Electronic Circuits and Projects Forum

Ok,
So the LM339 sees the reference voltage at the negative input (of each comparator) and as long as the voltage at + is lower it will keep the output pinned to ground (or low...)

So what I would need to work out is how to make sure that when over-current occurs the sense pin will raise the voltage to above a certain level... Should I slap a multimeter on there and short the circuit out briefly to measure what comes through?

I've made up a design to incorporate these two comparators and it sorta makes sense to me but the resistors required are beyond me.

I would imagine the initial feed from the 555 circuit needs to be lowered from +5v since that's the comparative voltage and I don't know if the voltage out of the current sense pin will exceed that?

Also, the 0.1uf cap will provide the timing... 100ms = 0.1uf? I'd nearly like the timeout to be 1000ms since times to find faults would exceed this easily and I'm happy for power to be cut off for longer periods.

Another nice advantage would be an LED when the voltage is cut... is there a chance to use an inverter between the two comparators (as the 7404 has been doing, although it's all used up) to slap an LED in (as seen in the 555 circuit) to show the overload status?

Thanks again for your help.

Hi again,


Do you know how to calculate resistor dividers to get a lower reference
voltage from a higher voltage like 5v?

The idea is to make the first comparator section sense the voltage across
the 0.47 ohm resistor so you need one divider set to that voltage, and
that gets compared to the sense voltage (voltage across 0.47 ohms resistor).
That discharges a cap and the next comparator senses that and disables
the L298. No 555's required.

The charge time of a resistor and capacitor is equal to the time constant
(sometimes simply called TC) and that is the time it takes the cap to charge
to 63 percent of the supply voltage. If the supply voltage is 5v, then
63 percent of that is 3.15v, so we would make one divider that produces
a voltage of 3.15v and apply that to one comparator input. One time
constant is easy to calculate too so we can figure the cap value and its
resistor: 1 TC=C*R, with C in farads and R in ohms. It takes t seconds
for the cap to charge to 3.15v then t=1TC=C*R. With a 1uf cap and
100k resistor this works out to 0.1 seconds (100ms). If you want 1 second
then a good idea would be to go with a 10uf cap and 100k, or 5uf and 200k.
Since the cap is a bit large it would also be a good idea to beef up the
output of the LM339 with a single NPN transistor. This will speed up the
response to an overcurrent.

To test the circuit for overcurrent to see what voltage you get across
your resistor, it would be a good idea to know what the current is
normally. In other words, what is the current when everything is
working normally? You would want to measure the voltage across the
sense resistor when the current is just above this level or when
whatever you are powering (motor, etc.) is running normally. You
dont want it to trip when it is running normally, but you do when
something goes wrong. Depending on the current you may also have
to switch the 0.47 ohm to something else, like 0.1 or whatever.
An adjustment would be nice too, so the current trip point could
be adjusted during some tests.


Yes adding an LED wont be hard to do.


Perhaps i should make a schematic and post it here for you to take a look
at? I need to know what the normal current draw is for whatever you are
powering first.

Originally Posted by MrAl

Do you know how to calculate resistor dividers to get a lower reference voltage from a higher voltage like 5v?

I have never really done this form of electronics math but do want to learn... am a software engineer by trade and haven't delved into hardware too much.

Originally Posted by MrAl

No 555's required.

The 555 I refer to is th e cut-off circuit that already exists on the lower half of the diagram to force the enable pin high if there is no incoming data.

The enable pin (when pulled low) does disable the output. I tested this by disconnecting the input to it and grounding it intermittently.

Originally Posted by MrAl

I need to know what the normal current draw is for whatever you are powering first.

Currently the system takes around 500mA per train on the track (but this varies depending on brand/headlights/etc...)

I'd be happy to limit the booster at 4Amp.

My multimeter currently can only show up to 200mA and when two locos are sitting still it's at 154mA and as soon as I set a throttle or headlight on either the multimeter just shows "1" since it can't go any higher.

I'm about to slap the multimeter on the 'current sense' pin and see what happens when I short the tracks.

Voltage on Pin 1 rises to 0.91v when the tracks are shorted
...this would mean that I need to lower the input voltage into the first LM339 + to anything between 0.1 and 0.5 (as the voltage on pin 1 climbs gradually, although quickly, to 0.91) ??



Thanks heaps for your help on this.

Here's the latest.... cutting the circuit at the line marked 'cut' will stop the enable pin from working... i.e. the whole trip circuit just stays tripped for some unknown reason... it could also be that i'm not bringing the enable pin high enough due to my use of incorrectly valued resistors... since it turns out that my 0.47 resistors are actually 0.27R/5W... I tried adding in another 0.27 in parallel to the 2x0.27 (i.e. 0.52 in parallel with 0.27) as that's what i'm cutting off when i remove the feedback circuit but that didn't help either.

I need to source some 0.47 resistors from somewhere

On the up-side... everything still works great when both the feedback circuit and trip circuit are connected... I get the ~1.1s timeout and then it all comes back to life... no arching (well, buzzing) from the shorted item when across the tracks anymore.

Hi again,


You see that LM339 pin 2 that is connected to the 555?
That section needs to have pin 5 and 4 swapped.
That is, connect the sense resistor to pin 4 and the
resistor divider to pin 5. Test with that circuit alone
as you did before, that's a good idea.
You should see your train or whatever run normally until
there is an overcurrent, and then it will appear to shut
off. If it shuts off when the train starts to move,
that means the current sense is too sensitive. You
might try paralleling some 0.47 ohm resistors (or whatever
you have on hand) to beef up the current trip point.
This also keeps power dissipation in the sense resistor
low too, but if that's not a problem (sense resistor does
not heat up) you can also adjust the resistor divider a bit.
Before you do that however try to get this working with
one train alone.

BTW, with that 10uf cap instead of 1uf you might get
more like 10 seconds between current 'tries', if that's
ok then dont worry about it. Note also that you may
not see this happen, because the LM339 is pretty fast,
so it would turn off fast, then turn on for possibly only
10us, then turn off again. You might not see this happen
but it will be working. Once the short is removed,
up to 10 seconds later the train will start moving again.
It may take the full 10 seconds to start again due to the
10uf cap as noted above.

If you want an LED to show when the current is being
cutback (turned off) you can connect an LED to a
2.2k resistor to +5v (anode toward +5v) and connect
that resistor to the output of another inverter, then
connect the input of that new inverter to the output
of the 555. When the current shuts off, the LED will
light. Note however that when there is a short it
may appear to stay lit and not go out, even after 10
seconds, because the pulse will be very short. After
the short is removed the LED should go out after
a max of about 10 seconds and the train should start
to move again.

There is also the possibility that when the train starts up
the surge current trips the current sense. We'll have
to deal with that too if that happens.

What power rating are your sense resistors?