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Author Topic: INRUSH CURRENT LIMITER  (Read 7241 times)
N8CHR
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Posts: 34




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« on: December 31, 2012, 11:59:19 AM »

HOMEBREW AMP USING AN 813 TUBE. WANT TO ADD AN INRUSH CURRENT LIMITER. WHAT IT IS AND HOW TO I INSTALL? IS THERE A CIRCUIT FOR BUILDING IT OR IT IS A THERMISTOR TYPE PART.
THNAKS  TOM N8CHR
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KH2G
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« Reply #1 on: December 31, 2012, 01:09:14 PM »

Inrush current limiting or step start on a Amp power supply are one and the same. the Object is to delay having full power on the filter caps until they are charged up a bit. Caps when voltage is first applied appear as a short to the DC current.  There are many ways to do this but the essence is to have a small resistance in the primary which gets bypassed by relay contacts when the charge is sufficient on the filter caps. This can be done by a relay energized when the level is met. This whole action is only a second or two but it saves expensive caps. 
regards
KH2G
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KD0REQ
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« Reply #2 on: December 31, 2012, 01:24:18 PM »

or a one-piece part, the old thermistor

http://www.digikey.com/catalog/en/partgroup/j201-j204-series/6374?mpart=B59201J0140B010&vendor=495&WT.z_ref_page_type=Part%20Search&WT.z_ref_page_sub_type=Part%20Detail%20Page&WT.z_ref_page_id=0

for one example.  because the part continues to generate a tub of heat after it has done its protection thing, switching it out with an RC or 555 timing circuit with a relay will be friendly to everything around it.
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KE3WD
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« Reply #3 on: December 31, 2012, 01:51:39 PM »

And -- Inrush limiting for an 813 or two likely isn't needed at all. 


73
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AA4HA
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« Reply #4 on: December 31, 2012, 09:45:56 PM »

Inrush current limiting or step start on a Amp power supply are one and the same.

They are really not the same, the end result may be similar but this can be two completely different approaches.

Current inrush limiting may be done with a negative temperature coefficient resistor like the CL-90. These work well for low current loads like a receiver where you can anticipate what the running current will be. They are sized to drop just a few volts while the radio is in steady operation. During power-up when the inrush current is greater these devices can be dropping 10-20 volts. They run a tad bit on the warm side. One of the better known uses is on tube based receivers like the Hammarlund SP-600 or the Collins designed R-390A.   http://www.mouser.com/ProductDetail/GE-Sensing-Thermometrics/CL-90/?qs=sGAEpiMZZMsgSGrx0WqTbD2qPKFzydYx

Another type of inrush management is step-start where you use the a relay and resistor that is in series with the power supply for a few hundreds of milliseconds to seconds while the power supply caps charge up. Sometimes the step-start is on the mains side of the power supply, in other applications it is on the secondary side inside of the supply, before the capacitors.

Another variation of the inrush current relay would be a circuit that only switches during the zero crossing of the sine wave, when current is at a minimum. This is seldom seen in amateur gear but sometimes is in high power industrial/commercial.

On a transmitter or amplifier the inrush current limiter is a bad idea. The current during transmit is many times the inrush current. You will end up with a transmitter/amp that does limit inrush current for those many milliseconds and even looks good when idling. As soon as you key up and start drawing real amps you end up dropping lots of volts across the NTC thermistor and the radio goes into something similar to "brown-out" and may just turn off. Additionally that NTC thermistor gets roasty hot. Think about something the size of a large disk capacitor trying to dissipate 10-100 watts. Even in normal service thermistors should not be soldered into a circuit as the heat can melt the solder off. Normally it is suggested that they be crimped to lugs and the lugs mounted to a screw terminal.
« Last Edit: December 31, 2012, 09:52:51 PM by AA4HA » Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
KE3WD
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Posts: 5694




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« Reply #5 on: January 01, 2013, 06:35:20 AM »

Still gotta wonder how Grampa (okay, ladies, Grandma too!) used all those ~150W and higher transmitting tubes without Inrush Current Limiters to protect whatever in the power supply. 

The answer is that they did not, because there was no problem found in the first place. 

A hunnert-an'-fifty W ain't QRO.  Neither was three-hunnert in the daze when AM was king and the high level modulation was oh-so-nice sounding. 

Design the power supply properly and inrush current theories about electrolytics drawing "infinite" current at turn-on doesn't seem to be such a problem. 

Can't draw more than what's available.  Literally millions of transformers did it day in and day out, all those radios, TVs, and yes, Amateur Radio transmitters and linear amps testify to the truth of this.  Don't let the occasional failed old boat anchor lead you to ignore statistical results. 

73
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G3RZP
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« Reply #6 on: January 01, 2013, 06:56:20 AM »

Back in those days, most HV power supplies were choke input, and the filter caps of the order of 4 or 8 microfarads. Further, plate transformers were relatively high resistance windings, usually full wave bi-phase. So inrush current wasn't that much of a problem. Come SSB amplifiers, where a fairly large capacitance is needed because of the peak to mean variation of plate current, we started seeing voltage doubler circuits and large amounts of smoothing capacity, combined with silicon rectifiers and low transformer winding resistance to keep the regulation. The result is that you can get a mighty large current surge when you switch on as the capacitors charge, and so the use of 'step -start' is common, with a surge limiting resistor switched in series with the transformer primary until the HV is perhaps half of final voltage, or sometimes, just done on time. Then it's shorted out. (We did that on 22kV 3 amp supply for a travelling wave tube back in the 1960s - the resistors were about 5 ohms, wound of edge-on nichrome  strip and about 15 inches long and 3 inches in diameter  - looked like some of the resistors on electric trains)

Thermistors in HV supply primaries are not a good idea. They may limit surge current, but once they are hot, they are still a resistance in series and hurt the regulation.
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K8AXW
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« Reply #7 on: January 01, 2013, 08:05:32 AM »

While I agree 100% with 3WD in that "there isn't a problem" I'm throwing in my 2c worth for a inrush delay circuit suggestion.

I built a delay circuit using a 0-10sec. time-delay relay.  They're expensive new but available at a reasonable cost at many hamfests and on eBay.
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KH2G
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« Reply #8 on: January 01, 2013, 08:23:40 AM »

I suppose I should have asked which inrush current was he desiring to limit. I assumed the HV power supply. For high levels, my personal preference is step start. The "old days" with vacuum tube rectifiers like the 866's etc since no current flowed until the filaments got warmed up, it wasn't really needed.
Regards, 73
KH2G
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K1ZJH
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Posts: 887




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« Reply #9 on: January 01, 2013, 08:53:32 AM »

Back in those days, most HV power supplies were choke input, and the filter caps of the order of 4 or 8 microfarads. Further, plate transformers were relatively high resistance windings, usually full wave bi-phase. So inrush current wasn't that much of a problem. Come SSB amplifiers, where a fairly large capacitance is needed because of the peak to mean variation of plate current, we started seeing voltage doubler circuits and large amounts of smoothing capacity, combined with silicon rectifiers and low transformer winding resistance to keep the regulation. The result is that you can get a mighty large current surge when you switch on as the capacitors charge, and so the use of 'step -start' is common, with a surge limiting resistor switched in series with the transformer primary until the HV is perhaps half of final voltage, or sometimes, just done on time. Then it's shorted out. (We did that on 22kV 3 amp supply for a travelling wave tube back in the 1960s - the resistors were about 5 ohms, wound of edge-on nichrome  strip and about 15 inches long and 3 inches in diameter  - looked like some of the resistors on electric trains)

Thermistors in HV supply primaries are not a good idea. They may limit surge current, but once they are hot, they are still a resistance in series and hurt the regulation.

Plus 1...   'RZP pretty much hit all the bases.

Pete
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G3RZP
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« Reply #10 on: January 01, 2013, 11:01:42 AM »

Mercury vapour rectifiers such as the 866 and 872 need a good warm up period, as otherwise you strip the filament emitting surface. You have to get the mercury vapour pressure right, which is also why they were limited in operating temperature range - too cold or too hot and the pressure was wrong and you either stripped the filament (too cold) or had flashovers if too hot. Xenon rectifiers weren't so bad - the 3B28 only needs 5 seconds of filament before applying the AC.
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AA4HA
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« Reply #11 on: January 02, 2013, 02:10:40 AM »

NTC thermistors are not bad for predictable, fairly low currents (1-12 amps), I use them in boat anchor receivers. They need to be sized according to current demands and the voltage drop you can tolerate. They do need space as they are tiny little heaters and if you try to run too much current through them they can get hot to the point of fire starting temperatures.

http://www.ge-mcs.com/download/temperature/920-325C-LR.pdf

If this is the way you want to go, two suppliers are Mouser and Newark. I am certain there are others.

For a transmitter or amp a Step-Start relay with some sort of time delay is a great idea. There is some debate on if you want to use them on filament transformers for power amplifier tubes. I would definitely leave the filament transformer through the step-start on smaller tubes just to minimize the thermal shock to the filaments. (remember there was a product known as light bulb savers that were little disks that you put into incandescent light fixture bases?) All that I would be trying to blunt with Step-Start is the EMF to the transformer, any sort of current peaks on power switches and to ramp up capacitor charging so the rectifier does not go up in smoke.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
KD0REQ
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Posts: 840




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« Reply #12 on: January 07, 2013, 01:43:23 PM »

"light bulb savers" were disk rectifiers, you got DC at the bulb.  cut half the wave off, reduced light.

old HS buddy W0SHR made his own Heathkit power supply for the HW-101 with a 150-watt light bulb in an open-base socket on the chassis.  the bulb was in series.  turn on AC power, the bulb was in series, and after you blinked at the light a few seconds you flipped up the second switch to bypass the bulb.

the technique has been used since at least the 30s as a test bench staple to see if the radio under test had a high current short in the power supply.

you can come up with a dozen ways to automate and shrink the circuit... NE555, Amperite relay, surplus dial-it-in variable time relays... but it all works the same in the end.
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WB2WIK
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« Reply #13 on: January 07, 2013, 02:01:16 PM »

Hmmm.

I've owned many commercially-built large power amplifiers, and have homebrewed several from scratch, based on my own designs, and my opinion is that most step-start or inrush protection circuits mostly make it easier on the power on-off switch. Wink

My homebrew amps use a small switch powering a LARGE relay, and the relay has contact arc suppression, so I don't worry about the switch.

My Henry amp uses a 30A double pole circuit breaker as the on-off switch, and I don't worry about that one, either.

But small and cuter desktop amps often use a marginal switch, perhaps so it fits and looks nice, or maybe to save money on the switch.

I don't step-start or inrush protect my big power supplies (4-1000, 8877, etc) which use 15kV rated bridge rectifiers (150A 8.3 mS surge rating) charging 53uF, 5000V oil-filled capacitors and I've never lost a rectifier or a capacitor, or anything else, in any of them after 25-30 years and thousands of switching cycles.
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AC7ZN
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Posts: 41




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« Reply #14 on: January 08, 2013, 05:56:41 AM »

Inrush current limiting is not just to protect the power switch or other internal components.  It is needed to prevent transients on your AC line from damaging other equipment.  I watched our nearby big screen TV power supply fail as I turned on my computer, which had particularly bad inrush current (enough to make the lights in the room flicker).  Troubleshooting revealed the TV power supply switching chip had failed.

As we usually have little control over our AC line regulation or the susceptibility of other devices, keeping AC line current transients within reasonable limits is a good idea.

 73,
Glenn AC7ZN
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