I'm trying to get parts for this voltage boost schematic and I was curious about the .27ohm resistor. It seems like an odd value, what would be the purpose of such a small value resistor?

Thanks

 

Typically they are used as a shunt for current sensing by placing them in series with a current to be measured or monitored.

Edit: In your referenced circuit it likely is used by a current limit circuit in the PWM controller.

 

They are also used in old power linear supplies where several big pass transistors are installed. They are put in the emitter circuits to hellp minimize current differences caused by slightly different Vbe from one transistor to the next.

 

You can bridge this resistor, if short circuit condition will be very unlikely, or if you use electronic transformer, with short circuit protection. The one's I have cut off the output voltage + blink LED.

If you need current limiting, then use it as sensing resistor. It may sound weird but you can actually make 0.27 Ohms resistor using about 3 metres bell wire. If you use thinner magnet wire, or wire wrap wire, much less length is required. The point is it does not have to be exactly 0.27 Ohms...

 

I'm trying to get parts for this voltage boost schematic and I was curious about the .27ohm resistor. It seems like an odd value, what would be the purpose of such a small value resistor?

Thanks

By "odd value", do you mean that it is 0.27 instead of something nice and round like 0.25? If so, then the reason is that 27 is one of the standard component values.

The '27' is introduced in the E12 series, also known as the 10% tolerance series.

E12: 10 12 15 18 22 27 33 39 47 56 68 82

The idea is that you have components that have a +/-10% manufacturing tolerance. Thus there is no point making a 100Ω resistor and also a 110Ω resistor because a given 100Ω resistor could be as much as 110Ω while a 110Ω resistor could be as small as 99Ω. So what values should you use that permit you to span one decade (a factor of 10) while keeping the overlap/gap of the tolerance ranges between adjacent values to a minimum. For the 10% band it turns out you need, ideally, ~11.47 standard values. So that is rounded to 12.

The point being that don't think that just because it seems 'odd' in this sense that it means, for some reason, that it is important that it be that particular value. It's very possible they would have liked to use a 1/4 ohm resistor and this is simply the nearest standard value to that without going to precision resistors.

 

Thanks for the info everyone... By odd, I simply meant so small but that is a useful explanation nonetheless.

 

Pin 7 of that regulator limits the peak current through the inductor; it's turned off when pin 7 reaches about 300mV below Vcc, give or take 50mV. So, with a 0.27 Ohm resistor, current would be shut off at about 300mV/0.27 = 1.111...Amperes.


takao21203 wrote:

<snip> It may sound weird but you can actually make 0.27 Ohms resistor using about 3 metres bell wire.<snip>

Adding extra wire in the Vcc path of a switching regulator is not a good approach, as the parasitic inductance will cause problems. It is essential to use NON-INDUCTIVE resistors.

In some cases, the power dissipation in current sense resistors is high enough to require the use of wire-wound resistors. The use of a common wirewound resistor is also problematic; however Vishay/Dale and other manufacturers have lines of non-inductive wirewound resistors that can be used.

 

Not that it would make particular sense to limit the current for a LCD backlight to 1.11A- on a PCB.

 

I'm trying to get parts for this voltage boost schematic and I was curious about the .27ohm resistor. It seems like an odd value, what would be the purpose of such a small value resistor?

Thanks

according to the data sheet its for short circuit monitoring

http://www.sparkfun.com/datasheets/IC/MC34063A.pdf

 

I would simply use 4 1ohm resistors in parallel to achieve this value.

 

I would simply leave it out, when I want to power LCD backlight.

1A will already do considerable damage in case of a short circuit.

So it's useless. Calculating an useless circuit element to a point where this component has to have the right value...well...is of no use?

And if a 1m piece of wire has sufficient inductance to bother a 200uH main inductor, I have doubts about this as well.