need a circuit that will limit 2.5a flowing to the probe pins

[jdupz]

The circuit is intended to limit the current to 2.5a per probe pins to avoid burn. I have a design using optoisolator to drivemy power mosfet. The problem with thiis design is that it will fail if installed at vcc pin.however, no issue at out pin. Fyi,the circuit is to be installed at probecard between pins to test a wafer. please shed some light.thanks in advance guys.

 

[BradtheRad]

It makes a difference to mosfets and transistors where you place the load. It can be put on the emitter leg or the collector leg.

Then how well the configuration works depends on whether it's an N-type device (bias referenced to the more negative leg), or a P-type device (bias referenced to more positive leg).

 

[jdupz]

Thanks Brad...actually i used pchannel mosfet and load current was measured at drain. Input voltage is at source so that when supply increases the vgs will remain constant. I can limit the current in this connection but as i have mentioned cannot pass if placed at vcc pin. The rdson for mosfet is around 0.065 ohm but when connected to probepin it csn measure around 1 0hm. What is needed is actually at least 0.3 ohm or better. I donot know why i get 1 ohm is maybe bcoz of loading effect? Can suggest other method?

---------- Post added at 23:04 ---------- Previous post was at 22:28 ----------

I have another ckt to try but im not sure it will work. Im planning to use 0.1ohm resistor to sense the 2.5a then shut off current or shunt once 2.5a is detected then convert this to voltage and in this case is 2.5ax0.1=0.25v then feed this to buffer amplifier. I have use buffer coz of high input impedance to guarantee to effect on ckt if current is less than 2.5a. Output of buffer is then feed to comparator wherein ref voltage is 0.25v. When output is 0.25v(2.5a was detected) output of comparator is 0v then i will feed the output of comparator to inverter then to optoisolator to shunt the current to gnd. The ckt should work on positive and negative supply and should work on high current/high voltage as high as 60v. The input voltage/current varies,so what is needed the ckt should be dynamic. Im not sure it will work given buffer typically accept less than 15v as input and the comparator im stuck. How can i implement the negative supply/current detection? Do i need to have separate detection for negative and positive. Less components the better. Thanks in advance again.

 

[dick_freebird]

Does your SMU not have a built in current limit / compliance
that you can set?

 

[BradtheRad]

Thanks Brad...actually i used pchannel mosfet and load current was measured at drain.

Drain implies the positive leg of the mosfet. If you inserted an ammeter here then that changes the sensitivity of a p-mosfet. It could account for your obtaining a high (1 ohm) 'on' resistance (per next statement):

Input voltage is at source so that when supply increases the vgs will remain constant. I can limit the current in this connection but as i have mentioned cannot pass if placed at vcc pin. The rdson for mosfet is around 0.065 ohm but when connected to probepin it csn measure around 1 0hm. What is needed is actually at least 0.3 ohm or better. I donot know why i get 1 ohm is maybe bcoz of loading effect? Can suggest other method?

I have another ckt to try but im not sure it will work. Im planning to use 0.1ohm resistor to sense the 2.5a then shut off current or shunt once 2.5a is detected then convert this to voltage and in this case is 2.5ax0.1=0.25v then feed this to buffer amplifier. I have use buffer coz of high input impedance to guarantee to effect on ckt if current is less than 2.5a. Output of buffer is then feed to comparator wherein ref voltage is 0.25v. When output is 0.25v(2.5a was detected) output of comparator is 0v then i will feed the output of comparator to inverter then to optoisolator to shunt the current to gnd. The ckt should work on positive and negative supply and should work on high current/high voltage as high as 60v. The input voltage/current varies,so what is needed the ckt should be dynamic. Im not sure it will work given buffer typically accept less than 15v as input and the comparator im stuck. How can i implement the negative supply/current detection? Do i need to have separate detection for negative and positive. Less components the better. Thanks in advance again.

If I have the right impression of what you're doing then it might require you try a few methods to see which is most feasible, to operate your current detector:
(a) as 2 mosfets in something like class AB mode, or
(b) as 1 mosfet in class A, or
(c) use 1 op amp in detector stage, or
(d) use 2 op amps in bridge configuration, or
(e) other method.