neg voltage relay driver (for an idiot)

John, there is a single signal line that can be at -24VDC, 0V, or

+24VDC. But I have close access to +24V supply power if needed, which is how I powered the NPN relay driver circuit for +24V state.

I guess I am looking for the inverse of:

that operates when -24VDC appears at the input.

I'm a little unsure about Andrew's circuit. It apparently performs amplification for both -24 and +24 signals, using 2 NPN's and a PNP?

Chris

Excellent. Thanks a lot to the both of you.

The relays draw 6.3mA, 3.9K Ohms coil if I recall correctly. Should I use 20-50k as Rin for the circuit instead of 10k? I was going to use

2N3904 NPN's, and the equivalnet PNP I guess, so something like Hfe of 30 gives requirement of .21mA base current to turn on the NPN. So I doubled that and said around .5mA, so 48K Ohms? Or somehwere in between...

10k does work in the circuit I am using now. I suppose it doesn't matter that much, but I was thinking might as well keep the base current lower if it is possible. Not that I have any idea why.

Chris

--
I\'m confused.  Do you have a single power supply that you want to
use for the relay coils and you want to turn one relay on if your
input signal line is at +24V, the other one on if the input signal
line is at -24V neither of them if your input signal line is at zero
volts,  or do you have a single input line which can be at either
+24V, -24V or 0V and you want to use that as the supply to turn the
relays on with?

Thanks again Andrew. I'll throw a 20k in and see what happens. Now I need to go buy a PNP, I think I have the other NPN's on hand.

Last question, the "backing diode" is just the diode strapped across

+/- on the relay, right? I know, that should be obvious.

Chris

--- Yes. With +24V on the input, the bottom NPN and its relay will be turned on, but the ~ 0.7V drop across its base-to-emitter junction will reverse-bias the middle NPN, turning it off. That will cut off the flow of base current into the PNP, keeping the top relay turned off. When the input goes negative, however, that will forward bias the middle NPN, allowing current to flow out othe PNP's base-to-emitter junction, turning it on, allowing the relay to be energized. At the same time, the drop across the middle NPN's base-to-emitter junction will reverse bias the bottom NPN, turning it and its relay off. Clever circuit.

-- John Fields Professional Circuit Designer

Here's one way of doing it (view in a fixed pitch font) :

V+ | | |< .-------| | |\\ | | | | | |/ .-----| | |>

| | === | GND | | | | | ___ | |/ Vin ---|___|-------o------| |>

R | | === GND

Connect one relay (with backing diode) from the open NPN collector to +24V Connect the other relay (with backing diode) from the open PNP collector to GND R could be about 10k

As you say, it's not that critical, but I suppose 10k is a bit low.

Yes. Be sure you connect them the right way round or you'll blow the transistors.

Arggh. So I am indeed an idiot. Andrew's circuit works fine...

It's too bad I was wrong about the circuit requirement. I did the measurements a long time ago, and I didn't write my notes out correctly.

So here's the difference.

Relay "1" should turn on when the input becomes +24VDC to ground.

Relay "2" should become active when input becomes 0V == GND. In this state, the input also is -24VDC to the +24VDC source (positive meter on input, neg on +24VDC line). This is where my notes were wrong.

Both should be *off* when input is floating, very high impedance to ground. Basically unmeasurable.

Any more ideas?

Thanks again. Apologies for my bone-headedness.

Chris

Looks great John. It is sort of what I was thinking, that a comparator would be needed, and started looking them up last night. But alas, I am worse than a novice at this stuff.

The relay driver from the first circuit, which worked for one relay, required a rather large (80k) resistor on Vin. The smaller resistors were apparently dragging the power supply down too much, even with the relatively small base current.

With the 80k it worked like a charm. Are there changes to your circuit that I could implement that would reflect the smaller base driving ability of Vin?

Thanks yet again,

Chris

---

+24 | +--------+-----+----+------+-------+ | | | | |K | | | | | [DIODE] [COIL] | | | | | | [100.0k] [10.0k] | [10k] +-------+ | | | | | | +----|-\\ | C | | | >--+----B npn Vin---+-------------|+/LM393 E | | | | | GND | | | | | | +24 | [20.0k] | | | +------+-------+ | | | |K | | | | [DIODE] [COIL] | | | | | | | [10k] +-------+ | | | | +-------------|-\\ | C | | | >--+----B npn | +----|+/LM393 E [100.0k] | | | | [10.0k] | | | | | | +--------+-----+-----------+ | GND

Depending on the relays, you may want to get comparators capable of driving the relay drivers a little harder than LM393's, which are spec'ed to have a worst case drop of 0.7V with 4mA of collector current in their output transistors.

-- John Fields Professional Circuit Designer

Ok. I can calculate the resistance required I think. 24uA seems quite small, it should be ok.

Ahhh....the curiosity is killing you, eh? ;-)

It's the solo/PFL logic in a large scale recording console. I am currently in the midst of removing and replacing the master section with an aftermarket discrete summing amp and passive switching for source/montor selection.

When an individual channel is SOLOed, the PFL ENABLE (Vin) line goes to

+24VDC. This is via a transistor pull circuit in each channel. Anyways, if a channel is PFL'd (pre-fader listen, like solo with no fader or panning), the Vin becomes 500 Ohms impedance to ground. Otherwise, it is essentially floating.

The SOLO function requires that all other channels be muted. This is achieved by sending -7.5V down another line in the master ribbon. For PFL, there is a separate PFL mix buss that is swapped in place of the L/R buss.

The reason for relays is because in the PFL situation, the audio itself needs to be switched, and sealed relays are pretty much the cleanest way to do this.

I suspect the reason the SOLO/PFL logic is so wacky is because there is a SOLO/PFL switch on the stock master section that forces all SOLO's into PFL functionality. PFL is pretty lame, really, so I am removing that switch altogether. However, PFL ability is still needed in the master section because grouping channels and effects-returns do not have SOLO buttons, only PFL.

Some day I intend to change the PFL's in the effects returns and groups to SOLO as well. For this, I'd need to bypass the PFL buss send in each return and make a MUTE enable that engages the mute when it sees

-7.5V on the SOLO mute line, unless the current channel is the one being SOLOed.

That should be enough detail. It's a Soundcraft TS12, in case anyone else out there is searching for details about messing with a TS12's SOLO/MUTE/PFL logic.

Thanks again for all the help,

Chris

Now I'm confused. With a low resistor value the other circuit we discussed earlier (forgetting about the fact that the neg relay wouldn't work), 4.7k, 10k, and 22k wouldn't click over the relay. When I went up to 100k, the circuit required less current and was able to prevent the power supply from collapsing. that is my understanding.

Are you talking about using no resistor somewhere in your new circuit, or in the original one...?

Chris

--
Because of the 100k ohm resistors, all your Vin has to do is be able
to sink or source 24µA while staying away from the rails by less
than 6V.

If you have problems you could raise the value of the resistances to
1 megohm, then you\'d only have to source or sink 2.4µA.

Where\'s your Vin coming from, anyway?

--- Actually, the deal is if your Vin is capable of supplying 24µA into a 100k ohm resistor, and not dropping much below 24V, then you shouldn't need a resistor at all. It's easy enough to try if you want to, just ground one end of a 100k resistor, connect Vin to it when Vin is at 24V, measure Vin, and see what you get. I'd be willing to bet (almost ;) that it won't move much...

-- John Fields Professional Circuit Designer

The other problem is getting the LM393. I don't think I can get that locally. The only place I could think of has 0 on hand. Does radio shack stock something that will work? Mouser and Digikey both take about a week to get to me, and paying for overnight shipping seems ridiculous for an IC chip...

Thanks again,

Chris

Ok. All they had at the RS I went to was 555's. The web site said they had 339's. I, of course, saw your post after I got back, and I was in a hurry so I just grabbed the 555's because they are pretty useful for all kinds of stuff.

I know a 555 can be used as a comparator, can I swap that in instead of the comparators you spec'd? They can source a lot of current, so that could be a help. I could find a configuration out there I'm sure for a

555 comparator implementation.

Thanks again. Deciphering these circuits and knowing the application is helping me learn a lot about this stuff.

Chris

Radio Shack carries an LM339 which is a 4 comparators just like the two in an LM393, so you could spare out the two you don't need (short the inputs and the outputs to ground) and use the other two. They also have a TL082 which you could use by getting rid of the pullups and connecting the otputs to the bases of the relay drivers through 3900 ohm resistors.

--
John Fields
Professional Circuit Designer