did read where they get quite hot; I'll position in an open area and perhaps do a lug mount. I'll order a few to experiment with.

Quote:
I am preparing to build a power supply for my ART-13. Are you working from a schematic? I could use a good design.

Mike, I'll PM you, I can share the several schematic and docs I found online - the legwork they did of the ART-13 connector pin-out gave me a jump start.

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 Post subject: Re: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Aug Mon 19, 2019 2:09 pm 
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Mike Bracey wrote:
I am preparing to build a power supply for my ART-13. Are you working from a schematic? I could use a good design.

73,
Mike


Mike,

Also take a look at Electric Radio magazine Dec. 03 and Jan. 04 issues. There is a good power supply design and some other very useful information but be sure and get both issues because there are a couple of errors in the Dec. schematic that are corrected in the January 04 followup and one of these errors will cause damage. It is an excellent basic power supply design and there is some other useful information for anyone putting one of these back on the air.

Rodger WQ9E


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 Post subject: Re: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Sep Thu 23, 2021 12:10 am 
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rsingl wrote:
I use NTC inrush limiters for this purpose, simpler to implement than a step/start relay/resistor system and they are perfectly adequate for what you are doing. You will want to choose an inrush limiter where the expected steady state current flow is around 75-85% of the device rating. I use these in my Johnson Desk KW, Viking 500, and several other medium and high power transmitters here.


I will be replacing the HV filter capacitors in my Heathkit DX-100B. I would like to add an inrush current limiter for the plate transformer. Has anyone here done this with a Heathkit DX-100 or B model or the 100W Johnson Viking? Did you find that there is a current rating and resistance that works well for your transmitter? Did you also install an ICL in the lead to the primary of the main power transformer?

Brad K4RT


Last edited by bfo on Sep Thu 23, 2021 5:30 am, edited 1 time in total.

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 Post subject: Re: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Sep Thu 23, 2021 5:30 am 
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Location: Edmonds, WA USA
The plate transformer in the DX-100 is start up current limited by original design. It is very rare to find a bad HV transformer in a DX-100. The Viking Valiant does benefit from inrush current limiting although I love the thump when turning on high voltage on mine. Dig into Heathkit literature on line and you find a lengthy article about the design of the DX-100. The B version is little different with a more modern cabinet.

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 Post subject: Re: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Sep Fri 24, 2021 6:11 am 
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I would like a bit more current limiting than what may be there already, if any.


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 Post subject: Re: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Sep Fri 24, 2021 12:10 pm 
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General in-rush comment: New AM operators seem concerned about power supply in-rush. I think it's because most AM rigs with separate power supplies for the RF final PA and modulator switch into transmit by keying the primary line to the h.v. supply transformer. Hams accustomed to operating SSB and CW with linear RF amplifiers where the h.v. is on all the time but the tubes biased into cutoff on receive, are disturbed by this keying method, even though it's been used with AM forever.

Generally, if the power supply filter is choke input, and the choke is at least 10 Henries, you don't need any in-rush limiting because of that, plus the filter capacitance value is usually low because the AM carrier load provides regulation.

Things change a bit with a separate supply for the modulator, but not because of any stress on the iron. I have a 2.2 KV supply that powers transmitter modulator plates only, and because audio is a dynamic load, I have 32 mfd capacitance following the choke. I determined that was the minimal value needed for good audio regulation. But on transmit, the line fuse on the modulator supply kept blowing. The fix is NOT to up the current value of the fuse. So on that supply a thermistor was needed in the AC line to the transformer to protect the fuse not the transformer.


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 Post subject: Re: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Sep Fri 24, 2021 6:27 pm 
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K5UJ wrote:
The fix is NOT to up the current value of the fuse. So on that supply a thermistor was needed in the AC line to the transformer to protect the fuse not the transformer.


Basically the reason I want to add one here. Occasionally on transmit the fuse opens. I don't know of any design flaw, I have found no failures upstream or downstream of the transformer, and I don't want to use a fuse higher in value than what Heath specified. Fuses are inexpensive but when it happens during a QSO it becomes an annoyance since the fuse holder cannot be instantly reached.


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 Post subject: Re: ART-13 Power Supply - Inrush/Soft Start necessary?
PostPosted: Oct Sat 02, 2021 10:23 pm 
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Here's my ART-13 power supply without any kind of inrush protection so far after 5+yrs its still humming away just fine. D3-D4 are diode strings, and the 400v supply transformer primary is hooked up to the 117v primary not 120v. Once the power supply was connected to the transmitter the 400v sagged down just a bit too far so hooking up the primary to the 117v taps brought the final voltage back up. The Meanwell supply does produce some RFI hash probably if I added some filters it might solve the RFI problem luckily the frequencies where I park myself there's no RFI hash. Have fun with your ART-13 they're great rigs and built like a tank they should last for another 70yrs of course by then my daughter probably would have thrown mine in the trash by then. :cry:


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