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Author Topic: WIll a 12vdc/35 amp PS wired in parallel to a car battery yeild more current?  (Read 5135 times)
N2EYE
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« on: August 21, 2012, 07:06:05 PM »

Hello folks,
I got a 4 x 2879 amp  as part of trade and  am interested in using it for 10 meters inside my home shack.  I have an extra Astron 35 amp PS that does not have enough current for the amp to operate and I thought that if I added a car battery in parallel it may work. I have read that a PS with >12vdc can be used to charge car batteries.  I also have a Car Battery Charger but I would expect that device to have no filtering or veryt little regulation.

I read on the Web where people have done this but it was not on point regarding transmitted RF. I also read that some folks advised  inserting a diode in the  PS Pos lead to the battery.  I   want to drive it low for cleanest signal possible.   Are there any fusing methods I should be aware of?  AMp rated at 80 amps draw max but it should still work with lower current.

Thank you,
neil, n2eye, NY City
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W5DXP
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« Reply #1 on: August 21, 2012, 07:46:42 PM »

I thought that if I added a car battery in parallel it may work.

That's essentially the setup I have for my SG-500 amp. The Astron 35 amp supply will not overcharge a deep-discharge marine battery. I also have a gigantic automotive audio capacitor in parallel - like one of these.

http://images.search.yahoo.com/search/images?_adv_prop=image&fr=yfp-t-701-s&va=car+audio+capacitors
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W8JX
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« Reply #2 on: August 21, 2012, 08:35:00 PM »

I thought that if I added a car battery in parallel it may work.

That's essentially the setup I have for my SG-500 amp. The Astron 35 amp supply will not overcharge a deep-discharge marine battery. I also have a gigantic automotive audio capacitor in parallel - like one of these.

http://images.search.yahoo.com/search/images?_adv_prop=image&fr=yfp-t-701-s&va=car+audio+capacitors

This guy has his ducks in a row!   Smiley
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KB5ZSM
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« Reply #3 on: August 22, 2012, 12:24:17 AM »

It should work fine BUT I agree about adding the diode to protect the power supply. If the diode drops your voltage too much then open the power supply and adjust to compensate. Be sure to select a diode that can easily handle the current. I used a 50A stud diode mounted on a heat sink.
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AA4PB
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« Reply #4 on: August 22, 2012, 05:20:57 AM »

The reason for the diode is that if the AC power goes away then, without the diode, the battery can discharge back into the power supply and that has the potential to damage the regulator circuitry.

The battery/power supply combo will yield more peak current, but you still can't draw more than the power supply rating on a continuous basis. It works well for modes like CW and SSB where the average current during transmit is low or for modes like AM or RTTY if the transmission time is short and intermittent.
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W8JI
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« Reply #5 on: August 22, 2012, 05:26:39 AM »

Is this a clean amateur amplifier or is it a dirty CB amplifier that you are planning on using on ten meters?

My bet is you have a CB amplfier without proper bias and filtering. If so, leave it for the CB band.

Well designed high power 12 volt amplifiers are all bad enough for IMD, and CB amps are virtually all terrible.
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WB2WIK
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« Reply #6 on: August 22, 2012, 10:07:58 AM »

  I   want to drive it low for cleanest signal possible.   Are there any fusing methods I should be aware of?  AMp rated at 80 amps draw max but it should still work with lower current.



Most of those amps I've seen and used work best at higher voltage, like 14Vdc, rather than 12Vdc.  By "best" I mean not only more power output, but also better IMD even at reduced output power.

I'd always shoot for the highest voltage possible within the transistor ratings, and then drive the amp to about "half power" or so, and not more than that.  Usually reduces IMD and prolongs transistor life, so it's a double bonus.

I think the ubiquitous 2SC2879 is rated 18Vdc max.  The data sheet I have indicates -24 dBc IM3 at 100W output.  That's not great.  But four of them running 200W output (50W each if they're well balanced) might yield better results.
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AD6KA
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« Reply #7 on: August 22, 2012, 11:33:33 AM »

Hello folks,
I got a 4 x 2879 amp  as part of trade and  am interested in using it for 10 meters inside my home shack. 
Thank you,
neil, n2eye, NY City

I hope you are going to wait until you upgrade
to General before you put that amp on the air.
That's assuming the amp qualifies to be put on the ham bands at all.
A disappointing number of entry level hams (not saying YOU are
one of them) see nothing wrong with putting CB amps on 10m.
Good luck with your project.
73, Ken  AD6KA
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AA4HA
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Posts: 1434




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« Reply #8 on: August 22, 2012, 01:07:43 PM »

Hello folks,
I got a 4 x 2879 amp  as part of trade and  am interested in using it for 10 meters inside my home shack.  I have an extra Astron 35 amp PS that does not have enough current for the amp to operate and I thought that if I added a car battery in parallel it may work. I have read that a PS with >12vdc can be used to charge car batteries.  I also have a Car Battery Charger but I would expect that device to have no filtering or veryt little regulation.

I read on the Web where people have done this but it was not on point regarding transmitted RF. I also read that some folks advised  inserting a diode in the  PS Pos lead to the battery.  I   want to drive it low for cleanest signal possible.   Are there any fusing methods I should be aware of?  AMp rated at 80 amps draw max but it should still work with lower current.

Thank you,
neil, n2eye, NY City

I will not get into the unrelated questions regarding license class, who you bought the amplifier from, etc.. Here are some things to consider;

1. Look up a data sheet on the type of battery that you are going to use. All 12 volt batteries are not created the same. You want to know what is the peak voltage that the battery is at when completely charged.

2. You want to use a diode for isolation for the very reasons given.
    a. if your power supply fails you do not want the inoperative supply sucking power back out of your battery.
    b. having DC presented to some power supplies that are off can cause them to fail.
    c. make sure the diode is rated for some % greater current than the power supply is capable of supplying
    d. fuse that line from the power supply to the battery, somewhere below the maximum rating of the supply.

3. Your power supply needs an adjustable output or be prepared to add a few diodes in series to drop the supply voltage to within a few tenths of volts of the batteries maximum charge voltage.
    a. we want the battery to be "happy" at what is called the "float charge level", just a few tenths of a volt above maximum charge. Do not let this go more than .1 to .3 VDC higher, the battery will just have to burn off that excess potential as heat. Cooking batteries is a bad idea.

4. Fuse the combined output of the battery and charger at some % greater than the maximum transmit power but some % less than the internal fusing on the radio. We want this fuse to open up in case there is a problem and not to cook the radio with a temporary, extraordinarily high combined current from the battery and supply. Nor do we want to be using the internal fuse as the primary safety device as they may not be rated correctly and you do not want purple smoke coming out of your radio with the unmistakable smell of frying semicondctor, circuit board or melting plastic.

5. This battery is also going to effectively operate similar to a capacitor in cleaning up the DC/DC converter output of the supply. In fact, it is going to act like a GIGANTIC capacitor. If you check out the ripple of the DC supply (use a DVM set to measure AC volts across the DC supply with and without the battery, look at how many millivolts of ripple go away when that battery is attached). (this ratio can be expressed as a % of ripple too, kinda neat)

Your idea and this approach is an excellent way for many amateurs to be operating their stations. The battery is going to clean up the AC ripple on the DC supply and unless you are "Amperes handicapped" by the battery size you will be able to operate even when the power goes out. Your radio will not even flicker when the power goes out.

(hint: that battery, acting like a giant capacitor, is also acting similar to a surge protector device for the radio power supply)

Important Note: if this is not a lead acid type battery chemistry and is maybe a Lithium, NiMh or NiCad then just floating the battery may be a bad idea. Some cell chemistries will learn of what is expected of them and over time, become incapable of doing anything else (memory effect). For some batteries they prefer to cycle to a discharged state and sometimes even an "equalization charge (overvoltage)". Be careful doing battery equalization with attached radios unless you want to become intimate with replacing radio components. Gel-Cell batteries are bad for this type of behavior.

I prefer to use AGM (activated glass mat) batteries rather than wet cells or gel cells. Nothing to spill, great current density and last for years if you are not in the habit of running the battery dead. I use NiMH and rechargeable lithium chemistries for portable devices like backpack radios and portables.

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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
AC5UP
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« Reply #9 on: August 22, 2012, 02:24:00 PM »

Minor Quibble:
Assuming we're talking about the typical deep charge trolling motor kinda' battery and a stock Astron 35, I would have some concerns about loss of regulation at anything over 20-ish Amps.........

Consider: The Astron is good for 13.8 VDC and the battery is more like 12 VDC (or less) under load. Assuming the amplifier does pull some serious current on SSB peaks the Astron will fold back under load, the supply voltage will drop to whatever the battery can maintain, then try to bounce back during every pause in the modulation. Is this an issue for the radio or amplifier? As previously mentioned the IMD on many 12 volt lin-yars is not good to begin with and will degrade rapidly as the supply voltage heads south. Aside from subcompacts, the typical automotive alternator is good for 50-ish Amps with some capable of 80 Amps or better. At highway cruise in a Dodge Ram pickum-up truck with the Heavy Duty Bubba option that's westbound and down it is possible for the amplifier to see all the current it needs, but an Astron 35 isn't exactly Imported From Detroit...

If a large value auto stereo capacitor can hold the peak draw voltage above the nominal 12 volts of a deep cycle battery as W5DXP has suggested, what's the battery doing? Aside from waiting for the Astron's output voltage to drop down to its level...

( ? )

BTW: The FCC sez N2EYE holds a Technician class license and that tells me two things:

1)  SSB from 28.300 to 28.500 MHz is Ohhhhhhhhhhh-Kayyyyyyyyyyy.
2)  N2EYE is either a noob, an underachiever, or both. Either way that's curable.   Cheesy
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W5DXP
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« Reply #10 on: August 22, 2012, 02:51:53 PM »

If a large value auto stereo capacitor can hold the peak draw voltage above the nominal 12 volts of a deep cycle battery as W5DXP has suggested, what's the battery doing?

My capacitor is 8 farads. With my configuration, I can suffer an AC power failure (common on Lake Palestine, TX) and never even notice. My deep-discharge battery is somewhat more than 12 vdc with a full charge.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K5LXP
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« Reply #11 on: August 22, 2012, 03:07:12 PM »

Has anyone considered the voltage sag between a nominal 13.8 V supply/float voltage vs the 12.75V or so where the battery will start to supply current?  That's over 1V difference which I would think would impact linearity quite a bit.


Mark K5LXP
Albuquerque, NM
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N2EYE
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« Reply #12 on: August 22, 2012, 03:57:59 PM »

Thanks for all the the Replies gentlemen.  I read elsewhere that the PS voltage should be .5-1 vdc lower than the battery (new, small Car lead acid type).  As for the diode between PS and Battery on Pos lead which way is it conected?  ANode to battery of vice versa?    Am I correct in believing that keeping a charger hooked-up during operation is a bad idea?  A regulated PS sounds better out of common sense to this 'noobie.'

BTW  I was first licensed as a  Novice in '84 after passing the 5 wpm code.    Back then, the Tech written element was the same as the General written exam  element, right?
neil
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W5DXP
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« Reply #13 on: August 22, 2012, 04:20:11 PM »

I read elsewhere that the PS voltage should be .5-1 vdc lower than the battery (new, small Car lead acid type).

A 13.8 v power supply won't overcharge a 12 v lead-acid automotive battery. The charging system on my GMC pickup puts out ~15 v.

A power supply one volt below a 12 vdc battery will not power a lot of transceivers including my ICOM which is specified for 13.8v +/- 15%.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
AC5UP
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Posts: 3864




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« Reply #14 on: August 22, 2012, 04:38:14 PM »

As for the diode between PS and Battery on Pos lead which way is it conected?  ANode to battery of vice versa?

Download this and give it the hairy eyeball:  http://www.hammondmfg.com/pdf/5c007.pdf

Look at the direction of the diodes in every example and which terminal the " arrow " is pointing to.  Do that and you'll always remember the direction of forward current flow...........  Grin
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