PWM Chip substitution

Thread Starter

R!f@@

Joined Apr 2, 2009
9,918
Hey guys.
I was abt to repair a PSU when I came up with a dead PWM controller
Find the attached data sheet.
I have replace MC3842 with 3843 no problem.
But I do not have 3845.

I checked the specs, note the second paragraph.
I am having doubts on swapping 45 with 42 or 43.
Will it work or do you like to see me blow the PSU bridge and FET. :D
 

Attachments

Thread Starter

R!f@@

Joined Apr 2, 2009
9,918
oh well!!!
it seems this is also my only reply thread.
No matter... it works.
u can swap 42, 43, 44, and 45.
Cheers
 

SgtWookie

Joined Jul 17, 2007
22,230
Ahh, they're different IC's for a reason.

They have different thresholds and different maximum duty cycles. The '45 has a low threshold and a low duty cycle limit, while the '42 has a high threshold and can go to 100%.

It may work for awhile, but it would take a thorough analysis of the original circuit to determine whether or not such a replacement is acceptable or not. It may work OK in your circuit for a while, but under a heavy load, you may see smoke and flames coming from your supply.

The designer probably had a good reason for using the '45 vs the others.
 

Thread Starter

R!f@@

Joined Apr 2, 2009
9,918
That is what I like to know, the duty cycle thing.
I compared the data sheets and every thing else is almost the same.

I know what duty cycle is, but the thing is when it comes to analysis I go berserk.
What I like to know is that it says that it will work with in 100% duty cycle, and other one says with in 50% duty cycle.

The duty cycle is the mark space ration of a PWM, the thing is "with in", I am having a hard time grabbing what it means, I think it's because my native is not English, and somethings are hard to understand for me, even though I may seem pretty confident in English.

But the good news is the supply is working and I have replaced the power switch and Vcc resistor plus Isense resistor in accordance with the data.
I tried to quickly power up and down and now still it is powered and loaded and seems fine. So I guess we'll now when the max load occurs how well it can handle. right?

Thanks Sgt
 

Thread Starter

R!f@@

Joined Apr 2, 2009
9,918
Well Sgt.
I know you'll be smiling after reading this.
you know I could just ignore but I am not that type, If I made a boo boo I will admit it .
and then I will learn. And then my chest will come up...well!! .u know what I mean.
So here go's.

The Bloody PSU smoked at the customers hand.
Returned and checked and found the FET, PWM, All the low R's ..kaput....gone....Pffffffffft!! :eek:

But ...nooooo!!! I ain't gonna stop. Wait till I get my hands on some new FET's and then we'll see who has the last laugh :cool:.

This is not the End...It is the beginning.............................................................. more to come
 
Last edited:

SgtWookie

Joined Jul 17, 2007
22,230
I find no joy or satisfaction in failing to convince you to install the correct replacement part.

I hope you took some photos of the board while you were working on it; at least that will make the re-build much easier.
 

Thread Starter

R!f@@

Joined Apr 2, 2009
9,918
It's not that you get joy... I was just humoring....u know me...
OK let's see photo's ....coming up.
I went to buy FET's... no luck.
 
Top