# resistor rating?

Discussion in 'General Electronics Discussion' started by qweets, Jan 2, 2011.

1. ### qweets

48
0
Sep 26, 2010
hello everyone.

i have a diagram here for a h bridge. there is a resistor '0.5 ohm' between pin 11 and 13 but I am unsure of what rating ( watts) to select. i will be using a 10v 900ma motor and pwm at 30kh with this circuit.

any ideas how to calculate this?
peace.

2. ### Resqueline

2,848
2
Jul 31, 2009
Can't really do that properly w/o knowing what the datasheet says (the resistor is surely there for a particular reason), but 0.9A * 0.5ohm * 0.9A = 0.45V * 0.9A = 0.405W.

3. ### qweets

48
0
Sep 26, 2010
: )

Thanks for your input on this Resqueline, here is a link to the data sheet, i'm just a little scared to over heat the resistor and melt my vero board : ) or hurt my motor : / any help would be great.

http://www.alldatasheet.com/view.jsp?Searchword=A3953SB-T

4. ### Resqueline

2,848
2
Jul 31, 2009
Ok so it's an Allegro A3953SB-T.
The data sheet tells the story, up front, and continuing on page 6, quote:
The peak value of the current limiting (Itrip) is set by the selection of an external current sensing resistor (Rs) and an external reference input voltage (Vref).
The internal circuitry compares the voltage across the external sense resistor to the voltage on the reference input terminal (REF) resulting in a transconductance function approximated by:
Itrip = ( Vref / Rs ) - Iso
where Iso is the offset current due to the internal base drive current (33mA).

During braking, with the MODE input high, the peak current limit can be approximated by:
Itrip brake mh = Vref / Rs
CAUTION: Because the kinetic energy stored in the motor and load inertia is being converted into current, which charges the Vbb supply bulk capacitance (power supply output and decoupling capacitance), care must be taken to ensure the capacitance is sufficient to absorb the energy without exceeding the voltage rating of any devices connected to the motor supply.
Brake Operation - MODE Input Low. During braking, with the MODE input low, the internal current-control circuitry is disabled.
Therefore, care should be taken to ensure that the motor’s current does not exceed the ratings of the device.
Unquote.

So by using the suggested resistor (or rather 0.47 ohm) and setting Vref to 0.933A * 0.47ohm = 0.44V - both the motor (0.9A) and the resistor (0.41W) should be safe.

5. ### qweets

48
0
Sep 26, 2010
**************************************8

Wow ! well done , thank you, i am amazed. i have only been working on bread boards for just under a year. i was lost on this.

i shall purchase the parts and try it all out.

you made my night, thank you Resqueline : )