Replacement Transistor In A Ring Of Two Circuit

[mrd]

In a ring of 2 circuit with 32V and limiting at 130mA.. I'm using a KSD1273 for the power transistor.. can I change this for a TIP120? No other changes to the circuit are needed are they?
The 15k possibly but I'm not 100% sure how to work that out.. because the darlington and KSD1273 are both similarly high gain, am I ok to leave as is? (as darlington is higher gain, it'll definitely work ok with 15k right? Even if a little overkill...)

I presume the circuit will still limit the current at the correct 130mA as that's set by the emitter resistor.
Just a little unsure as I've always used a KSD1273 but now it's discontinued.

**broken link removed**

Many thanks!

 

[ericgibbs]

hi mrd,

This is what LTSpice shows.

E

EDIT: a 5R resistor gives close to 129mA

 

[dr pepper]

The 15k resistor being the one needing changing if the circuit didnt work, you need enough current to drive the base of the tip 120 into satuaration, but not so much as to overcurrent the 4401.

 

[mrd]

Thanks Eric.
Do you think it's the darlington changing the current limit? Or the simulation incorrect?
I guess in principle I'm ok to just try it and adjust the resistors accordingly if it draws too little current...

Or is there an appropriate replacement for KSD1273 that people are now using?

 

[ericgibbs]

Thanks Eric.
Do you think it's the darlington changing the current limit? Or the simulation incorrect?
I guess in principle I'm ok to just try it and adjust the resistors accordingly if it draws too little current...

Or is there an appropriate replacement for KSD1273 that people are now using?

hi mrd,
Changing the 15K has minimal effect on the 5.5R current.

If you have a 52R or 47R resistor, connect it across the 5.5R if the current limit value is too low.

E

 

[mrd]

Great stuff. Thanks you two. I'll jump in and try then...!

 

[unclejed613]

this is a constant current source. the action of Q1 as a voltage reference is balanced against Q2's Vbe, which is 0.7V for a regular transistor, and 1.4V for a darlington. it is this difference in Vbe that is changing the operating point

 

[ericgibbs]

this is a constant current source. the action of Q1 as a voltage reference is balanced against Q2's Vbe, which is 0.7V for a regular transistor, and 1.4V for a darlington. it is this difference in Vbe that is changing the operating point

hi jed,

The presence of the 5.5R emitter resistor sets the current, the 0.7Vbe versus 1.4Vbe has negligible effect upon the emitter current.

Eric

 

[alec_t]

The choice of transistor for Q1 is key, since its Vbe has a significant effect on the current limit.

 

[ericgibbs]

The choice of transistor for Q1 is key, since its Vbe has a significant effect on the current limit.

As I understand it the OP is still using the 2N4401, its the new TIP120 that was being queried.

At the very low collector current in the 2N4401, I would say the transistor would start conducting around 0.65Vbe, so with a 5.5R, that would be approx 118mA, [which LTS shows] not the 130mA the OP was expecting.

Which would mean a Vbe switch on of 0.715V which is far too high.

 

[Roff]

The 15k resistor being the one needing changing if the circuit didnt work, you need enough current to drive the base of the tip 120 into satuaration, but not so much as to overcurrent the 4401.

Under normal operating conditions, the TIP120 will not saturate.

 

[dr pepper]

If the circuit is limiting yes I understand, however if theres no load on the circuit wont the 120 saturate?

 

[ericgibbs]

If the circuit is limiting yes I understand, however if theres no load on the circuit wont the 120 saturate?

hi,
With no load path to 0V for the required base biassing current, the TIP will not be conducting, it will be off.

E

 

[dr pepper]

I was looking at this as though it was a series pass transistor within a power supply with current limit transistor and sense resistor, probably not the right approach.

 

[MikeMl]

It is a pretty good current sink for a wide range of voltages, but its temperature stability sucks! It makes a pretty linear thermometer

Here is a plot of sink current vs voltage (X-axis) at temperatures from -55degC (lt. green) to 85degC (dk. blue).

 

[dr pepper]

Doesnt this circuit also create a fair amount of noise.