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Author Topic: Which capacitor to use?  (Read 1018 times)
N4NOO
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Posts: 106




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« on: June 04, 2009, 10:57:08 PM »

I have an SGC 500 watt hf amp in the rear of my car.. I also have an Optmia battery at the amp that is conected to the front battery. My amp will "fault" on low voltage even with the engine running at highway speeds.  I am thinking about adding a capacitor.  Question: Should I use a 1 Farad or a 2 Farad cap?
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K0BG
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« Reply #1 on: June 05, 2009, 05:47:19 AM »

What you need to discover first, is WHY the amp is kicking off, the low-voltage LED notwithstanding.

If the SWR is too high, the amp will draw more power than is normally would, which will indeed trip it out, and with the low-voltage LED lit.

If you have common mode currents flowing on the coax, you can have the exact same problem.

If you overdrive the amp (70 watts is about all you can drive it before the attenuator kicks in), you'll also get the same problem.

You didn't state how large the wire between the batteries was, but it has to be at least #6, and preferably #4 if the run is over 15 feet or so.

I own three of these amps, and about 5 years experience with them. Been there, done that!

In any case, the cap won't help!

You also might go to my web site and look under Amplifiers.

Alan, KØBG
www.k0bg.com
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N4NOO
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Posts: 106




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« Reply #2 on: June 05, 2009, 06:37:32 AM »

Thanks for your thoughts Alan.
I am using an Icom 7000 with the Better RF system and a Hi-Q 6/160 RT-MC-3-H antenna.  When the antenna is tuned to 1.1:1 on 3.980MHz with the amp in by-pass the SWR doesn't move on voice peaks.  But I have noticed that when I turn the amp on the SWR will be very high on voice peaks but is that just the input to the amp circuit causing this? I only use 45 watts of drive from the 7000.  I have several of the correct type ferrite clip-ons at both the amp output and at the antenna input.  This piece of coax is only about 5 feet long and was just replaced. This did not change anything.
The interconect wire from battery to battery is #2 and the wire from the battery (Optima)to the amp is only 1 foot long and is a #4 size.
The Martel volt meter is attached to the rear battery and shows voltage drops to below 13 volts on voice peaks when the amp is on.
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K0BG
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« Reply #3 on: June 05, 2009, 07:12:56 AM »

I sent you an e-mail. If you didn't get it, let me know.

Alan, KØBG
www.k0bg.com
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KM3F
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Posts: 525




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« Reply #4 on: June 05, 2009, 06:14:19 PM »

Trying to diagos from your circuit discription leads me to believe the connecting leads from the front charging system has to much voltage drop.
The battery by it'self will not hold up the voltage by it's very nature.
500 Watts is roughly 13.6 x 38 amps.
What you think is a simple arrangementt is not so simple once you break it down and see how it works.
The alternator regulator has to try to follow the audio rate of change for which it was not originally intended to do for auto service.
The regulator has an internal reference that it sets the alternator output from.
When you talk, the regulator has to adjust the alternator output to satisfy the battery in addition to the amplifier.
The extra battery does add added system capactiy but also takes current from the alternator as well.
Next question is, is the alternator large enough in capacity to handle the added load?
Emg vehichles have very large alternator systems to handle their extra loads of lighting and other needs to the paticular application to be reliable.
Adding large capacitance at the amp will tend to help as it is seen just like a battery and still needs to take a recharge during the RF 'off time' between words etc to provide that function each time.
I would evaluate the charge system and the cable sizes vs voltage drop for the main issue, even then you will not completly eliminate the 'droop' (system hardness).
In addition there are some other items to consider.
The vehichle's other functions also powers off the charge system. Any night time, A/C use drives up the total system current the alternator has to supply.
The general rule for large loads is the alternator should have a capacity of about 150% of average loading in the worst case as well as the battery/s have large capacity to hold up the supply voltage within reason.
Lastly, an alternator's capacity should be quite large bacause they are not designed to run full output much longer than to charge at peak for a short time until the batteries begin to recover their charge and their voltage rises reducing the alternator output to a level that is safe for the long term without overheating.
Another way of looking at the total system is the resistances involved.
R= E/I.  13.6/ 38 = .36 ohm.   You can see from this that any significent cable drops, connection drops that add up to more than .25 ohm will drop a significent amount of voltage to the load and tend to cause you your hassle.
Hope I have provided enough tech insight for you to be able to find the issues and improve them where possible.
These high cirrent application have more to them than meets the eye.
Good luck.
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KM3F
Member

Posts: 525




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« Reply #5 on: June 05, 2009, 06:14:38 PM »

Trying to diagos from your circuit discription leads me to believe the connecting leads from the front charging system has to much voltage drop.
The battery by it'self will not hold up the voltage by it's very nature.
500 Watts is roughly 13.6 x 38 amps.
What you think is a simple arrangementt is not so simple once you break it down and see how it works.
The alternator regulator has to try to follow the audio rate of change for which it was not originally intended to do for auto service.
The regulator has an internal reference that it sets the alternator output from.
When you talk, the regulator has to adjust the alternator output to satisfy the battery in addition to the amplifier.
The extra battery does add added system capactiy but also takes current from the alternator as well.
Next question is, is the alternator large enough in capacity to handle the added load?
Emg vehichles have very large alternator systems to handle their extra loads of lighting and other needs to the paticular application to be reliable.
Adding large capacitance at the amp will tend to help as it is seen just like a battery and still needs to take a recharge during the RF 'off time' between words etc to provide that function each time.
I would evaluate the charge system and the cable sizes vs voltage drop for the main issue, even then you will not completly eliminate the 'droop' (system hardness).
In addition there are some other items to consider.
The vehichle's other functions also powers off the charge system. Any night time, A/C use drives up the total system current the alternator has to supply.
The general rule for large loads is the alternator should have a capacity of about 150% of average loading in the worst case as well as the battery/s have large capacity to hold up the supply voltage within reason.
Lastly, an alternator's capacity should be quite large bacause they are not designed to run full output much longer than to charge at peak for a short time until the batteries begin to recover their charge and their voltage rises reducing the alternator output to a level that is safe for the long term without overheating.
Another way of looking at the total system is the resistances involved.
R= E/I.  13.6/ 38 = .36 ohm.   You can see from this that any significent cable drops, connection drops that add up to more than .25 ohm will drop a significent amount of voltage to the load and tend to cause you your hassle.
Hope I have provided enough tech insight for you to be able to find the issues and improve them where possible.
These high cirrent application have more to them than meets the eye.
Good luck.
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AD5X
Member

Posts: 1437




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« Reply #6 on: June 05, 2009, 06:50:16 PM »

I wonder if you are faulting your amp because you are over driving it when you first speak.  I don't know about the IC-7000, but when the IC-706MKIIG has its power reduced via the menu it still puts out a full power spike until the radio's ALC throttles it back.  I see this at the beginning of each transmission, and on two IC-706MKIIG radios that I have.  I fixed this problem by putting an in-line attenuator between the radio and amplifier.  This way I can leave the radio at full power all the time, eliminating the initial speech or first "dit" spike.  The attenuator obviously doesn't impact S/N, and only minimally reduces receiver sensitivity.  Info is at www.ad5x.com.

Phil - AD5X
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AD4U
Member

Posts: 2186




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« Reply #7 on: June 17, 2009, 09:48:19 AM »

Also you are NOT drawing 38 amps @13.6V.  Yes 13.6V X 38 amps equals around 500 watts, but this amp is not 100% efficient.  Assuming 50% efficiency (typical) your amp will draw around 76 amps @ 13.6V to achieve 1000 watts input for 500 watts output.  But then no auto battery I am aware of will "hold" 13.6V while drawing 76 amps.  This causes the battery voltage to sag way below 13.6V, which is probaby causing your amp to shut down.

If all the other suggestions offered so far do not solve your problem, I recommend you replace the Optima with a heavy duty battery like those used in diesel equipment or an AGM deep cycle battery and connect it to your amp using very short lengths of #2 or larger welding cable.

If everything else is OK, this should solve your problem.

Dick  AD4U
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K6AER
Member

Posts: 3535




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« Reply #8 on: June 17, 2009, 10:25:03 AM »

Even though the SGC-500 would indicate the power output is 500 watts the SGC amplifier is capable of much more. The amplifier runs 8ea.  2SC2879 transistors and is capable of 800-900 watts peak out RF. No mind you if you run the amplifier at this level your IMD level will greatly increase and your signal will have distortion.

At 850 watts out with 55% efficiency the input power could be as high as 1545 watts input DC on voice peaks. 1545 divided by 13.8 VDC is 111 amps of DC current. Most automotive alternators are rated at no more than 125 amps under ideal conditions. Your vehicle will need about 30 amps under ideal conditions for overhead, maybe another 30 amps to charge the battery after starting the car for the battery to recover over the next 20 minutes. This leaves 65 amps if the charging system is in good health.

Don't forget your HF radio is using 20 amps on voice peaks. You have even less for the amplifier. Don't turn on the head lights.

As you can see you don’t have much left over for running an amplifier. A new healthy low impedance battery will help, having a number 4 gage wire for the amplifier as Alan has suggested will also help but the bottom line is unless you reduce your power output from the amplifier you don’t have enough alternator capacity.

You might consider installing a larger alternator such as 220 amp after market alternator.

http://powerbastards.com/proddetail.asp?prod=Fitzall%2D220

Also install a new battery and make sure all electrical connections from the alternator to the battery to the Engine block and to the amplifier are solid. Run the transceiver electrical directly to the battery and don’t share the amplifier DC line.

Good Luck.
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K6AER
Member

Posts: 3535




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« Reply #9 on: June 17, 2009, 10:28:09 AM »

Something I forgot to add…make sure your antenna will handle the power. Most mobile antennas are fairly lossy and with 800 watts of power you will be setting your loading coil on fire unless the antenna is designed for that power level.

Ham Sticks and Hustlers need not apply.
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