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relay driver for a 556

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vielle568

Member
Can someone please give me some advice on how to drive a relay from the output of a 556 dual timer. I have configured both halves of the 556 in monostable mode to put out a single short pulse; these pulses will then be used to trigger a small 12 volt bistable relay.

The 556 is functioning OK and giving out 1/2 second signal pulses; these are currently powering indicator LEDs. I've tried using these output pulses directly to the relay without success; perhaps the coils are a little too powerful for the IC? I have tried using these output signals to control transistors to drive the relays but without success; the 2N2222A transistors seem to get very hot and burn out; other types (2N2904, BC549, 2N3819) don't seem to work at all.

The 556 and the relay are running on 12 volts from a 7812. If there's a standard means of driving a relay from this IC can someone please explain how it should be done? Thank you.
 

fingaz

Member
Hi,

It really depends on the specs for the relay. I would guess that when you had it wired using the 2N2222 that either a) the transistor was not turned fully on, so limiting the power to the relay or b) the relay was just drawing too much current for the transistor.

It would be really helpful if you could post a schematic, or explain exactly what you want to do.
 

MikeMl

Well-Known Member
Most Helpful Member
Hello again,

Here is how I would do it: It assumes that you want the relay pulled in when the 555/6 pin 3 is high.

If the relay coil resistance is less than about 15Ω, the 2N2222 may get warm to hot. I prefer the NFET switch. Any Power NFET capable of switching > 2A, with a Drain-Source breakdown of >25V, and a Gate-Source Breakdown of > 20V would work. (IRF530, for example).

The Snubber could be a 1N400X.

The resistor to the base of the 2222; Rb = E/I = (12-Vbe)/Ib, where Vbe is about 0.65V for a 2222.

Ib=Icoil/β, where Icoil is the relay current, and β is the current gain of the 2222.

Halve the computed value to guarantee saturation when the 2222 is turned on.
 

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oldrocker

New Member
Not a 556 but should be similar. I used a 3904 for the transister. I used an Omron 12v and the 9 volt relays from Radio Shack and both worked.

 
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vielle568

Member
Thanks for all the suggestions and hello again to MikeMl. Yes, it's a development of the same project I was working on a couple of weeks ago for the wind turbine charge controller. The output from the original 555 that senses the battery level has been connected to another 555 configured as an inverting buffer. This then gives out signals for both the high and the low charge states, and these are then sent to both sides of a 556 to create pulses to trigger a small bistable relay that in turn will connect/disconnect the main system relay.

I do have some power MOSFETs (BS170) in with my transistor collection so I'll try this configuration first and see what happens. I'll let you know later.
 
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vielle568

Member
Thanks again to everyone for the advice. The 556 relay circuit is now operational but it wasn't easy to figure out in spite of all the help. It seems I'd put the wrong type of diodes across the bistable relay and as a result current was around passing the coil and causing the transistors to get hot/burn out. I used a MOSFET as you suggested MikeMl and I added indicator LEDs from oldrocker's design. Once I'd found the error everything fell nicely into place. Mind you, although both sides of the 556 have the same R and C values one side has a trigger pulse twice as long as the other; I can't figure that one out! However, it toggles the relay and that's all that really matters.
Vielle568
 
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Hero999

Banned
If the relay coil requires less than 200mA (>60Ω @12V) then you don't need a driver, just connect the relay directly to the output of the 556 with an appropriate protection diode in parallel.
 

vielle568

Member
Thanks for your information hero999. The relay coil is low impedance and below 60 ohms, but I'll certainly bear in mind what you mentioned for future reference. I have the circuit running OK now; I'd used the wrong diodes as snubbers and consequently the relay didn't function correctly.
 
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