Looking for ideas on electronically switching resistors. 2

[itsmoked]

I've got a regulator that controls to a specific operating point via current thru a resistor.

I want to be able to select between a couple of resistors to set, say, three operating points. My problem is that the resistors will have between 300mA and 1500mA running thru them and are somewhere around 0.1 ohms to 0.5ohms.

Digital pots are out cuz they don't do amps.

So I thought of using MOSFETs switching in resistors or perhaps shunting them. There are lots out there with sub 0.0x ohms ON resistance. But I'm thinking they'll be a problem since they all have fairly large thermal drifts. Just the thing to have around a thermally active regulator.. All their charts also seem to peter-out down around 1A so I'm not sure what happens when I'm only pushing 300mA thru them. The controller point looks to keep the top of the resistor at about 300mV which again isn't much of a source/drain voltage. Again that's off the charts.

Got any suggestions?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Hi IR: I'm still struggling with the vagaries of transistors. The sense resistors are 0.5 ohms down to about 0.1 ohms. While the 1 milliohm might work the 2 bucks sure won't when multiplied by 15! That's adding $30 to the product cost. The cheaper ones you found end up having about a 10% contribution of error. If I could mess around with the resistor values to cancel them out we still have the temp coefficient rearing it's head. That will bring a 25% change in the Rds over this thing's operating temp.

Skogs.. I've used one of those 545As! CLICK CLICK CLICK. I understand your need.

So "C" is where you're going here? The switches are again outside of the sensing scheme.. I get it. I think it would probably work as long as the incurred delays don't destabilize the whole control loop.

But. If I'm bringing in op-amps maybe I should just dump the switches and use a $0.30 digital pot to pick up 64 different settings.. "D" The digital pots come in at less than transistors of the type contemplated...

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

OK, so you could use one resistor for the whole current range?

Then I don't see why you should work on the measurement (actual value) side at all. It is a lot more convenient and reliable to vary the reference value aka setpoint. Then you don't have to worry about differing dynamics and stability at different settings.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[IRstuff]

Since you only have a single pot in "D", the summing amp is not necessary, and you could use a single non-inverting op amp and cut the op amp cost in half. In fact, have two op amps may work against you, since the op amp offset will show up as an error in the current sensing. Something like

http://www.ti.com/product/op07c

might be a good place to start.

As for temperature effects, even the sensing resistor itself would be problematic, as would the offset voltages and currents of the amplifiers. In some respect, the transistor switch may actually be better, since the transistor will be in hard saturation, and the channel resistance is even in the picture. You'll be dominated by the diffusion and metallization resistances, which would be similar for any possible alternative. The thermal compensation is something that would have to be built into the controller itself. Or, you might need to build something like a Wheatstone bridge to eliminate the sensing resistor changing vs. temperature.

TTFN
faq731-376

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[itsmoked]

You're right IR. A single SOT23 should do the trick nicely. I've found fast ones for 50 cents. Some even come with digital gain adjust for a few shekels more.

Great. Thanks guys. I think I've got it now.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

No worries...

Beats yapping about lawyers, guns, and money ;-)

TTFN
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