current limiting circuit

[spirosd]

I am trying to set up a current limiting circuit in my design and have set-up the following using an TIP 127 and an MPSA 56 (gotten this particular circuit on the web).

I am using two 9V batteries in parallel (line on the left of the diagram), as I am powering a step motor (center taps, line on the right of the diagram). When I place any load on the circuit, it is meant to act as a chopper circuit and limit the current going to motors (by switching off the supply).

In practice, it completely eliminates any power to the motor (checking voltage with a volt meter), when any load is applied. Does anyone have any insights as to what I am doing wrong?

Thanks for your help

 

[Exo]

It's not a good circuit for a motor. A motor requires a large startup current wich will trigger the circuit, preventing the motor from starting.
There should be a delay allowing the motor to start first.

 

[spirosd]

The 9V batteries are providing about 5.5 Amps, and the motor is rated to 2.4 Amps (I am limiting it to 2 Amps with this circuit).

Would that be sufficient current to start the motor?

 

[Oznog]

The 9V batteries are providing about 5.5 Amps, and the motor is rated to 2.4 Amps (I am limiting it to 2 Amps with this circuit).

Would that be sufficient current to start the motor?

Well, if you're talking about 2x 9V alkaline batteries, you're a bit ambitious! Those have very tiny cells inside, they are only good for a few hundred mA. Putting batteries in parallel is also a bit unwise. The voltages never match exactly, particularly if they're not at the same temp, and it causes one to slowly discharge into the other one. Similarly they don't discharge totally equally under use. The one getting discharged more gets warmer and generates a higher voltage, further tipping the discharge in favor of that cell.

 

[Sebi]

the motor is rated to 2.4 Amps (I am limiting it to 2 Amps with this circuit).

quote]
If the motor need 2.4A, why do You think the 2A current limit is enough?
Anyway i agree with Exo and Oznog.

 

[spirosd]

I tried using a computer power supply (12V - 3Amps) and nothing!

I am beginning to believe that this is another wiring blunder on my part. I have checked the wiring, the datasheets over and over again.

Checked the voltage without any load, and it works as advertised. When I try to step the motor (place a load on the circuit) the votage to the motor drops to practically 0.

This is the circuit that I am trying to implement https://www.bbastrodesigns.com/currentlimit.html .

Thanks for your help.

 

[Sebi]

For proper work, the PCPSU always need minimum load on the 5V output. Put a dummy-load (e.g. a bulb)about 1...2A to 5V. Try change the sensing resistor to 0.1ohm.

 

[audioguru]

Hi Spiro,
Are you actually using little "transistor radio" 9V batteries? With 2 in parallel, 200mA is a very heavy load on them. You need 6 "D" cells in series to supply 2A.
I think that the 2A of limited current is killing the voltage from the batteries. Try adding a huge (10,000uF) capacitor across them to supply starting current.
If the motor is rated for running on 6.0V, its resistance is 2.5 ohms for it to draw 2.4A when stalled. When the current-limiter operates at about 2A, the voltage across the motor should be about 5.0V.
The current-limiter is not a switch, but is a linear circuit that continuously adjusts itself to keep the load current constant. Its output voltage will be zero only if the load is a dead short, or if the battery voltage sags 'way down.

 

[mechie]

Some checks to try

spirosd
Can you clarify a few things about your circuit ...
The current limiting circuit feeds the stepper common rail?
... you therefore have switching for each phase on the negative side (as per my re-drawn circuit?)

Can you power up the circuit with the load attached so the current limiter seems to be operating and then measure the base-to-emitter voltage on each transistor in turn.
If the current limiter is operating you should see 0.7v on the left-most transistor and maybe less than 1v on the Darlington transistor (TIP127).
disconnect the load and you should see nothing (0v) on the left-most transistor and something like 1.4v on the Darlington.
The above should indicate (not guarantee) that the current limiter is working - if so maybe the 0.3 Ohm current sensing resistor is faulty - you should see about 1v across it when the load causes the circuit to limit.
Try a smaller resistor (or briefly short it out :? ) if this seems to be the problem.

Failing the above - we would need more info on the phase-switching circuit - could this be open-circuit ?