Resistor value helpalues

[nogger]

Hi all, these carbon film resistors always confuse me for some reason, been reading resistor values over 50 years now but i seem to have a mental block when it comes to these.

This is used in a power supply alongside a TOP256EN chip which i suspect is dead. This resistor is open circuit , i think it is 0.68 ohms (source resistor ?) but of course BLUE/GREY/ORANGE would be 68k although the orange bar is spaced away from the BLUE

I don't want to put a 0.68 in if it is 6.8k
One of you will know i am sure.
Thanks in advance
Nogger

[petehall347]

Re: Resistor value helpalues

is there another one with same markings ?

[nogger]

Re: Resistor value helpalues

No, that would be too easy BUT there is another of a different value on the board.
I removed it and measured it and it says 47 ohms (see attached pic)
So maybe it is 68k although i have just traced it and it connects from the positive of the reservoir cap (quality CapXon construction) to the Drain of the TOP256 off line switching chip

Nogger

[vinceroger69]

Re: Resistor value helpalues

Any data sheets for the top256 show the typical values of the resistor used?

[stj]

Re: Resistor value helpalues

maybe it is not orange but brown?

[nogger]

Re: Resistor value helpalues

Any data sheets for the top256 show the typical values of the resistor used?

Yes one has 4 example circuits but various values, in one its 100 ohms and another its 68k and 68 ohm

I attached another pic, 3rd band is clearly ORANGE so i am beginning to think it's 68k after all

nogger

[vinceroger69]

Re: Resistor value helpalues

i would guess 68k too as that whats the online calculator says
https://www.digikey.co.uk/en/resourc...or-code-4-band

[nogger]

Re: Resistor value helpalues

Yes,i think that seeing as the other reads 47 ohms i will go for 68k ohm.
I think the worst that can happen is it just wont start up
Thanks
Nogger

[budm]

Re: Resistor value helpalues

https://www.badcaps.net/forum/attach...4&d=1530965311
47 Ohms

https://www.badcaps.net/forum/attach...2&d=1530981728
68K, Looks to be for snubber network. If this one goes open circuit, the MOSFET inside the IC will be damaged.

[momaka]

Re: Resistor value helpalues

So maybe it is 68k although i have just traced it and it connects from the positive of the reservoir cap (quality CapXon construction) to the Drain of the TOP256 off line switching chip

Looks like a 68 KOhm to me as well.

Are you sure that's how it's connected, though?

Budm suggests is used for the snubber network, which makes sense given the resistor value. But if it's part of the snubber, you won't see that resistor connected simply between the positive (+) on the input bulk cap and Drain of the TOP256. Most snubber networks consist of a resistor like this in parallel with a ceramic capacitor, and the two in series with a diode. This whole network is then placed between the main winding of the power supply's transformer (which on one side happens to be connected to the positive of the input bulk cap and the other to Drain of the IC).

That said, are you sure the resistor is not connected to the Voltage-Monitor (V) or Multi-Function(M) pins (if your TOP256 has these)?

In any case, 68K looks like a safe bet. Worst case if you use a resistor with a value much higher than intended is that the device won't work. Now doing the reverse (using a resistance much lower than what the circuit calls for) can cause much more damage.