# mc34063 current sense resistor question

#### uraniumhexoflorite

Joined Oct 23, 2016
216
Hello! I'm working on a buck converter that uses the MC34063 IC. It's regulating the voltage perfectly, but the current is nowhere near what I want. I want to pull at least 700 ma, but I've only been able to get about 70 ma. I understand that this is because I'm using a current limiting resistor that's too large, but the current limiting resistor is the battery connector. It's about 1 ohm. Any ideas on how I can lower the resistance even more? Thanks.

#### LesJones

Joined Jan 8, 2017
2,843
Start by posting a schematic plus information on the power source and load.

Les.

#### uraniumhexoflorite

Joined Oct 23, 2016
216
Here's a schematic of the circuit. The component values are, for the most part, identical. R1 is about 1145 ohms. R2 is 3300 ohms. Ct should be 560 pf (has the number 561 on it, which should mean 56*10^1 pf). Co is 100 microfarads. Input capacitor is 660 microfarads (2 330 in parallel). Rsc is a battery connector with about 1 ohm of resistance.

#### R!f@@

Joined Apr 2, 2009
9,740
Explain a battery connector with "1Ω" resistance.

#### uraniumhexoflorite

Joined Oct 23, 2016
216
I calculated that to get 700 ma out, Rsc, the current sense resistor, needs to be just under 0.22 ohms. I discovered that the connector that I was using to power the circuit was about 1 ohm. I decided that that was the closest I could get to the 0.22 ohms of resistance that I needed.

#### R!f@@

Joined Apr 2, 2009
9,740
And you figured 1Ω is same as 0.22Ω?
And how long do you think that a contact will stay at that resistance over usage and time.

Find a low value and put it there.

#### uraniumhexoflorite

Joined Oct 23, 2016
216
It's the closest I could get. My multimeter says it's 1.2 ohms. I have resistors that should be 0.27 ohms, but my multimeter says that they are 1.5 ohms. I'll double check with another multimeter.

#### R!f@@

Joined Apr 2, 2009
9,740
1.2 + 0.27 = 1.47
Do you see where I am getting at ?

If you do not, try adjusting the 0Ω adjust of the VOM (If analog)
if DMM, try subtracting the probe resistance from the measured value

#### uraniumhexoflorite

Joined Oct 23, 2016
216
I see. The multimeter's own resistance is messing with the reading.

#### uraniumhexoflorite

Joined Oct 23, 2016
216
The current output isn't what it would be with such low resistance. My calculations say that there is just over 2 ohms on Rsc. I found the equations on the MC34063 datasheet. Do you think the breadboard could be increasing the resistance?

#### R!f@@

Joined Apr 2, 2009
9,740
Possible.

#### uraniumhexoflorite

Joined Oct 23, 2016
216
I'll test the resistance.