• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Has anybody got an off delay timer circuit I could use?

Status
Not open for further replies.

bytraper

New Member
I wondered if anyone had any circuits that I could add on to this circuit for my project!

I've designed a current limiter for my speed control using a LM358. The output stage is shown below, and it works fine and triggers like it should.

The problem however, is that when its triggered, its only triggered momentarily. So the motor turns off when its over current then straight back on, then straight back off etc.

The output is 5V which at the moment is connected to the Shutdown pin of the driver IC.

Can anyone think of any way using discrete components that I could add an adjustable time turn off delay in this? I would like 5-10 seconds if possible?

The circuit is run from 12V, and the output from the circuit is 5v with about 14ma which is enough to trigger the shutdown.

View attachment 60567
 

alec_t

Well-Known Member
Most Helpful Member
add an adjustable time turn off delay
Do you mean a delay before turning off the motor, or a delay before re-starting the motor after the turn-off? Presumably the latter if you want to protect the motor?
 

bytraper

New Member
I just simply need to delay the signal to the Shut-down logic on the motor driver chip.

The motor driver only disables the mosfet outputs while the disable pin is held high (+5v logic).

So, what is happening now is that every time the comparator triggers an over current situation, the comparator triggers which sends a high signal to the driver chip (+5v) which shuts down the mosfets, but almost instantly once the mosfets are shut down, the over current situation is gone, and so the comparator see's the current load gone and drops the signal low again and the mosfets fire up. This basically repeats. But its so quick that its kind of like a jack hammer.

Over current
Shut down
Under current
start
repeat

That is what's happening at the moment.

So I am looking for a circuit that I can add, which once the comparator triggers (+5v 14ma), the timer circuit holds it at its +5V output for an adjustable amount of time (between 1 sec and 5-10sec) so the driver stays off rather than switching straight back on when the comparator senses the over current is gone!

I hope this is clearer, thanks
 
Last edited:

bytraper

New Member
All good now, thanks for the suggestions. I ended up building a analogue circuit out of transistors to give me the delay. Now I have between about 0.5s and 20 seconds.
 
Last edited:
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top