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use ni-mh battery as power supply for filament

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LAN_403

Thu Apr 13 2006, 10:37AM

Registered Member #245 Joined:
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Posts: 6

hi !
i try to find a power supply for filament, with enough instant power :

i read here that i can use may be 9 Ni-mh battery :

3 x 3 [ Ni-mh 1.2V-2A ] => 3 [3.6V-2A ] => 3.6V-6A

it's seems to be enough for 1 sec ? :)

but it's DV : i need to convert to AC !?

Steve Conner

Thu Apr 13 2006, 10:41AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

No, the filament should work off DC just as well as it does off AC. And, you shouldn't need 9 batteries, just use 3 in series. The 2 amp-hour rating doesn't mean that the battery can only supply 2 amps: it means that if you draw 2 amps from it, it runs out after about 1 hour.

You could also try two D size alkaline batteries in series. Good brands like duracell should be able to supply several amps for a short time.

robert

Thu Apr 13 2006, 12:54PM

Registered Member #188 Joined: Thu Feb 16 2006, 05:18PM
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Posts: 67

The filament drafs around 15A, so you need batteries that can supply this.
That will mean at least sub-c size for NiMH/NiCd and paralelled D cells for alkaline...

Fri Apr 14 2006, 12:45AM

Registered Member #245 Joined:
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Posts: 6

ok so if i plug directly the "+" part of a 3v DC power supply directly on the filament input of the magnetron (and the "-" part connect on the ground ) that will work ???
great :)

just one ni-mh 1.2V can supply 2A for 1 H --> so it can supply 20A for 6 min

with 3 ni-mh in serie -> 3.6V, 20A for 6 min ? :)

ragnar

Fri Apr 14 2006, 02:27AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

C'mon guys, we should all know that the capacity figures stated are typically at a 20hr discharge rate.

Don't expect more than half the capacity at 1H discharge rate, let alone less. Google your battery manufacturer to find time/capacity curves - you'll find the graphs disappointing.

</pessimistic>

If you need big currents, grab D cells, get the voltage you need, and make two or three strings in parallel.

Why on earth do you want a portable magnetron + filament supply? Who is the to-be victim of this backpack HERF?

Steve Conner

Fri Apr 14 2006, 11:01AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

agence: The magnetron should have a plastic insulator sticking out of it, with two spade connectors on the end. You connect the filament supply between these two connectors. The negative end of the HV supply goes to either one of those connectors (doesn't matter which) and the positive end of the HV goes to the metal case of the magnetron. It's wise to arrange the case of the magnetron to be ground- nobody wants to be electrocuted while they're being cooked.

blackplasma: Yes, Peukert's Law is a pain in the butt.

I didn't want to complicate things even more by mentioning it, though.

Marko

Fri Apr 14 2006, 07:24PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

honestly, I don't think that couple of small 1,2NiMH's could heat up magnetron filament.
You would need lots of them in paralell, and you would ruin them quickly by repeated current overdraw.
I had problems to heat up filaments of small vacuum diodes with new 9V battery

Look for heavy duty NiCd's/NiMh's (ones used in battery drills) or small lead acid battery, and then PWM the filament (as you can hardly find such batteries for 3V)

Or just do the same with the main source, I dont think setting simple PWM regulation would make much trouble.

Sat Apr 15 2006, 02:51PM

Registered Member #245 Joined:
Location:
Posts: 6

for this job, D cells are better that ni-mh ?

with i acid battery, I'm afraid that i don't have enough instant power :(

i read here that (if necessary) the filament can support more that 3v ?

FastMHz

Sat Apr 15 2006, 05:39PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

RC NiMH battery packs are designed to provide high current for short times and take abuse. You could get one of those 3600mah 7.2v packs and remove a couple of cells if you need lower voltage...get a sport pack, they're cheap at Tower.

Marko

Sat Apr 15 2006, 06:03PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Lead acid batteries (even small) can actually supply far more instant power than most usual batteries. High power (rc/drill) Nicd/nimhs are also good choice.

i read here that (if necessary) the filament can support more that 3v ?

For very short periods it could, with drawing a lot more current and possibly shorted lifetime.

You can also use PWM, with big capacitor to provide current trough ON time and put less stress to the battery.

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