R8600: Powering On/Off Question

[nanZor]

.. The only time the power supply has failed is when a friend wanted to use it to charge a gell-cell - he did what all non-electrically trained people do - connect the battery then turn on the power supply at the main power switch! Popped one of four MJE2955's to a dead short and popped the fuse next - I guess the gell-cell survived!

Have to comment here since this is a hidden classic problem. I've done it myself with a series of events until I found out what's wrong:

1) Attached a *fully charged or very well charged* 12v battery acting as a simple backup-battery Astron power supply floating at 13.8v. No problem. Even tested it by removing AC. Looks like a great simple backup.

2) Power actually does go out for many hours, faithfully discharging the backup battery.

3) AC power returns, and POOOF. Crowbar or other protection circuits smoke and fail.

Why?

A normal power supply is not truly a battery charger. When the battery is adequately discharged, it looks like a total SHORT to the power supply.

What fools many is that when the battery has a decent enough charge in it, and is placed across the supply, it doesn't fail and float charges just fine. It's the major discharge and subsequent power-on that sees what it thinks is a dead short.

Some online videos show guys hooking batteries up in just this fashion with no problem. Yet there is usually no followup 6 months later when ac power is actually lost for a non-trivial amount of time and the supply smokes when the power returns.

Note: sometimes you don't even have to wait for the power to return to smoke the supply. Merely turning it off can blow it, as supplies aren't engineered to see voltage at their output terminals when turned off.

At any rate, you just don't want this happening when you've got an 8600 at the other end of the power cable.....

 

[nanZor]

Re: *never* turning off a power supply or other equipment once in awhile - ie 24/7 till it croaks...

Not a good idea. Gear needs to be turned on and off at least once in awhile to extend the life of the electrolytic caps. So every 38.7 days .....

Seriously, if you turn on a piece of gear and let it run 24/7 without ever turning it off, when you are forced to cycle the power for some reason, that gear may fail suddenly, rather than last 10 years or more with at least a *few* on/off cycles. Commonly seen in the broadcast industry where once a piece of gear goes in and on the air, it NEVER gets turned off! Then comes the day for a rack or building move years later, and blam. Toast / nada / zip on power on. Better have some backup parts. The very same gear that DID get the occasional power cycle seems to survive.

The electrolytic caps can be thought of as small batteries with polarized plates, electrolyte and so forth. Very *tiny* batteries that is. Cycling them with a power cycle once in a blue moon keeps them healthy.

Without going too far into it, imagine if you got a new cell phone and kept it fully charged on the charger for years and NEVER allowed it to discharge at all. When you actually do, the battery just crashes real fast. Kinda / sorta the same thing for caps.

Is there any formula for this? No. Just a best practice kind of thing to do once in awhile if you have the chance.

 

[majoco]

as supplies aren't engineered to see voltage at their output terminals when turned off.

as hertz said.

The way round this is a fairly large diode backwards across the main series regulator - you'll end up with a flat battery again as it discharges back through the diode and the discharge resistor which you hopefully have installed across the reservoir capacitor. The regulator only has the forward voltage drop of the big diode across it so remains cool, calm and collected! A series resistor solves the charging problem - for 7A/hour gell cell I use a car brake light lamp - about 32 watts and it just glows dimly.

My large-ish power supply ( I have no idea what the max current is - it's electronically limited to about 12Amps - but the secondary winding is BS #8 wire (which is 1/8 inch dia.), a 35Amp continuous diode bridge and a 24000uF 60volt computer grade capacitor) has a small transformer to supply +12 and -12volts to the regulator 'works', a 741 op-amp and a few transistors. My theory is that the main large supply has such low resistance that it charges the 24000uF in the first half-cycle to the full voltage of the winding which is about 18 volts. BUT the little transformer with it's greater secondary winding resistance hasn't charged the two 1000uF capacitors in the 12 volt +/- supplies yet. The main regulator (4 x MJE2955's and another as the driver) by way of the uncontrolled base feed resistor now turns on to the max output, +18 volts or so with no electronic current limiting - as I said, the overvoltage SCR probably popped the 20Amp fastblow fuse but it was too late - the weakest MJE2955 popped first and overcurrent usually melts the innards to a short circuit. The gell-cell would have seen an enormous current spike but fortunately only for a few milliseconds.

The original schematic for this power supply was a document by J. Lindsley-Hood of amplifier design fame and often published in "Wireless World" - now defunct I think. You may think it strange that the main regulator uses PNP transistors which would appear to be the wrong polarity. Usually the collector goes to the supply side and the emitter is the output - I puzzled over this for a while until it was pointed out to me that the collector is usually the bigger junction and is bonded to the mounting plate - thus better heat transfer. Gain is not a problem, in fact there is excessive gain - I had to add a few capacitors to stop the local broadcast band station modulating the output!

 

[prcguy]

For home made battery backup I prefer to use a separate float charger on the battery and a high current relay with the coil connected to the power supply. The power supply normally powers the equipment and the relay coil with the battery being float charged and isolated. When the power fails the relay transfers the radio from the power supply to the battery and everything keeps on working. When the power returns the power supply runs the radio and the battery goes back on charge. The relay switching is so fast I never had a glitch in any equipment.

When I ran a bunch of commercial repeaters I used this method and it was very reliable, as a trip to the mountain top was time consuming and expensive.

 

[KB4MSZ]

Of course, there are other proven methods.