yes, requirements are the same.....Why do you say that doesn't work?

What SGT said...and it still doesn't have any way to differentiate between a blown fuse and an empty fuse holder.

Ken

 

Thanks for all your help Sgt.
Can I ask what program you did that schematic/simulation in?

 

Thanks for all your help Sgt.

You're welcome.

Can I ask what program you did that schematic/simulation in?

Circuitmaker Student. It is an old program that is no longer supported by it's authors. It is limited to 50 components in a simulation, and you cannot add components to the library.

I simply use it for "quick and dirty" simulations here, as the library has a good number of "hobbyist"-type components, and the interface is a bit more intuitive than Linear Technology's LTSpice. LTSpice, however, is far more powerful, is fully supported by Linear Technology, and there are user's groups where you can obtain even more components to add to the library fairly easily.

 

...and it still doesn't have any way to differentiate between a blown fuse and an empty fuse holder.

Yeah, Ken - but it would be tough to determine an empty fuse holder vs blown fuse unless the fuse holder had some kind of switch.

In my opinion, just having an indication that there was no connection through the fuse holder would be adequate; one way or the other, the load won't get powered until someone replaces/inserts a fuse.

 

Yeah, Ken - but it would be tough to determine an empty fuse holder vs blown fuse unless the fuse holder had some kind of switch.

In my opinion, just having an indication that there was no connection through the fuse holder would be adequate; one way or the other, the load won't get powered until someone replaces/inserts a fuse.

I agree, but the OP said their spec's haven't changed, and as you say (and I said early on) you would need a special holder. Otherwise, your original bridge/LED circuit would work. Maybe they can tell us why an empty fuse holder is to be ignored.

Ken

 

OK, now that I've gone back and looked at our OP's requirement statement yet again...

...monitor the state of up to 2 fuses and energize a relay if either one of the fuses blows but not energize the relay if only one fuse is installed in the fuse holder and the other position is not filled (unused)...

I realize that all I'm doing is indicating the blown fuse by turning on an LED; there are no provisions to energize a relay.
That's what I get for doing this stuff when I'm not feeling very well.

 

I must have stated something wrong along the way or my requirements were misunderstood with regards to blown/empty.
I just didn't want the LED to turn on when no fuse was installed AND NO load was present. My project might have up to 3 fuse positions and only 2 may be used so I didn't want to trigger and LED if NO load and NO fuse was installed.

If a load is attached then your right it's imposible to tell if the fuse is blown or missing. But the loads won't be attached at all to those unused positions.

Again thanks for all your help guys.. Time to crack open the "Art of Electronics" book and start learning... I've been told is a great book to get into learning electronics.

 

Funny you say my schematic with the PnP/NpN transistors doesn't work as I just breadboarded it and it works just fine. "No transistors were hurt in the making of this schematic."
Attached is the exact circuit I breadboarded.

 

If I measure voltage on each transistor with a blown/missing fuse (one lead on base and one lead on emitter the voltage is approx .7vdc) What am I missing?

 

If I measure voltage on each transistor with a blown/missing fuse (one lead on base and one lead on emitter the voltage is approx .7vdc) What am I missing?

(1) Your LED polarity is wrong.
(2) Q1s (PNP) polarity is wrong.

Your schematic is to large, which makes it difficult to read.
The black background would be more tolerable if the white traces were wider and brighter. To read your schematic, I had to resize it in a photo editor... a bit of a pita.

 

mcgyvr,

I now disagree with SGT. about the reverse Vbe problem. What you saw was true. Since the PNP and NPN B-E junctions are connected in anti-parallel there will only be the 0.7v reverse across the non-conducting junction.

I redrew your schematic to make is more clear...for me. If you got your circuit to work on a breadboard...with both polarities...your schematic is mis-drawn.

The top one is your circuit.

The second one is what you were trying to do with supply-positive. Though the biasing on Q2 is correct and the polarity for the LED is correct, D4 is reverse biased and will not conduct.

The third one is what you were trying to do with supply-negative. Though the biasing on Q1 is correct, D4 and the LED are both reverse biased and will not conduct.

Now, if you reverse the polarity of D3 and D4, and replace your LED with your original bi-color LED, it will work.

That said, SGT's bridge/resistor/LED seems more straight forward...but, that's just my opinion.

I'm still hung up on the no-fuse/blown-fuse requirement.

And, I'm very curious about what your whole system is: You may have up to three "loads" that are polarity sensitive, and the fuse detector circuits must work on either polarity because the power supply can be connected either way.

Need more coffee!

Ken

And why do you have two resistors, R1 and R2, in parallel?

 

I agree with Ken; the original schematic was drawn in a way that was very confusing.

I started to re-draw it, but I've been under the weather for a few days, and am not "firing on all cylinders" so to speak. Ken's re-draw is most helpful.

Ken,
Each transistor should have it's own base resistor. 80k per transistor is very likely too high, particularly if a relay is to be added.
It would also be a good idea to have a diode across each transistors' be junction to take care of the Vbeo breakdown voltage problem.
On the NPN, cathode should be connected to the base.
On the PNP, cathode should be connected to the emitter.
Our OP still wants a relay. That should be connected in parallel with the LEDs and the 2.4k resistor, and will need a pair of Zener diodes back-to-back in series across it to absorb the reverse EMF pulse when current is discontinued. Otherwise, the transistors may be destroyed.

 

I'm back.. Thanks again to Sgt and Ken for their help so far..hoping for just a little more here... I'm just wondering if this schematic will work and if not can you please explain why it doesn't (I'm away from my breadboarding supplies to fumble through testing on my own) I believe it should work just fine to light that LED when the fuse blows with a supply of 24 to 48 vdc and either polarity.
Rememer total novice here...
Eventually I will expand this to include having multiple fuses on the same supply and a single LED for each fuse and all tied to 1 single common relay that is energized when any of the fuses blows.. But I'm moving in little steps as I am learning. Thanks again.

 

I haven't traced out the circuit to see if it works...but...by sticking the bridge in there you might as well eliminate R2/Q1/Q2/D3/D4 and you would be back at SGT.'s circuit in post #5. Simple and effective. And if you add an optocoupler in series with each LED the output transistors can be paralleled in a transistor "OR" circuit to drive a relay if any fuse, with a load, is blown.

Ken

 

ohh the learning I have to do... I assume you just picked 5V as its a common relay supply voltage. I already have a 24 to 48 V relay I commonly use in other circuits..
I kept looking at SGT's post #5 circuit and couldn't figure out how to get it to work with multiple fuses to a single relay.. Everytime I tried all LED's would light up when only 1 fuse failed.. Seems like the missing link was the optocouplers.

Is the upper resistor by that last transistor (base to 5V) absolutely needed. Or just it just for stability and to ensure the transistor stays OFF when it should.. I'm learning.

I do like the optocoupler schematic you just posted.. less components.

Now if I wanted to add 1 more fuse type to this circuit that was an indicating fuse type where a mechanical arm pops out (attached to source voltage) when the fuse blows would I just simply attach that to another optocoupler LED or can I simply tie directly to the relay.

and thanks again

 

Hey guys, lets us not lose sight of what a fuse is for. Some of these concepts should employ a separate mA fuse of their own! If anyone is not sure of what I'm saying, just look through some of the schematics. One of them has nothing but semiconductors between the source and gnd.

The reason the Neon lamp method (for higher voltages) was so accepted was because R was very high and Neons drew microamps. The likelihood of either of these components becoming a short was very unlikely.

 

mcgyvr,

Yes, the 5V was just for convenience in the schematic.
Yes, the emitter-base resistor is to assure turnoff. For 10 cents and one very small component why leave it out.
If the pop-out fuse behaves like other fuses just add another LED/bridge/resistor/photocoupler circuit. Do you have a link to one?

CDRIVE has a very good point. The fuses are there to open a circuit "completely" under fault conditions. The question is: what's the risk if a small current...maybe 5mA...continues into the faulty load.

Ken

 

CDRIVE, the small current from the alarm circuit doesn't cause a problem to the protected equipment. I expect to get calls about it but only because they measure voltage at the output of the fuse holders without the fuses in place but I'll explain that as a " current limited sensing voltage for the alarm circuit since the fuses are non-indicating"

Ken,
Here is a link to an indicating fuse type. http://www.cooperbussmann.com/pdf/14c23c39-0c37-4fb1-9709-7854f709d165.pdf
Difference with this is it has its own visual indication (colored tip pops out from the holder) so I don't need the LED. plus I could have up to 20 of these fuses in my device so I don't want a bridge/resistor,opto, etc.. for each one. These fuses simply blow and the metal front of the fuse touches a terminal in the holder and I typically just gang the alarm terminals of all the indicating fuses together and run it right to a relay coil with a ground connection to the other side. Then use 1 set of the relay contacts to illuminate a single LED for the whole gang of fuses and the other for remote dry contact indication.

I figured thought that with your opto schematic I would just use a single bridge/resistor/opto-iso just like the non-indicating section for the whole gang of fuses and ditch the LED and add it into the transistor OR logic.

Thanks again for all your help guys. This really got me started learning about transistors/opto's. Great forum here. I know enough to do some damage now. mmuuhahahahahah. My life was much easier when I had an EE sitting next to me.

 

mcgyvr,


CDRIVE has a very good point. The fuses are there to open a circuit "completely" under fault conditions. The question is: what's the risk if a small current...maybe 5mA...continues into the faulty load.

Ken

This was not exactly my point. Some of the schematics had at least a resistor to limit current but at least one had no such limitation if the sensor circuit itself failed. From a safety point of view, the sensor circuit should have its own fuse. This is to insure that excessive current will not find a path through the defective device, or the sensor components direct to Gnd.

 

Ken, Maybe I'm missing something. Is there supposed to be a connection to a -GND on your transistor OR section of the schematic? I see the connection to +5vdc but nothing to a ground?