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

 

[Kido]

can you state:
min and max supply voltages and curents
what relay you intend to use and the specs for that relay
can you get a different supply? as aposed to changing an existing one.

 

[tensi]

I have to re-use existing used relays. I'm not sure that PWM will help to reduce the consumption. I've read that the hold current must be at all times above the minimum holding current.So, each time it would be under that value the MAX peak should be higher. This means that I win nothing. The charging cap solution is not applicable because of the lak of place.

 

[automatic2]

You say you've found your circuit. Even though you may not be able to post it, please explain so we can see how you've created power where there is none.

 

[melone]

I would be very interested as well! Unless you have some sort of booster, or a postive and negative supply, those relays need the voltage to work.

 

[buzzp]

For using PWM, the switching time must be less than the release time, then it will work fine as long as the average (DC) current is above the minimum hold current/voltage.

Something worth mentioning, if your planning on energizing all of these relays at the same time with power source you have, I do not believe it will work. At least with solenoid type coils, they can pull up to 20 times the nominal current at energizing. Your power supply will not like energizing all of these at the same time.

If you do not need all to come on at the same time, of if you do, you might try some large capacitance in parallel with the source. The cap will charge up to power supply voltage. When the switch between the coil and supply is closed, the cap will supply some extra power to pull-in the relay. Presumably, the power supply votlage will fall off when relay is introduced in the circuit but the cap will already be charged to the highest voltage available. Note: you may have to put some small resistance in series with the cap to limit the current when the power is applied across the cap (depends on cap size and mfg).

 

[tensi]

have a look at

http://www.piclist.com/techref/io/relays.htm

and but there is a fault in the schematic (the cap near the relay must be take off and there must be a free wheel diode on the relay coil also the diode near the undesirable cap must be also be taling off)and the left side of the resistor must be connected to +12 and at the other side of the resistor must come the collector of a transistor; its emittor to the ground , and a resistor at its base. So the circuit can be driven with +5Vdc. I hope this is clear... sorry if not. But I even doesn't need it.

I don't agree with automatic2. There is power and energy but not enough. Just remember that I have 200mA MAX.The trick is not to switch the relay at the same time. One relay per box, thus 10 relays at time. Each relay need 16.8V to pull-in (I have mimnimum 21V) and 8.12mA (the minimal coil value is 2070 ohms) thus 81.2mA for 10 relays (during 20ms). After that hold voltage: 8.4V and 4.06mA (the coil value didn't change) thus for 10 relays 40.6mA. Now the next 10. Thus, 40.6mA + 81.2mA = 121.8mA (during 20ms) after that 81.2mA. The next 10. Thus, 81.2mA + 81.2mA = 162.4 mA (during 20ms).After that 121.8mA. Now the 10 last. 121.8mA + 81.2mA = 203mA (during 20ms) here overflow of 3mA during 20ms ! (I resolve this with a capacitor of 330uF) After this time 162.4 mA. And now I have 40 relays in the ON state ! What do you think about it?

 

[tensi]

The relay type I use is a ZETTLER AZ 764-1AE-24DSF

 

[automatic2]

Your little trick may be feasible, but is vastly different than where you began. If your referring the the 'junk box' circuit, I'd like to know how you get the -5 volts (-12 in your case).

 

[melone]

He is absolutely right! The 47u is fully charged to 5V. Once the transistor turns on (which must be fast), one side of the cap gets forced to ground. Since the cap will resist instantaneous voltage change, it will cause the other side of the cap to be 5V lower, i.e. -5V (since the 100 ohm resistor prevents a direct connection to ground on the inductor / coil side). For a short time, the voltage across the coil will be creater than 5V. The only things that I see that prevent this from being a straighforward solution is:

1) The 5V control / supply signal for the coil must be able to be quickly turned on / off & supply enough current to not be "browned out".

2) The turn on time of the relay must be fairly small, like 100 micro seconds. Increasing the cap could increase the negative supply.

3) The turn on voltage must be as low as 8V. This circuit will not produce exactly -5V, but closer to -4V on the bottom of the coil. Therefore, to have some margin, the coil should energize down to around 8V.

Otherwise, this is a wonderful circuit and I will keep this one in my personal library! Tensi, you get a star for sticking to your guns and backing up what you claimed!!!

Excellent thread!

BTW, I was able to verify the theory by running a simple simulation with PSpice...

 

[tensi]

Hello everybody! Thanks a lot for the star I receive!
I've tried another solution but have to test it deeper before; just to be sure...
Using PWM can spare a lot of energy. A relay may so be driven at a frequency which may not be under the 100Hz. I made a try at 10KHz with a duty cycle of 50/50. So the current used by the relay is half of what it need!
I made already some measurements and what I measure is near what I calculate. But like I said upwards(?I'm not sure of these word) I have to try it. See You after that.

 

[tensi]

Hello! I've made a lot of tests! You know already the context... The schematic is the following (where can I ask if it is possible to put a schematic or ask to devlop something that make it possible?) The power supply (+) line go to one point of the relay coil, the other side of the relay coil go (with a diode 1N4148 parallel on the coil with its cathode connected to the (+)) to the collector of a BC549 the emittor connected to the ground. The base with its resistor (1K) receive a square wave signal (f= 10Khz). There is a buffer capacitor 330µF/35Vdc between the + and the ground of the "power supply" . One capacitor for 4 relays. By variating of the duty cycle you can give the current that the realy need to work. When you measure you will see that the power supply just give a little more than the HALF of the current used by the relay. That's what I need ! By the way I choose a Zettler relay of 12Vdc i.p.o. 24Vdc because a 24Vdc relay need 22.3V to pull-in @+55°C and the lowest voltage I can get is 21Vdc. OK a 12Vdc relay need more current but with this system it is no more a problem ! ;-D
I hope this can help others. Don't be shame to write me @ yves_tensi@yahoo.fr if you need more infos/datasheets about relays. I think I will assemble all the infos and put them on a web page. I'm sure this will be helpful. Best regards to everybody who take time to answer my questions. I will be tracking this till the end of july. After that no more.

 

[automatic2]

So you've gone with a couple of the suggestions, PWM and the purchase req., great, don't be ashamed to think you might be humbled by speaking with some of us 'field workers'.

 

[tensi]

MIssunderstanding again! What I wanted to say is that you don't have to hesitate to ask me things. I give my e-mail because it is more easier to send files than to write a long explanation. I'm really soory if I hurt someone ; it was and is nver my goal.

 

[buzzp]

Do you notice any audible noise from the relay with a certain PWM frequency? I have seen circuits like this but never had the need for one. The ones I have seen, at some audible noise associated with them. I am sure there is some PWM freq where the noise is not heard. I would like to see a schematic of what you have come up with. You can email me at buzzybooper@excite.com. I would appreciate it.

 

[tensi]

buzzp, The schematic is very simple: Foom the positive point of the power supply dedicated to the relay(s) go directly to the coil of the relay the other side of the relay coil is connected to the collector of a transistor (BC547 e.g.) the emitter of the transistor is connected to the ground. Put on the base of the transistor a resistor calculated to give enough current for the relay. The other side of the resistor go to the µC. For experimentations you can replace the µC by a square wave generator. Don't forget to put a diode // on the coil with its cathode to the +. That's all. The frequency must of course not be too low. For my part I used a frequency of 1KHz. The trick is to play with the frequency. The way of measuring is also important. Just add a resistor between the + and the relay coil to measure the current which flows through the relay coil. Put a Ameter at the + of the power supply. You will see the difference ! I will try to scan the schematic and send it to you. Please be patient. Regards.

 

[buzzp]

Tensi,
Okay it is all clear to me now. I now understand what you were trying to do. A schematic is not necessary. Thanks.