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.
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.