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Current sense resistor in SMPS

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Blueteeth

Well-Known Member
Hey,

I'm designing a negative LCD bias with the MC34063. I have yet to find an easy way to get hold of a current sense resistor, of 0.22 to 1 ohm. I opened up a car phone charger, and that used the same chip, but no low ohmic resistors in sight, instead I tihnk they used a PCB track 'made' of solder, or at least they covered it in solder to increase its current capability. Anyone know if I really need this resistor? or where I can get hold of one/substitute with something else?

Cheers,

Blueteeth
 

Hero999

Banned
Go to any large electronic component distributer like RS or Farnell.

You could always parallel 4 1:eek:hm: resistors to get 0.25:eek:hm:.
 

Blueteeth

Well-Known Member
Hero999 said:
Go to any large electronic component distributer like RS or Farnell.

You could always parallel 4 1:eek:hm: resistors to get 0.25:eek:hm:.
I was tihnking of using parallel combinations, but the lowest values I have are 4.7ohm (will need at least 10 :( ). I tend to shun both farnell and RS, unless I have a large order to justify the delivery charge. OO I just found a 0.47R in a broken floppy drive, guess that will have to do, its 0805, but I've used those on stripboard before. Still going through my 'broken bits box' for motor contorllers and PSU's etc..

thanks

Blueteeth
 

Hero999

Banned
You could always try wire.

http://en.wikipedia.org/wiki/American_wire_gauge

Wikipdeia says the resistance of 40 AWG wire has a resistance of 3543.307/km so 0.1:eek:hm: won't be too long; work it out for yourself.

How much current does this have to take?

I wouldn't use it for more than 50mA or so.
 

Blueteeth

Well-Known Member
Hey, thanks again, for 0.22 (just to be sure) that would need, by my calculations, 62mm :D I assume the inductance of this is a worry though?

Being an LCD bias, it can't be more than a few mA, the data sheet specifies '4mA max', so I guess 6 would be enough to cover it. Thats on the output though, which should probably be about -18V. Rough guess at input current.. 35mA max. so I won't need a big inductor or anything, and I'm sure the wire would do just fine. I have just desoldered a few 0.1R's and 0.01Rs from a old broken motherboard these should do, although I'm completely ignorant when it comes to working out the value for the Rsense (reading the datasheet over and over still).

Cheers for the tips, and sorry for bringing such a basic quesiton to the forums, but you know, thinking 'out loud' (or typing it) helps with the process :D

Blueteeth

edit: just worked out the Rsense value. I think I'll just use good 'ol trial and error, with a pot on the output to adjust the load. Measuring the voltage output, and adjusting the pot should provide a fairly accurate cut off output current (god bless V=IR). And I'll just keep reducing rsense until its sweet...first SMPS design right here :D
 
Last edited:

Hero999

Banned
I know you've probably sorted it but this is just for you information.

Blueteeth said:
I assume the inductance of this is a worry though?
Only if the frequency is high enough. The formula for inductance can be found on Wikipedia. Also don't forget the harmonics will be higher than the switching frequency.
 

ericgibbs

Well-Known Member
Most Helpful Member
hi blueteeth,

If you prefer to wind your low value resistor, you can use a 0.125W resistor as the winding former.
Just solder your wire to one end, wind on the 62mm, and solder the wire end to the other end of the resistor.

The standard 0.05 thru 1:eek:hm: are readily available, but they maybe physically to big for your project.

I have see 0.1 and 0.2:eek:hm: in surface mount.

EriG
 

Hero999

Banned
Winding it on a former is a good idea as long as you are aware of the fact that it will increase the inductance significantly.
 

Blueteeth

Well-Known Member
Hey guys,

Hero999, cheers, I'll play about with inductors, I've got a few 47uH, 100, and 220's, one of those shuld work a treat, any inductance in the rsense might not make much of a difference using the above inductors (ie: the inductance of rsense is comparably small).

Ericgibbs, great idea! carbon resistors would also be easy to file a groove or two in them to make things a bit neater. I love idea's like that, nothing fancy, but practical, quick, clever, simple solutions for when you're stuck on something :D Now I have plenty of options, something to while away the time on my sick-leave...

If the addded inductance of the winding does become a problem, I could just wind a caduceus/bifilar winding.

Cheers guys, I'll let you know how I get on!

Blueteeth
 

ericgibbs

Well-Known Member
Most Helpful Member
hi blueteeth,

You don't need to file grooves in your resistor, just double wind two wires side by side, nice and tight, on the resistor.

When you have done, varnish them with a little 'ladies nail lacquer' it works fine. The colour is optional!.

When its almost dry, carefully remove one of the windings, leaving a wire diameter spacing on the permanent wire.

As you say, if you want minimum inductance, bifilar would be OK.

If you are using tinned copper wire, you can stretch it, within limits, to increase its resistance per meter.

EricG
 

Blueteeth

Well-Known Member
ericgibbs said:
...When you have done, varnish them with a little 'ladies nail lacquer' it works fine. The colour is optional!.EricG
LOL, well, I've always been partial to 'plum' meself. I'll try out the SMT resistors I nabbed from old boards first, but that idea will come in very handy for future projects. As I got 20 MC34063's....precise low ohmic values will come in handy, and I'm always up for the 'true DIY' approach, saves money, and only takes a little time. Hopefully I'll set up a website one day to include all these tips'n'tricks (citing references of course) :D

Now wheres my nailvarnish....

Blueteeth
 
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