Short circuit protection problems

[Exo]

Hello again ppl,

The circuit below is a motor driver using a H-bridge. Current is measured using resistor R1 and divided by R2, R3 and R7. This way we can use R3 to set the working point of transistor Q1 roughly between 0.9A and 5.3A.

When Q1 is triggered C13 will provide a small delay (to filter out little crap) and then the set reset (made up by IC4A and IC4B) is reset causing the H-bridge to shutdown by taking the enable signal away.

This all works great using a resistor as a load, but when trying with an actual motor causes problems.

I was testing with a very small unloaded motor wich only draws 35mA when running and i need to crank R3 all the way up to 4A or the motor isn't allowed to start by the SC protection.

Now i realize that a motor will need a larger startup current, but a 35mA motor requiring a 4A startup current is something i don't believe (hard to measure).

So, Any of u guys know what might be causing the protection to switch off so fast. To my belief the protection should never act when using such a small motor cause i dont believe it actually uses that much current for starting, more something like 250mA.

But like said, with a resistive load it works :?

 

[Nigel Goodwin]

Try measuring the DC resistance of your motor with a multi-meter, before it starts turning the resistance is likely to be very low - it may well take a 4A surge when you apply power!. If the motor reads 1 ohm, and the motor supply is 12V, ohms law tells you the current will be 12A - you might also try measuring the stall current?, connect a ammeter in series with the motor (set to a high amps range, 10A or 20A), then stall the motor with your fingers - read the current off, this will be similar to the start current pulse.

You really need to apply a fairly long start delay, before you start monitoring the current!.

How big is the motor?, normal little DC motors (as used in small models) usually have a higher stall/start current than dry batteries can provide! - with NICAD or lead acid, it's a different story!.

 

[Exo]

A

It's a motor with a little reductor built onto it so stopping it, with anything, is thoug. And the reductor cannot be taken off (its a single piece)

Its has 40 ohms when measured directly, and the supply is 24V.
(0.6A ?)

fysical size: It's 5 cm long and diameter is 3.5cm, not including the reductor.

It uses 35mA , so a startup current of 0.6A would be realistic. But 4A is more then 100 times as much ...

Update: I tried programming the pic to make a softstart. The motor is started with a PWM signal with a low duty cycle, and then slowly increasing the duty cycle (10% to full on in 1.5 sec). This way it works, even on the lowest setting (0.9A)...