need some help with resistor values

[petehall347]

have a battery charger here that blew itself up like they do .only trouble is i cant find a schematic for it .its a 18v workzone one .so far there are 2 obvious blown resistors and maybe another one , plus the fet is totally shorted which i could do with a cheaper sub for as expensive to buy .
here are some pics .will post more in next post .
ah yes unknown resistors are R9 R5 R11.

[petehall347]

Re: need some help with resistor values

more pics .oh the fet is 4n80

[petehall347]

Re: need some help with resistor values

just had a thought .wondered if i might pick up a plug in charger and simply connect the wires to charger socket to save money ?
if so any special charger needed ? i am guessing not .only 2 connections to battery +-

maybe this then i could put dc socket into the case of charger box . https://www.ebay.co.uk/itm/18V-1-5A-...kAAOSwZvRaYY-o

[R_J]

Re: need some help with resistor values

If R5 (gate resistor) is blown, it is likely that ic1 is also defective. R9 is likely a low value 1Ω or less, R5 could be around10~30Ω R11 seems to be 100 which would be 10Ω

[petehall347]

Re: need some help with resistor values

yes i wondered about the ic cant see what it is though yet .

edit ...
looks like 6B55322 . the 3 might be a 1 .

[petehall347]

Re: need some help with resistor values

clear ish pic of ic

[R_J]

Re: need some help with resistor values

The ic seems to be a Cr6855 60w current mode PWM control chip

[petehall347]

Re: need some help with resistor values

what frequency should it be at to gate ? am thinking i could remove the shorted fet to test the frequency and voltage ?

edit think i found correct value at 65khz .

[petehall347]

Re: need some help with resistor values

wondering if this is correct mosfet ? https://uk.rs-online.com/web/p/mosfets/7925754/

[petehall347]

Re: need some help with resistor values

r11 is actually 10k and still good .its R4 i cant figure out .i see it starts with a 4 .
reads about 280k in circuit

edit .removed from circuit it measures 1.3 meg . which doesn't seem right as first digit is a 4

[petehall347]

Re: need some help with resistor values

i looked at the frequency and everything seems to be at 50hz which is of course the mains supply frequency.

[stj]

Re: need some help with resistor values

that's strange, it has a rectifier so even if the cap is fucked it should be 100Hz

[petehall347]

Re: need some help with resistor values

that's strange, it has a rectifier so even if the cap is fucked it should be 100Hz

might be my meter playing up .only way i can get any sense of frequency measurements is to use just one meter probe am sure it used to work fine if say i put red into live and black to neutral on the mains . i even did something silly and happened to have black probe in my hand and touched red to gate out on the ic . it sure felt like 50hz through my hand .
will have another play with it shortly .

[stj]

Re: need some help with resistor values

probably read 50Hz in the air, radiating from all the wiring in the building.

[petehall347]

Re: need some help with resistor values

ok new findings and things blowing up .
i replaced R9 with a 1k resistor just to have something to make up the circuit . checked frequency at gate out and now zero hz according to my meter had one probe on main cap minus and other on gate out pin of the ic . then a few seconds later 2 rectifier diodes blew up .didnt half make me bloody jump . bearing in mind the mosfet is removed . i have no idea if its because of that or not

edit
ok tacked in a couple of 1n4004 i think or might be 1n4148 plus botched 5 amp fuse wire accross the blown fuse .anyway that doesnt matter right now .back to 300v dc and zero hz .
parts count is adding up now . as need 4 diodes and a fuse including . am guessing rectifier was on its way out anyway .

[petehall347]

Re: need some help with resistor values

diodes marked M7 so am i right thinking they are 1N4007 ?

[R_J]

Re: need some help with resistor values

R9 will be the source resistor for the fet, The ic monitors that line to control the current, If you used a 1K, and the fet turned on, the voltage developed across the resistor will blow CS input of the ic, Max input on that pin is usually 5~7 volts
R9 is usually less than 1 ohm, Also if the fet shorted drain to gate, and blew R5, I can not see how the ic would handle 300vdc going to it.

[petehall347]

Re: need some help with resistor values

the fet is out of circuit .was hoping to see if the ic was working or not . can this be done without the fet in circuit ?

[R_J]

Re: need some help with resistor values

Without the fet, the ic will not see any voltage on the CS pin, it might try and output a few cycles, then shut down, it depends on how the ic operates.
A similar ic would be the ON-Semi NCP1251, its pdf explains how it operates and it may be similar to the Chip-Rail ic works

[petehall347]

Re: need some help with resistor values

so are you saying a NCP1251 will be a direct replacement ?