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G8MNY  > TECH     29.04.22 07:35z 146 Lines 6895 Bytes #13 (0) @ WW
BID : 991_GB7CIP
Subj: Small Alafontronica 500W UPS
Sent: 220429/0706Z @:GB7CIP.#32.GBR.EURO #:991 [Caterham Surrey GBR] $:991_GB7C

By G8MNY                                    (Updated May 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

This small UPS, is "supposedly up to 500W" of power. It is a 50Hz pulse width
stepped square wave mains inverter all from small 7AH 12V battery when in
backup mode (e.g. 55A @ 11V ?)

      FRONT                          INTENAL SIDE VIEW
旼컴컴컴컴컴컴컴  heatsink                                      ______
넬컴컴컴컴컴컴커  複複複     reg            Freq     Volts    쿝ELAYS  DATA
납On      On[]납  =======================================================PORT
납Backup      납  幡賽賽                                     __
납Charge Off[]납   ~~~~~~~~~~~~~~~~ 旼컴컴컴컴컴컴컴컴컴컴컴컴컨커
냅컴컴컴컴컴컴켸      __________                                
                      MAINS                                       IEC
                      TRANS-              12V BATTERY             OUT
                     읕컴컴컴컴켸                                    IEC
                                             batt fuse               IN
                   읕컴컴컴컴컴컴컴켸 읕컴컴컴컴컴컴컴컴컴컴컴컴컴켸

The transformer has tapped primary & 2 secondaries, one into a bridge & SCR for
charging. And the 2nd is the centre tapped inverter input winding. Two relays
control the mains side, one for mains tap & other for mains connection.

On power fail the mains relay releases & the 2 large Tab FET switching
transistors sandwiched in the heatsink, run at 50Hz giving the waveform below.

On mains return the 50Hz is 1st phase locked to the mains, before switching off
& connecting back to the mains. Another relay also changes the output tap on
backup mode.

OUTPUT WAVEFORM with load (ringy from transformer & filter)  

  +340V  旼컴 Peak Voltage depends on Battery state & Load current!
  +230V      Pulse width varies to get 230V RMS?
   0V  늙켸   읕컴   旼
  -340V          읕컴

Mainly for computerised kit, that might suffer file corruption, or even HDD
damage on some power fails. But note a "non on line" UPS can still be fooled by
bad "brown outs", where the mains voltage does not go straight out, but has a
few cycles close to the kit's PSU safe minimum voltage (e.g. drops to 150V for
1 second while someone is blowing a fuse & then out!) So kit PSU becomes
desperate for power & the UPS does not cut in, in time to stop the kit's PSU
giving illegal & corrupting voltages especially to CPUs, memories, & HDDs.

As with most UPSs, this UPS is not suitable for an unmanned site. This is
because, when the battery is emptied/fails on a long discharge, not only does
the output fail, but the UPS then stays in OFF mode with no output power, even
when the mains is power restored!

Initially I designed 2 modifications to solve this...

I wanted the UPS to come back in automatically when the battery has recovered
after discharge. On scoping around the PCB, I found the ON button was just -12V
on a low current circuit, to the ON logic. There was also a pulsed +ve 18V
charging waveform feeding the charge LED via 1k, that diminished to around 1V
when the battery was OK. So a little scratching of head I came up with this

18V 100Hz      1N4148   1N4148
Charge 컴쩡220캑>쳐쩡컴쩡<쳐컴쩡컴컴컴>ON LOGIC
Pulses  1K          +          ON
        __     10u=== 100k      /button
      컴컨컴컴컴컴컴좔컴좔컴컴컴좔컴컴컴 -12v

When the pulses reduce, the 10u discharges through the 100K to level were the
12V ON electronics starts up, doing a self test & back on. Although with this
circuit the UPS does not come straight back on when the battery might be usable
after a UPS exhaustion event, but leaves the load on normal unprotected mains
via my 2/ mod. However it does come back automatically eventually & hopefully
with a usable well charged battery.

A label was added to warn of the modification & how to turn the unit off.
e.g. POWER OFF UNIT WITH MAINS ON & then disconnect mains input.

By adding a small 5A double pole mains changeover relay (low current 30k AC
winding) to the load side (on IEC socket), when the output fails, the relay
releases & the load is then connected direct the mains.

Mains L 컫컴컴컴컴컴컴컴컴컴컴컴커nc
        憫컴컴                   \_____ L
             쿗        10k
         UPS 쿽ut- 渼>쳐좔>첼        LOAD
             쿾ut  쳐훀ELAY컴        UPS
             쿙    윱>쳐쩡>쵬        BYPASS
             쳐컴컴컴컴컨컴컴컴커 _____
        읓컴컴                   /      N
Mains N 컨컴컴컴컴컴컴컴컴컴컴컴켸nc

But nothing is that simple, as my relay did not operate correctly & tended to
Buzz on UPS changeover. So I added a 4 diodes bridge & a series R of 10K (do
not use series cap on square wave mains!) This slowed down the action OK, from
10mS to <100mS.

Later after 50 days of trouble free use, a failure occured blowing the 3.12A
slow blow input mains fuse, I redesiged it with out this relay & also removed
the mains OFF relay & bridged that out with a wire link,

컴컴컴컴컴컴컴             In use this is bit better as there is less
Mains filter  쿝ear         arcing mains relays etc. But now there is a 2s
              쿽f           break on "power on", as the 50Hz Osc has to be
     Missing  쿛CB          in mains sync before the C/O relay connects
   Volts relay             the mains (whether inverter is on or off!)
     Removed               However this does not happen unless the
    OFF relay 쿛CB          battery has flatened & then on a full
              쿎onnector    recharge, you get a clean short break.
     Mains ON              
    C/O Relay              Anoying, but much prefured to blowing a fuse!

The 12V DC side has a 32A fuse, this is a lot of current from a 7AH battery!
The fuse holder & battery connectons need to be kept clean & greased to ensure
good operation & no corrosive heating at these currents. 

The UPS has the DC battery -ve is on the mains neutral, & not mains isolated!
I did all my testing with an isolated mains supply & a scope earthed to the
battery -ve.

Why don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP

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