defcon31

hello

i'm searching for a component/circuit that does the following:

i have a motor (DC, 24V) and the current to it may never be larger than 150mA. if the motor has something to pull that's too heavy, than the motor asks much much more current. now, how can i limit the current to the motor to 150mA??? the motor has always to remain pulling the weight (motor may not stop), but may not exceed 150mA (so i cannot use a fuse, because i always have to change the broken fuse).

i found some components like on
http://www.sprinters.zen.co.uk/wilot/project1.htm
but this is also not the good solution since in that case you have to turn power off to reset it all.... i need another solution.

any suggestions???

Oznog

There's polymer breakers which don't actually break the circuit, they just increase resistance. There are also self-resetting circuit breakers.

There are also schemes you can use with semiconductors. Op amps, transistors, and voltage regulators can be put in arrangements which limit current.

The breaker will stop the motor- but it's also very possible that if you overload the motor, it will stall even though 150 ma is applied. In general, speed is proportional to voltage, and torque is proportional to current. The max pulling capacity has a straight relationship to torque.

Klaus

You don't tell us exactly what the motor is pulling. Does it *have* to be an electronic current limiting solution?
You could fit a friction clutch that slips when the load is exceeded, the motor would never stall nor stop then. You can find an example of a friction clutch in powered screwdrivers.
Klaus

Roff

Like the others, I'm not sure an electronic solution is the best. Your motor will still stall if overloaded.
If you want to try an electronic limiter, I designed the one below. It has been simulated, but not tested. The 741 op amp is not my first choice, but it is available and will probably work. The MOSFET needs to have less than 0.1 ohms Rds(on). You should put a small heat sink on it - enough to handle 4 watts in the case of a direct short across the motor.

Attached Images

150ma_current_limiter.gif (10.4 KB, 873 views)

Nigel Goodwin

Your requirements don't make any sense, limiting the current to 150mA will stop the motor if the load increases, and it's quite possible that the motor won't even start under load.

What are you trying to do, and why do you have the 150mA requirement?.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

chemelec

An LM317 can be easily used to limit current to 150 mA, But there is no way to guarantee the motor will not stop. All it will do is supply a Maxium 150 mA.

Gary

defcon31

[quote="Nigel Goodwin"]the reason that i'm searching for this is that our customers often block the motor with their hands (without knowing that it causes extremely damage to the motor and the chip that has to feed to motor). we solved it with a fuse, but then they complain that they always have to open the machine to replace the fuse. so a current limiting circuit (or chip) of 150mA should protect the motor and the udn2981 chip that has to source to motor with 24V.

as some people told, with the lm317 it cannot be guaranteed that the motor can start or goes slower: this is not a problem... the only thing i want to protect everything.

so tomorrow i'm gonna study about the lm317-chip what i can do with it!
thanks in advance and if someone has better ideas (or remarks) for me as an electronics-newbee, always welcome!!!!

Roff

Sorry - brain fart.

defcon31

????

Roff

I guess that was somewhat cryptic. I posted what I thought was a solution to my understanding of your problem (startup current) then had second thoughts.

Question for the forum: What is the problem with a current limiter? Is it startup current? Otherwise I don't see the problem, so long as you are certain that the motor will never require more than 150ma under normal load.
The limiter I posted has a an output impedance of about 1 ohm at less than the limiting current, so you will lose less than 150mv under normal loading. This impedance could easily be made even lower, perhaps as low as 100 - 200 milliohms, with some simple changes.

Nigel Goodwin

The original question asked for 150mA current limit, with the motor never stalling! - which is obviously impossible.

As you say, the main problem is startup current, particularly under load, a delayed current limit would be more useful - say 0.5 seconds before it kicks in.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

Gaston

could you use a relay to drive the motor therefore isolating the motor from the circuit?

Roff

The idea I had, but rejected, was a mod to the circuit I posted which would delay the limiting when power was turned on. The problem I had was what to do if the motor was stalled, and then was released. Power-on delayed limiting would not handle this situation.
Sounds like a job for a PIC.