Relay driving 3

[tensi]

Hello,
I'm searching for a schematic which let me drive a +24Vdc relay with only +12Vdc. The relay characteristics are the followings:
Unom = 24Vdc
Upull-in= 16.8Vdc
Uhold = 8.4Vdc
Urelease = 2.4Vdc
Inom = 16.7mA
Coil resistance = 1440 Ohms +/-10%
UMAX Coil = 60Vdc

Thank You already for the feedback

 

[IRstuff]

If you don't have 24 Vdc, why are you using a 24 Vdc relay?

According to your spec, you need a minimum of 17 V to engage the relay. You'll need to either get 24 V or generate 24 Vdc using a separate switching regulator.

TTFN

 

[tensi]

I'm using a 24Vdc relay because the consumtion is lower than a 12Vdc one !!!
A switching regulator is useless there is a very simple and better solution and I found it. Thank You for your help at all. Bye

 

[tensi]

The solution is the following a transistor which put a capacitor (already filled with the power supply voltage) in serie with the power supply you have. So, you'll get the dubble of the voltage. The relay will go in the 'ON' state and at the "same" time the capaitor will be filled again.

 

[dpc]

The voltage doubler will only work if you can store enough energy in the capacitor to pull in the relay.

For the same relay, a 12V coil and a 24V coil will require the same amount of **energy** for pull-in and holding, regardless of the operating voltage.

Also, be aware that the relay coil will deliver a powerful voltage spike to your circuit whenever you de-energize it.

 

[Heydave]

Is there a latching version of the relay? If so, you only need to pulse the coil long enough to actuate the relay then no power is drawn at all. Such relays are usually magnetic latching. Single coil versions set their state depending on polarity while two coil versions require simpler drivers. The capacitor charge pump scheme should work fine as long as the capacitor has time to charge before cycles and does not disintegrate later due to high surges. It will still have many times the power consumption of a latching relay solution.

 

[automatic2]

' a transistor which put a capacitor (already filled with the power supply voltage) in serie with the power supply you have. So, you'll get the dubble of the voltage.'

Duh, I think you'll get some extra current available to pull in that 12v relay, but I doubt you'll see any voltage dubbling to operate a 24v coil.

Now you could jump your unregulated voltage over your regulator to drive the coil, but your power requirements are going to be similar regardless of your coil voltage.

 

[tensi]

All right. I will try to answer all at one time.
That's true and I have calculate the capacitor value (220uF). In fact a relay has to be driven with current, the voltage is not so important. So, after be pulled-in the relay consumption will be 5.85mA for a 24Vdc and...11.69mA for a 12Vdc! My problem is that the power supply can only give 21Vdc to 31.5Vdc (30Vdc typical) with a MAX current of 250mA (minus 50mA for the rest of the circuit = 200mA for the relayS (yes, there are 4 relays which must be driven)) I can't jump to the unregulated voltage (no acces, I just receive cables which deliver what I wrote upwards)
The problemS with a latching version are that it is MORE expensive than a "normal" one + that the relays must be OFF if the main power falls down and I don't have enough energy in this case to switch all the latching relays OFF + also I know there is a circuit doing this but then the devlopment become too expensive; it has to stay low cost.
Oh yes one more thing, a 24Vdc relay with a 1296 ohms coil (=1440 -10%) has a hold current of 5.85 mA thus a power of 49.14 mW a 12Vdc one has a power of 98.28 mW in the same situation (HOLD). At last but not least, I didn't forget to put a free wheel diode.
Best Regards to all of you.

Yves

 

[automatic2]

Now lets see, 24v x 5.85ma = 140, and 12v x 11.69ma = 140, kind of looks like similarl power requirements.

At 12 volts, your 4 relays are going to require 11.69 x 4 = 47ma. You say you have 200ma available.

Voltage is a equally important in relay coils as it produces the motion of your contacts. Current then holds the contacts in place.

Tell us again, why do you want to go with a 24v relay in a 12v circuit.

 

[tensi]

Yes, indeed you're right.But the 12Vdc relay has a max voltage of 30.6Vdc while the 24Vdc has a max voltage of 61.2Vdc. Because the voltage I receive is 30Vdc +5% -30% I prefered to choose the 24Vdc one. 30Vdc minus 30% is 21Vdc and that's also above the 12Vdc nominal of the 12Vdc relay, but just under the nominal voltage of the 24Vdc relay and enough above the 16.8Vdc that its requires to pull-in. So I find that the 24Vdc is the best one in this case.

 

[RavenJoe]

I don't know about the rest of you guys. But I am totally lost as to what tensi is talking about. Are you saying you have a fluctuating voltage ? What kind of system is this ? What is the application ? Perhaps a relay is not the answer for your circuit. Is this perhaps an undervoltage relay ?

 

[buzzp]

Seems like a voltage regulator (cheap) or even cheaper a series resistor with the power supply would be the way to go to reduce the voltage down to something useable for the 12VDC coil or even a zener.
I think your creating way to much work to use the 24VDC coil. Besides putting a cap in series with a 12VDC supply will not give you 24VDC, the cap simply charges up to 12VDC then current flow stops. The cap + is then tied directly to the power supply + (remember Kirchoffs law). If you put the cap in parallel with the supply it will give you a little more current capacity at relay pull-in but thats it.
Go with something to reduce the available DC supply to something reasonable for a 12VDC coil.

 

[melone]

I think that he is saying that his incoming power supply allows him to use a 24VDC coil without any type of conditioning. If that is the case, then 24VDC is the way to go.

To control it, use a transistor to activate the coil. The 12VDC logic signal can be voltage divided down to work with the transistor you choose. Let me suggest using a power device...

 

[automatic2]

the original question was '...drive a +24Vdc relay with only +12Vdc.' Some good suggestion where put forward, but for some reason the criterim keeps changing. The problem we have here is not with the circuit my friends. I've learnt years ago, that to effectively troubleshoot a condition, you need to understand the basics.

 

[tensi]

I'm sorry guys but I'm still looking for a schematic which let me drive a 24Vdc relay with only 12Vdc.
OK! By writing this I just understand my error! I'm really sorry for this ! Because englsh is not my mother language I was focusing on what I was writing in place of the data I have to give. Sorry again !
So, you must imagine a box with inside a controller, 4 leds, 4 push-buttons, resistors, capacitors (1 of 330uF/35Vdc for the relays) 4 relays, 2 connectors.
The power supply is the one I describe upwards and I can't do anything to it. Now, the worse (that I forgot to tell) There are 10 boxes this means 40 relays which have to be activated! The lower voltage I receive (and is guaranted) is 21Vdc (30Vdc -30%) the maximum is 31.5Vdc (30Vdc +5%) the current is 250mA and I reserve 50mA for the 10 controllers and the 40 leds. I have NO place to put al lot of components on the PCB (because the clearance between the very low power and the main that the relays have to switch) the PCB is about 75mm x 30mm and I have to use it dubble sided to design the traces because the relays have to switch 230Vac (+/-10%) 16Amps!
I know there is a schematic with only 1 transistor and its base resistor, a capacitor, ??? (the rst I don't remember) and this small circuit put the resistor (which is filled with the voltage of the power supply) in serie with the violtage of the power supply just to pull-in the relay. after that the relay stays ON because is hold voltage is lower than the voltage of the power supply which rests.
I hope this is more clear for you now. One more time sorry and be sure this is not a joke and I'm really into the shit now because time flows and I still have nothing to show to my project leader. Once I will remember the schematic I will give it to you also. I don't like to waiste my time and so I don't want to waist the time of others also. Regards to everybody. Yves

 

[buzzp]

Your request is still not clear. You say you have 30VDC available but you want to drive a 24VDC coil with 12VDC. Makes no sense. I purpose the following based on what you have.

So, you have 24VDC available but are using 12VDC because of the so called lower current draw? The current draw of most relays goes down as you increase the coil voltage. If you look at a basic relay,

http://www.midtex.com/pdf/midtex/pcb/pdf/496.pdf

the current draw of a 24 volt coil is exactly half of that of the 12V coils.

If you want to use 12V coils still, then use a regulator, zener, or series resistor to reduce the voltage to the 12VDC relay coil.

If you really need to drive a 24VDC coil with only 12VDC then you have some work to do. I have not heard of a simple circuit to drive a relay coil based on one transistor. Maybe your thinking of using the transistor as an amplifier or something. Still will not work.
Good luck.

 

[melone]

- Are you using 12VDC or 24VDC relays?

- How much current can your supply source?

- What are you using to turn on your relay (voltage level, current source / sink ability / frequency / etc.)?

- Why does it have to fit in this box that you describe?

 

[automatic2]

I hear what your saying about wanting to use the 24v relays to meet your current ratings. But here's the deal, calculate what your maximum power requirements are, ie: the max number of relays on at any one time. If possibly all 40, @24v coil / 5.85ma, plus your 50ma control/led, that requires approx. 5.7 watts.

Your power supply is capable from between 5.2 (21v x 250ma) and 7.8 (31.5v x 250ma) watts. At the 5.7watt end of things, you'll be browning out, and possibly sooner. If any safety issues are about you can double that.

If your indicating (it's not clear) that your power supply is falling from 30v to 21volts as you draw from it, then your supply is simply underpowered. Your only option then is to insert a capacitor large enough to average the duty cylce of your load, on your supply line. If your average power exceeds your supply, pull out your purchase aquisition form.

 

[tensi]

I never guess that it would be so difficult to make me comprehensible! I thought that what I was writing was clear. My first question was, in fact, a schematic which let me drive a relay (whatever which one) with the half of the voltage it need. I didn't give more infos because I thought it was not necessary. But... when I look at aal the responses !!! I was wrong and still be !!!
One more precsion now for "automatic2" The 50mA I have reserve are for the 10 boxes (10 microcontrollers, 40 leds)
The characteristics of the power supply I "receive" are what they are I can't do anything to that. So are the specifications. What I wrote is that the power supply is a 30Vdc -30% +5% (that are the min & MAX values it can reach)
So, I take the worst case for my calculation. The same with the relay. By the way I found one which has a coil of 2300 Ohms +/- 10% (2070 < 2300 < 2530) In this case also I take the lowest value because it is when the coil value is 2070 and the voltage 31.5 that it would take the most current. For me the worst caseS are first when I have only 21Vdc 200mA (50mA reserved so I don't count them) with which I have to pull-in and then hold 40 relays with a 2070 Ohms coil (at the same time (worst case also)) and at the other side 31.5Vdc 200mA and the same story.
From there my idea of a schematic which let me drive a relay with the half of its nominal value. I finaly found it. But I don't know how to put a schematic here. It is the first time I use such forum. My idea is even to use a 48Vdc relay so the pull-in is 33.6V and the hold 16.8V and the current is half of the current of a 24V relay.

 

[OperaHouse]

So what do you have to drive this? No, you havent provided much useul imformation! I've done a lot of these relay tricks to reduce power consumption but you usually pay wit a number of extra parts. If you have a microprocessor, you can turn the driver on full initially and then pulse it, PWM mode, to reduce the current. You can also have a parallel cap and resistor in series with power to the relay. Of course these work only with a relay that is the same as the supply voltage. Charging up a cap and then putting it in series is an easy trick if you are just using a switch. It is a lot more parts if you do it with electronics. I think the purpose of engineering is to do the design before you order the parts.