Thanks very much for the explanation Ron. It is counter intuitive for me but I think I get it now regarding D1/D2 remaining forward biased. It looks like I will have to go with CMOS for power saving and consequently stick with a 2N7000 as a switch for the higher current demand of the relay. I have 36V zeners on order so I am wondering if I use one of those in series with a 1N4004 across the coil will the 2N7000 be happy? I can't figure out the consequences to the device from the datasheet. Do I even need to snub the coil with a 2N7000 driving it?<snip>The 2N7000 should allow you to do that.</snip>
I'm not sure where SgtWookie wants to put diodes, but if you leave D1 and D2 in, you will slow down the turnoff of the 2N7000. Replace the diodes with jumpers, or low value (47Ω) resistors.Proposed new schematic.
http://pict.co.uk/556_RD_with_Relay_V2.gif
D1/D2 left in place as they are on the board already and it doesn't look like they will do any harm and are presumably analogous to the diodes suggested by SgtWookie. Output from 556 now driving gate of a 2N7000 mosfet. 2k2 pulldown resistors added from gate to source. I am hoping this will eliminate the longer pulse I observed with the mosfet in place, which I presume is stray capacitance holding the thing open longer than dictated by the RC timng components. Getting there?
Do you mean that you might want to drive one coil with both monostables? If so, you could wire the two MOSFETs in parallel (drain and source only), eliminate the diodes, and drive the gates separately. Or, you could use one MOSFET and connect D1 and D2 cathodes to the gate. The MOSFET will turn off slightly slower if you do it this way.Thanks. I'll add a 100K across the coli. Are these drain/source junctions bidirectional? I am presuming they are, so if I connect the drain and sources of both mosfets together to form a single output that pulses on each toggle of the switch I will have to move D1/D2 between the drains and the coil?
Be aware that the diode slows down the turn-off of the MOSFET slightly, which seems to be important to you.I do mean that - drive one coil with both monostables. Thinking about it I can see now that the diodes D1/D2 can go when using 2 mosfets cause when one mosfet turns on the other is off and affords no path to ground bypassing the coil. I will probably leave the diodes and use a single mosfet though as its easier to adapt the board that way.
I couldn't figure out how to do this with one monostable i.e. generate a negative pulse on the trigger each time the switch is toggled. That would have been nice and cut the component count somewhat, but glad to get something working at least. Thanks.
Yep that's what you would expect with a resistor value so low, larger than (or equalling) the coil current. In that case the back EMF calc is close to 100% accurate, because the speed of current change is slow (low freq) and the lossy high freq performance of the iron core has much less effect.I won't argue with your experience. I'll just tell you mine (from 10 minutes ago).
I added a 2N7000 between the 555 and the relay in my test circuit. The relay is a small unit with a 12V, 10mA coil. I snubbed it with 1kΩ, and the drain voltage peaked at about 21V, very close to Vcc+(Icoil*Rsnub).
Proposed new schematic.
http://pict.co.uk/556_RD_with_Relay_V2.gif
The board with the monostables was already built so I just added the MOSFETS on to the output terminals I had on the board. However I now realise that on the monostable board I have a common +ve for the coils and individual negatives from the mosfet drains. On the relay board it is a common -ve and individual positives. I can't think of a fix other than substituting the 2N7000's for p-channel mosfets. Am I correct in this assumption? If so can anyone suggest a p-channel MOSFET equivalent to the 2N7000? There are 36V zeners in the the coil snubbing loop so they would need a higher breakdown voltage than this.
Thanks for any suggestions.
Regards,
John
I am nearing completion of assembly and realised I made a pretty fundamental error in the board design. I was initially going to drive 6 relays with a common -ve ground connecting all the coil -ve terminals and multiple separate +ve feeds from each monostable output to the +ve coil terminals. The PCB was designed like this.
Then I realised I wanted to use CMOS for battery longevity and that I couldn't source the latching current from the 556 CMOS so added 2N7000 as per the simplified example here..
A PMOS high side switch requires that the gate go low to turn it on. Your 556 output goes high.Proposed new schematic.
http://pict.co.uk/556_RD_with_Relay_V2.gif
The board with the monostables was already built so I just added the MOSFETS on to the output terminals I had on the board. However I now realise that on the monostable board I have a common +ve for the coils and individual negatives from the mosfet drains. On the relay board it is a common -ve and individual positives. I can't think of a fix other than substituting the 2N7000's for p-channel mosfets. Am I correct in this assumption? If so can anyone suggest a p-channel MOSFET equivalent to the 2N7000? There are 36V zeners in the the coil snubbing loop so they would need a higher breakdown voltage than this.
Thanks for any suggestions.
Regards,
John
You can do that. The 2N7000 will be acting as a source follower. As such, it is self-snubbing, i.e., when the gate goes to ground, the flyback will only go 2-3V below ground, because the transistor conducts the coil current until it decays to zero. The down side of this is the coil inductance and capacitance causes ringing, which isn't destructive, but might be audible, depending on the resonant frequency. A reverse-biased diode across the coil pretty much eliminates the ringing, but slows down relay turn-off slightly. In case you're wondering, a zener will do nothing.As an afterthought, can I stick with the 2N7000 and connect drain to +ve and use it to switch the relay coil load between source (+ve) and ground -ve?? From what I can gather I can do this but the switched voltage will be less than the supply by the 2N7000 threshold voltage. So worst case 3V less. I need 12V for the relays so I could crank the supply up to 15V as the 556 should handle that. Is this the way to go? Is the breakdown voltage on the source side the same as the drain... i.e. would it handle a 36V pulse from the coil (via zener) in this configuration?
Thanks again,
John
That won't work as drawn. See post #38.Duh. Not thinking clearly patently. Would something like this work?
http://pict.co.uk/HiSideSwitch.jpg
If I supply the 556 at 14.5V I should get 13V at the output pin. I understand that the max supply voltage is 15V for CMOS so that should be OK I presume. I would then get around 13.5V at the output pin. When the 556 output goes high the mosfet turns on and the resistor divider formed by R1 and the relay coil keeps Vd and Vs at 9.1V which is above the relay minimum of 8.4V (ignoring the drain-source resistance as negligible). With Vs at 9.1 V when on, this is 3.9V below the gate (13.5V) so it should stay on until Vg goes low? Vth for a 2N7000 is listed at 3V max so should be fully on at Vgs=3.9V?
Does this have a chance of working? Any other ideas?
Thanks,
John
by Aaron Carman
by Jake Hertz
by Jake Hertz