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G8MNY > TECH 01.02.24 11:00l 75 Lines 3474 Bytes #999 (0) @ WW
BID : 393_GB7CIP
Read: GUEST
Subj: Power Devil Drill Charger
Path: IW8PGT<LU4ECL<I0OJJ<GB7CIP
Sent: 240201/0951Z @:GB7CIP.#32.GBR.EURO #:393 [Caterham Surrey GBR] $:393_GB7C
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jun 20)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
I was given one of these cheap hammer drills, hardly used, but the charger had
packed up. The 13A mains plug fuse as OK!!
THE PROBLEM
First I had to make a special screwdriver (ground a slot in the blade bit) for
the 4 case security screws.
The mains 50W transformer was open circuit. It has a 115øC thermal fuse burried
under a few layers of tape, that had tripped. So to make sure all was still
well with the transformer, I tried a full battery charge with the fuse
bypassed. All was well but the transformer was quite warm, but no where near
115øC, so it was worth replacing the special heat fuse.
The cause seem to be the stupid case manufacturing fault, where air vent holes
had been moulded into the top, they were not pushed right through in the
moulding to make the holes, & the underside holes were totally blocked by the
transformer core!
THE CIRCUIT
Link +25V RL 5A diode
ÚÄ´>ÃÄÄÄÂÄ==ÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄ>\ÄÄÄÄÄ´>ÃÄÄ> >ÄÄÄÄÄ¿+
L>ÄÄÄÄÄ¿ ³ ³ ³ ³ ³ green ³ _³_
)||( À´<ÃÄ¿ ³ ³ start|oÁÄÄ´>ÃÄÄ1KÄÄÄÄÄ´ _Ü_
)||( ³ ³ ³ push|oÂÄÄÄÄÂÄÄÄÄÄ´<ÃÄÙ _Ü_
230V )||(22V ³ ³ ³ ³ __³__ _Ü_
)||( ³ ³ +³ ³ [relay] _Ü_ 19.2V
)||(2A ³ ³ === 1K ~~³~~ _Ü_ 2 AH
)||( Ú´>ÃÄ)ÄÙ470u³ À´>ÃÄÁÄÄÄÄÄ330RÄÄÄÄÄÄ> >Ä¿ Ü Nicad
N>Äo-oÄÙ ³ ³ ³ 35v³ red \oÄ´
Trip ÀÄÁ´<ÃÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ> >ÄÄÄÄÄÙ-
115øC 5A diodes Thermal SW
HOW IT WORKS
The mains transformer is on 100% of the time, so the instructions mention to
"disconnect when not charging"! And also to limit charging to about 1 hour as
this is generally enough for a near capacity charge, 3 hours will be enough for
a full charge.
On connecting the mains there is no LED lights. With a cold battery inserted,
the green LED lights, trickle charging the battery at about 10mA.
On pressing the start push the red LED lights & 12V relay is operate through
the 330R & the battery's thermal switch. The relay contact shorts out the green
LED & apply the full whack to the battery.
When the battery pack has had enough of storing chemical energy it heats up &
at about 50øC its internal thermal switch brakes the relay hold circuit
dropping the charge back to the green 10mA LED trickle.
MY CURE
1/ Araldite in the new fuse to the mains winding outer layer & brown tape over.
2/ Felt pen the transformer black, this increases its dissipation by 10%.
3/ Cut away upper case vent holes internal plastic, unblock the slotted holes.
4/ Drill some new holes in bottom & top of case, not blocked by components.
5/ Change 15A link wire for 2A fuse wire on track of PCB, readily replaceable.
6/ Replace the 13A mains plug fuse with a 3A.
7/ Saw slots across case screws for ready access.
On testing all works OK. But I have not put in a totally flat battery pack to
see if the fuse wire survives yet.
As a good measure I also blackened the drill motor, as these tiny motors also
gets very hot & are easy to burn out on a stall!
Why Don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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