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DX-100/B Adding current inrush limiter




 
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Author Topic: DX-100/B Adding current inrush limiter  (Read 773 times)
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K4RT
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« on: September 23, 2021, 12:41:15 AM »

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?
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w7fox
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« Reply #1 on: September 23, 2021, 09:40:12 AM »

The vacuum tube rectifiers and choke input filters inherently limit inrush current.  If you are eliminating the vacuum tubes and changing to capacitor input filtering, divide the peak transformer voltage by the peak surge current rating of your rectifier and ensure you have that much resistance to limit the current.  My Viking 1 works fine with its original circuit.
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K6IC
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« Reply #2 on: September 24, 2021, 04:58:00 PM »

Twenty some-odd years ago,  replaced the PS filter caps for the LV and HV power supplies,  and the 5R4 rectifiers in my DX-100B.

Upon flipping the Transmit switch ON,  sparkies would fly around inside the 5R4s.

Assumed that the modern filter caps had a much lower ESR than the olde 1950s PS caps.

Installed a pair of NTC Thermistors in parallel,  in the switched AC lead to the Plate transformer's primary.

Problem solved.   Forget the value,  and the brand (Keystone,  perhaps) of the thermistors,  but RF Parts was the supplier.

YMMV,  and so on.   73  GL  Vic
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K4RT
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« Reply #3 on: September 24, 2021, 06:30:21 PM »

Thanks for the replies. Vic, occasionally on transmit the fuse opens. I haven't found any shorts or other problems in checking the transformer, filter caps, etc. I don't want to use a fuse higher in value than what Heathkit specified. I think adding a thermistor is worth trying. Did you use a pair in parallel for redundancy?

73,
Brad
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K6IC
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« Reply #4 on: September 24, 2021, 07:47:26 PM »

Hi Brad,

NO,  used parallel thermistors,  because,   as,  IIRC,  it was the continuous current rating of a single one would be exceeded by the max transmit operating current.

I had the main fuse open,  several years ago.  Replaced it with a new one of the same rating,  and have had no problems since.

Assumed that the fuse opened due to cycling  --  there was NO evidence that the fuse holder had gotten hot,  so just chalked it up to being an aged fuse.

We do not use the Dixie one hundred bee,  very often,  these days.   But,  it is a handsome and capable rig.

73,  have FUN,  Vic
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K4RT
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« Reply #5 on: September 25, 2021, 01:03:30 AM »

Ok Vic, thank you. I'll order some and try it.

I use my "B" on AM and CW. Lots of fun.

73,
Brad
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K6IC
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« Reply #6 on: September 25, 2021, 03:06:47 PM »

Hi Brad,

After thinking a bit more,   am almost certain that the supplier of the thermistors,  is the more obvious source:
Digikey.

The DX-100 is at another location,  as are the unused thermistors.   Can picture the bag in which they came,   and can picture the Digikey labeling.

Will look for that bag,  within a week.   73  GL,   Vic
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K4RT
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« Reply #7 on: September 25, 2021, 07:34:22 PM »

Hi Vic,

Thanks for offering to do that. I can probably find them on the DigiKey web site, but if you turn up that bag I would appreciate knowing the designation and specs. If not, no big deal.

73,
Brad
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N0YXO
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« Reply #8 on: September 25, 2021, 08:54:37 PM »

I just solid stated a T-Bolt amp on the high voltage and used them . They work well a friend got the values were from electric radio. I bought mine from Mouser


Charles N0YXO
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WD5JKO
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WD5JKO


« Reply #9 on: September 26, 2021, 12:13:46 PM »

I don't want to use a fuse higher in value than what Heathkit specified. I think adding a thermistor is worth trying.

Hi all, just a couple of comments on this topic.

The NTC thermistor concept is a good one when the HV supply is turned on and stays on. The initial inrush will be softened since the thermistor is cold. Then during operation, it heats up, and the resistance goes down, but never to zero ohms. Therefore sizing the thermistor to the application is important. Then if the power glitches off and back on, or if you just cycle the power switch OFF long enough to discharge the capacitors, the next turn on while the thermistor is still hot will not be surge limited, and the fuse may blow.

In the case where the HV is cycled as with PTT, the thermistor may still be hot after the initial PTT. The "hot" resistance of the thermistor may still be enough to prevent fuse blowing, but here you may be dropping several volts in the thermistor, even when hot. This scenario might be perfectly fine when the AC line voltage is on the high side. Playing with this is best done with several thermistor ratings above and below your first choice.

Another choice is to just replace the Heathkit desired fuse with a "Slo-Blo" fuse of the same current rating.

Jim
Wd5JKO
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