# Thread: Help choosing sense resistor for H bridge

1. ## Help choosing sense resistor for H bridge

Hi,
I'm trying to build a H bridge, MOSFET based. I have some IRF530 transistors and IR2104 half bridge drivers around, so I planning in use them. I want to implement current limiting using a classic design. Sense resitor from mosfets to ground and a comparator. I need current from 1A to 6/7 Amps max.

My question is about the value of the sense resistor. I believe this is related to the reference voltage (for the comparator) ,the voltage drop, and the dissipated power on the resistor. I know that the reference voltages are usually low (under 2 volts) and the resistor value are also low (from 0.05 ohm to 0,1 ohm).

Can somebody with more experience give some advice ?

Joaquim

2. hello:
you have to pic a current that u think you need as the comparitor will output if its inputs are either higher or lower than the referance exactly.
maybe use lightbulbs for now, a 100w bulb is approx 10 ohms (easy calculations ) use 2 , 4 bulbs whatever, this way u can calculate the current you are drawing.
put a sensing resistor say 0.1 ohms or so in series and measure the voltage across it, or mathimatically calculate it( perhaps both to see how close things are.
use a resistor dividing network, or a zener diode (better) as your voltage refference.
get it as close to the volts across the dropping resistor as possible.
if the comparitor has an inverted output you can use this to drive the gate of a pass mosfet, as it will always be high when the comparitors input is not matched and the fet will conduct.
add and subtract a few lightbulbs to adjust the current to prove your circuit before hooking it to the h-bridge. Bingo .... it should work .... good luck
i did the same type of thing in my 1988 dodge minivan and saved buying the \$600 SMEC (single modular electronic controller).(current to the altermator)

3. Well...let's step back a bit....pick a sense resistor that has a value much lower then the resistance that your FETs have when they are on rds. Otherwise you can plan on dissipating a bunch of heat and use up some of your valueable current....you could use it to keep your coffee warm.

Once you figure out the resistor value you can figure out the voltage versus current line. You can then setup a voltage comparator...or you may need to boost your voltage prior to the comparator (if the voltage is too low)....of course you could just feed it into a ADC....so, many options (you mentioned classical).

4. 050816-1140 EST USA

Codeman:

The sense resistor should have a value of R = V / I . The value of V is the trigger point of the comparator. I is the peak current at which you want the switch to turn off. If V = 1 v and I = 1 amp, then R = 1 ohm. At steady state of 1 v and 1 a this is 1 watt dissipation in this sense resistor. For 7 amps at 1 v R = 1/7 = 0.143 ohms. Steady state dissipation is 7 watts.

Hopefully you use a lower threshold voltage. If you used 0.2 v for the threshold, then at 7 a the power dissipation is 1.4 watts. This would be 0.029 ohms.

To use a low threshold will require good circuit design to eliminate noise.

Note: the resistance of a tungsten filament lamp is very sensistive to voltage. For example, a 90 watt bulb I checked had 11.6 ohms at 0 v, 66 ohms at 10 v, 95 ohms at 20 v, 115 ohms at 30 v, 190 ohms at 120 v. It was only dissipating 75.8 watts at 120 v.

Because of this high positive temperature coefficient of resistance Hewlett and Packard used a small incandescent tungsten lamp in their original audio oscillator to stablize the output amplitude.

.

• Sense resistor values depend on many things, but fundamentally they need to be large enough in value to overcome other circuit influences. Wiring method, comparitor or amplifier and residual electrical noise. A .05 ohm resistor at 5 A will generate 250mv sense voltage, reasonable level for a comparitor but not as forgiving on noise. But it's more suseptable to wiring IR drop error, layout is critical and at 1A there is only 50mv generated, noise becomes a huge enemy. Even at .2 ohm layout is critical, but voltage generated is higher. Bottom line, make it as large in value as you can handle power dissipation, while still having minimum effect in relationship to coil resistance.

• Boy, if he ain't confused by now....you can also use a current sense transformer.

• Thanks for all the response guys

I think I will use a 0.05 ohm 1% resistor
0,35V @ 7A 2,45 Watt
0.05V @ 1A 0.05 Watt
Then feed a TLC372 comparator (an enhanced version of the LM393) that is very stable. I think it will handle the 50mV without problems. If not I can allways use a preamp to boost the voltage before the comparator.

Wath do you think ?

• 050816-1410 EST USA

Codeman:

See if you can get a 4 terminal resistor at your 0.05 value. Put a differential amplifier immeadiately adjacent to the resistor fed from the two voltage terminals. Everything extremely short and tight to minimize the inductive loop area at the resistor and amplifier input. Now you can easily go to a 5 v threshold detector. Keep the threshold detector close to the differential amplifier.

A simple current sense transformer will not work in a dc circuit. It may extract some portion of an ac signal superimposed on a dc signal, but that is not what you want. Generally has a bandwidth problem.

A Hall device would work, but has drift problems. It may have bandwidth problems depending on the circuitry around it. Hall devices in a null balance system reduce the drift problem, but are generally very limited in bandwidth.

A resistor still appears to be the best and least expensive approach.

A four terminal resistor is built like a four terminal shunt.

One source of four terminal resistors is Vishay type CPSL.

.

• there are a lot of current monitors out there now, depending on your app. voltage the
enhanced lt1787 can go up to60 volts and is quite linear (lol) and only requires a small current resistor, i have used this chip many times and it works real well for me.solder it to an eight pin dip socket for easy handleing.

• Diff-amplifiers are a good choice. Current monitors to. Well they are basically diff-amplifiers with some logic. I will look in to it. Good idea the 4 terminal resistors. I'm not shure about the CPSL series however... These are wire wound resistors therefore inductive. I heard that we must use non-inductive resistors fot this kind of application....

• Codeman,
You may find the .05 ohm a challenge, keep us advised of your results.

• 050816-2125 EST USA

Codeman:

Look at the Vishay site as they have a number of different 4 terminal resistors. Yes the CPSL is listed as wire wound, but classified as low inductance. Usually a low inductance wire wound is done via a looped back winding. In other words start with a long "hair pin" and wrap this around a core.

In these resistance ranges you may actually be dealing with a straight solid wire with the two voltage taps attached near the ends.

Once I had to make a shunt for a high current circuit breaker test. We used 10 car batteries in series as the voltage source. Then thru welding cable and this shunt we shorted directly across the breaker. My recollection is that we were in the range of 1000 to 10,000 amperes peak current.

The shunt was home made from a bar of resistance material about 10 inches long. A coaxial cable connected the scope to the bar. Initially I had a very large spike at T=0. This was not expected because of the inductance of the cables, and did not correlated with the circuit and T+ measured current.

The problem was in my instrumentation because of the 1 turn loop formed in connecting to the shunt. By taking the center conductor of the coax and laying it as tight as possible to the shunt bar I was able reduce the area of the 1 turn loop so that there was insignificant induced voltage in the 1 turn coil from the magnetic field of the high current in the shunt.

It would have been better to have a tube for the resistance bar and the coax center conductor run thru the center of the tube. Then the current generated magnetic field would have been totally canceled.

Even if there is a small initial spike at turn on you may be able to work with a low pass filter at the differential amplifier input, or make the differential amplifier a low pass filter. Maybe a resetable integrator would be useful.

.

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