Getting appropriate voltage for relays

[JMan]

I have a bipolar psu circuit delivering +/-28vdc using standard LM317/LM337 regulators. This powers the circuit of a compressor, but I also need to (separately) send +24vdc to a couple of relays used for bypass. The relays do not necessarily need to be on the power rails of the audio circuit.

I tried sending the full +28 to the relays but they didn't seem to like that (although supposedly they are rated to handle it) and weren't switching when supplied with this voltage. So now I want to find a good way to get the appropriate +24v. I can only think of two options, and I want some input on if there are pros/cons to these or if perhaps there are better solutions that I'm not thinking of.

1. Just stick a decently rated resistor between the +28 from the psu and the relay. This seems like a coarse solution, but maybe it's all that needed...?

2. Add an additional, separate rectifier and LM317 circuit to the psu outputting +24 and send that to the relays instead. This seems like maybe it's overkill, but it strikes me as a surefire way of achieving the goal.

Thanks for any thoughts and input you have!

 

[Ricardus]

A current limiting resistor is fine. That's what they do on the SSL9000 preamps. The rails are 16v and the relays needed 12. You should be able to calculate the value just like you would for an LED.

 

[Ricardus]

Just make sure you calculate wattage also so you use a big enough resistor.

 

[Bobby Baird]

I have a bipolar psu circuit delivering +/-28vdc using standard LM317/LM337 regulators. This powers the circuit of a compressor, but I also need to (separately) send +24vdc to a couple of relays used for bypass. The relays do not necessarily need to be on the power rails of the audio circuit.

I tried sending the full +28 to the relays but they didn't seem to like that (although supposedly they are rated to handle it) and weren't switching when supplied with this voltage. So now I want to find a good way to get the appropriate +24v. I can only think of two options, and I want some input on if there are pros/cons to these or if perhaps there are better solutions that I'm not thinking of.

1. Just stick a decently rated resistor between the +28 from the psu and the relay. This seems like a coarse solution, but maybe it's all that needed...?

2. Add an additional, separate rectifier and LM317 circuit to the psu outputting +24 and send that to the relays instead. This seems like maybe it's overkill, but it strikes me as a surefire way of achieving the goal.

Thanks for any thoughts and input you have!

Yes use a limiting resistor. Are the relays in series with the +&- rail if so they would add up to 48v? Voltage-Drop Resistor Calculator
Can you post a small section of schematic?

 

[Monte McGuire]

Is a single 7824 regulator too complicated? It will 100% work with zero fuss...

 

[JMan]

Yes use a limiting resistor. Are the relays in series with the +&- rail if so they would add up to 48v? Voltage-Drop Resistor Calculator
Can you post a small section of schematic?

Thanks!

The relay coils are completely separate from the +/- rails of the main circuit (they are just functioning as bypass/in switching for the audio path), so I'm just sending them the v+ from the psu. I am not at home right now so I can't provide the schematic right now.

I used that calculator that you provided and plugged in some numbers for a random Panasonic 24vdc relay at Mouser and it looks like I need 482r (or whatever closest value) and .03W to get down to 24v.

 

[JMan]

Is a single 7824 regulator too complicated? It will 100% work with zero fuss...

You know, I hadn't thought of that, but it wouldn't be complicated at all. A CLR like what has been suggested above is obviously the easiest solution, but I will consider this as well. I haven't used a 7824 in awhile and can't quite remember what its requirements are, but I think it would work with the bits that I already have in place (thinking specifically about the rectified voltage going in to the 7824 -- I will have to look and see if everything lines up, but I imagine it will). Thanks for the suggestion!

 

[Monte McGuire]

You know, I hadn't thought of that, but it wouldn't be complicated at all. A CLR like what has been suggested above is obviously the easiest solution, but I will consider this as well. I haven't used a 7824 in awhile and can't quite remember what its requirements are, but I think it would work with the bits that I already have in place (thinking specifically about the rectified voltage going in to the 7824 -- I will have to look and see if everything lines up, but I imagine it will). Thanks for the suggestion!

The 7824 is basically like an LM317 except that it's fixed voltage, so you can forget the two voltage setting resistors. In general, these parts don't need output capacitors for stability, and the post rectifier caps on the 28V rail will satisfy any input cap requirements, so it's literally wiring a three terminal part in there to take the 28V and make 24V out of it. Easy peasy.

 

[JMan]

The 7824 is basically like an LM317 except that it's fixed voltage, so you can forget the two voltage setting resistors. In general, these parts don't need output capacitors for stability, and the post rectifier caps on the 28V rail will satisfy any input cap requirements, so it's literally wiring a three terminal part in there to take the 28V and make 24V out of it. Easy peasy.

When you put it that way, it’s pretty tough to argue with

 

[moamps]

I tried sending the full +28 to the relays but they didn't seem to like that (although supposedly they are rated to handle it) and weren't switching when supplied with this voltage.

That's very strange. Relay rated for 24V must work without problems with a voltage of + 28V. Maybe the relay coil you tried is polarized. Can you write which model it is?

 

[JMan]

That's very strange. Relay rated for 24V must work without problems with a voltage of + 28V. Maybe the relay coil you tried is polarized. Can you write which model it is?

Just got home and took a look. It's one of these. I don't think it's polarized, but I could be wrong.

 

[moamps]

Can you test the relay one more time by bringing +28V to pin 16 and - to pin 1 and vice versa?

 

[Newmarket]

You might want consider Zener regulation ?
Apart from that - bear in mind that the Turn On voltage will likely be significantly less than the rated voltage. Read the datasheet.

 

[JohnRoberts]

Just got home and took a look. It's one of these. I don't think it's polarized, but I could be wrong.

If it uses a DC supply, it needs proper + and - connections. It should not need regulated supply.

JR

 

[abbey road d enfer]

I tried sending the full +28 to the relays but they didn't seem to like that (although supposedly they are rated to handle it) and weren't switching when supplied with this voltage.

That's the first subject to address. This is not normal at all. Are you driving these relays directly or is there some electronics in between?

So now I want to find a good way to get the appropriate +24v. I can only think of two options, and I want some input on if there are pros/cons to these or if perhaps there are better solutions that I'm not thinking of.

1. Just stick a decently rated resistor between the +28 from the psu and the relay. This seems like a coarse solution, but maybe it's all that needed...?

2. Add an additional, separate rectifier and LM317 circuit to the psu outputting +24 and send that to the relays instead. This seems like maybe it's overkill, but it strikes me as a surefire way of achieving the goal.

Don't oversweat it. Relays are very tolerant. They need the full switch-on voltage but their position is maintained with a much lower voltage (typically about 15V for a 24V relay) Using a resistor and a capacitor achieves that.

 

[sahib]

You might want consider Zener regulation ?
Apart from that - bear in mind that the Turn On voltage will likely be significantly less than the rated voltage. Read the datasheet.

Indeed. For example I am currently using a 48V relay on something. It pulls (turns on) at 29V and holds at 15V.

 

[Newmarket]

Don't oversweat it. Relays are very tolerant. They need the full switch-on voltage but their position is maintained with a much lower volateg (typically about 15V for a 24V relay) Using a resistor and a capacitor achieves that.

In general you don't need the full voltage to turn on. There's often a "Must Turn On" voltage stated that is some volts below the nominal voltage. You can then drop that down further to hold the relay on. Again this limit is specified for a well documented device. Then a "Must Turn Off" voltage, although that is often done by leaving a connection open circuit or biasing the voltage applied to '0V'.
It's advisable not to exceed the nominal voltage for heat dissipation reasons. Although there is definitely something wrong if it's not basically working at 28V.
Supply may not have enough current on hand to maintain the voltage ? Or maybe something with a driving circuit as you point to.

 

[elskardio]

I use a simple Zener diode to drop the voltage to the appropriate value. You could use a 5.1v zener to drop your +28v and use a 24v relay.

 

[abbey road d enfer]

In general you don't need the full voltage to turn on.

That's why I wrote "full full switch-on voltage".

 

[warpie]

Wouldn't a dedicated regulator help with reducing the "click" noise while switching? Within reason of course.