What temperature can 3w resistors safely run at?

[bigal_scorpio]

Hi guys,

I have the circuit working pretty well now with 44uH I am getting 2A at 4.83v so not too much drop there.

One question though, is it ok for an inductor to get warm? I'm not talking red hot, only 31c, but is that ok or will it harm them in the long run and cause failure?

Also something which may be of interest to everyone here. For a nearly perfect 1A load at 5v you can use a standard 21w car indicator or tail light bulb. Not sidelight bulb. The indicator bulb gives a load of 1020mA at 5v and is often useful for testing PSUs and other things that need to be loaded with 1A.

BTW is it ok for the switcher to run at 50c? It has a reasonable heatsink and seems stable at that temp.

Al

 

[kinarfi]

sounds ok to me, you're not boiling water yet, LOL, glad it's working for you, got any place to post a photo and link it?
What the voltage & current, In & out? does the out put stay pretty constant as you vary the input voltage?

 

[bigal_scorpio]

Hi kinarfi,

I keep touching the heatsink and it does not seem to be getting any hotter but I will leave it overnight to be certain.

The voltage is pretty stable at exactly 5v with minimal load and only dropping to 4.85 at 2 amps load.

Strangely enough the other one I have made which has exactly the same components is a bit different in that it drops to 4.45v at 2 amps. Odd! Is there any way to tweak these switchers?

Also on the schematic 1000uF cap is used but I didn't have a suitable one at that value so I substituted a 2200uF 10v one on both my circuits, could this cause the drop to be more? I was thinking that it would be the bigger the better and also the 10v caps have better ESR than the higher voltage ones.

Al

 

[alec_t]

I see 2 inductors in the pic. Are they in series or parallel?

 

[bigal_scorpio]

Hi Alec,

They are in series mate and both 22uH. That seems to be the optimum size to give the right current but I didn't have a 44.

I know they look smallish but they don't overheat at all.

Al

 

[kinarfi]

Hi kinarfi,

I keep touching the heatsink and it does not seem to be getting any hotter but I will leave it overnight to be certain.

The voltage is pretty stable at exactly 5v with minimal load and only dropping to 4.85 at 2 amps load.

Strangely enough the other one I have made which has exactly the same components is a bit different in that it drops to 4.45v at 2 amps. Odd! Is there any way to tweak these switchers?

Also on the schematic 1000uF cap is used but I didn't have a suitable one at that value so I substituted a 2200uF 10v one on both my circuits, could this cause the drop to be more? I was thinking that it would be the bigger the better and also the 10v caps have better ESR than the higher voltage ones.

Al

It's been several month's since I was playing with these switcher, but I know of no way to tweak the switchers, how ever, if you measure the inductors and find one just a tad high, remove a turn from that one and see what happens, don't cut the wire, just unwind it. -- another thing that might make things equal is to shuffle the inductors around, and I agree that the larger caps are a good thing, if you have a scope, try looking at the wave forms and freq of each, if dual trace, look at them together.

after thought, if you take one turn off each inductor, it may lower the inductance just enough to make the voltage drop at max load less, just an idea.

 

[4pyros]

Hi kinarfi,

I keep touching the heatsink and it does not seem to be getting any hotter but I will leave it overnight to be certain.

The voltage is pretty stable at exactly 5v with minimal load and only dropping to 4.85 at 2 amps load.

Strangely enough the other one I have made which has exactly the same components is a bit different in that it drops to 4.45v at 2 amps. Odd! Is there any way to tweak these switchers?

Also on the schematic 1000uF cap is used but I didn't have a suitable one at that value so I substituted a 2200uF 10v one on both my circuits, could this cause the drop to be more? I was thinking that it would be the bigger the better and also the 10v caps have better ESR than the higher voltage ones.

Al

Looks nice.

 

[bigal_scorpio]

Thanks mate. Its been running on the car for about a week now and no problems. Touch wood!

And thanks for all the help from everyone, Al

 

[4pyros]

Thanks mate. Its been running on the car for about a week now and no problems. Touch wood!

And thanks for all the help from everyone, Al

Good job