paralleling relay coils

[[email protected]]

Pulling the base up to 12V would leave around 11.3V on the emitter, which
would be enough to turn the relays on.

One should also consider the expected ON/OFF ratio. If it is 50%/50%
or unknown, it does not matter, but if it is 1%/99% or 99%/1% the
driver circuit should consume current only at the shorter period and
select topology (PNP/NPN CE/CC) accordingly.

This topology also determines, if it can be driven by ordinary 3.3/5 V
TTL or if some special 15/30 V tolerant open collector drivers are
needed.

 

[[email protected]]

On Fri, 16 Aug 2013 17:46:06 -0700 (PDT),

Could be automotive relays with one terminal at frame ground, then he needs a high side switch.

In that case, use a PNP transistor at 12 V, the base goes through a
resistor to a 15 V tolerant open collector TTL output or to a NPN
transistor driven by a 3.3 V controller pin through a resistor.

 

[[email protected]]

https://picasaweb.google.com/106331...vh_NXi3AE&feat=directlink#5912784366042056882

Looks like he wants to switch the coils and regulate the coil voltage
down to 12. 18V is a bit much, 2.2x the normal coil dissipation.

One should remember that the relay turn on current is greater than the
hold on current.

Radio amateurs sometimes use "24 V" relays with 12 V car battery by
charging capacitors to 12 V at off state and when switched on, the
capacitor is in series with the 12 V feed, thus producing 24 V, but of
course, after a while, the voltage drops to 12 V, which is sufficient
hold for most 24 V relays.

In this case, I would not bother to regulate the 12-18 V input at
switch on (let the transistor be hard on), but after a while drop the
relay voltage to something like 6-9 V, which should give a sufficient
hold current. This will reduce the relay dissipation significantly.
Some of the dissipation moves to Q2 at steady state, but as the total
current drops, the total dissipation also drops.

 

[[email protected]]

Or no resistor, no measurement. The uP could drive the relays ON at 100% duty
cycle initially, but then drop down to something like 70% after a reasonable
pull-in delay.

PWMing 200 mA though big inductances seems to me as asking for
troubles, at least in EMC compliance testing, if not earlier.

If the processor only has digital outputs, then PWMin might be OK, but
I would still keep Q2 as a linear regulator, dissipating a lot of
power and low pass filtering the base drive to remove most of the PWM
waveform.

 

[Robert Baer]

I am connecting five 12V relay coils in parallel to switch five different loads. Is it necessary to have fly-back diode for each relay coil or just one 1N400x diode in parallel is sufficient? Each coil is 300 ohms.

-bhavj

Best for each relay to have a series resistor, and a diode in series
with resistor across the coil.
The diode-resistor combo is to snub flyback; the resistor decreases
snub current and thus allows faster relay release; prolly 30-200 ohms.
That coil-resistor/diode network should have that series resistor
mentioned, and should drop 5-10% of total (5V) voltage applied; these
tend to balance coil drives and isolate spikes - which decreases what i
call phase adding (more like EMI adding like crazy).

 

[Robert Baer]

It doesn't matter where the flyback diode is located.

Yep! Got one that came back to its pigeon coop; clipped its wings!

 

[[email protected]]

I've obviously led a sheltered life, since I've never seen a 12V relay

datasheet where pull-in wasn't guaranteed at well below 11.3V. Do you

have an example?



Cheers



Phil Hobbs



--

Dr Philip C D Hobbs

Principal Consultant

ElectroOptical Innovations LLC

Optics, Electro-optics, Photonics, Analog Electronics



160 North State Road #203

Briarcliff Manor NY 10510 USA

+1 845 480 2058



hobbs at electrooptical dot net

http://electrooptical.net

As a rule you can assume the relay pull-in voltage is just equal to the nominal coil rating at the maximum of the spec'd operating temperature range. This will be due to increases in magnetic circuit reluctance as well as winding resistance.

 

[amdx]

Thanks for taking time to draw them. Yes, as you said I am trying to regulate as well as switch the power to the coils. In the fig-1, why is that pnp needed instead of resisting to limit current?

BTW, is it automotive?

 

[[email protected]]

https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/Relay_Drivers.pdf

LM1117 doesn't have the Vin,max handling for the job. Bad choice.

 

[[email protected]]

BTW, is it automotive?

No. Its an industrial device.

 

[[email protected]]

The resistor value could be a problem. If it's too big, there will be a lot of

voltage drop in the resistor, because of the base current. If it's too small, it

will get hot when its low side is grounded by the little mosfet. There may be no

value that works.



Your relays need 200 mA. If the NPN transistor has a guaranteed beta of 100, you

could need 2 mA of base current. Assume that when you have 12 volt power, you

want at least 11 volts on the relays; the transistor drops 0.7, so that leaves

0.3 for the resistor. The resistors has to be 150 ohms. Now let the supply go up

to 18 and turn on the mosfet. The current through the resistor becomes 120 mA

and it dissipates over 2 watts.



If you think the relays will operate with less voltage, use a bigger resistor,

and it might work. But it's a messy compromise.





--



John Larkin Highland Technology Inc

www.highlandtechnology.com jlarkin at highlandtechnology dot com



Precision electronic instrumentation

Picosecond-resolution Digital Delay and Pulse generators

Custom timing and laser controllers

Photonics and fiberoptic TTL data links

VME analog, thermocouple, LVDT, synchro, tachometer

Multichannel arbitrary waveform generators

How does fig-4 work? The DEPL NFET part in particular.

 

[Martin Riddle]

Why do you keep saying stupid stuff, when data sheets are free online?

Operating Vin is only 15V, Device Max is 20v.
Vdrop is 1.4v at 800ma

I wouldn't use it at 18v.

Cheers

 

[[email protected]]

Abs max is 20, and 18 is less than 20. But the part I posted is obviouslythe

ADJ version. With 18 in and 12 out, the regulator only sees 6 volts.



It will lose just about a volt at 300 mA. One should check the relay datasheet

to make sure that's OK.





--



John Larkin Highland Technology Inc

www.highlandtechnology.com jlarkin at highlandtechnology dot com



Precision electronic instrumentation

Picosecond-resolution Digital Delay and Pulse generators

Custom timing and laser controllers

Photonics and fiberoptic TTL data links

VME analog, thermocouple, LVDT, synchro, tachometer

Multichannel arbitrary waveform generators

Circuit #2 produces neg voltage across coil on turn-off, bad choice. Circuit #5 also potential transient over-voltage at turn-on due to diode capacitance. Circuit #3 is really strange, hard to say but likely PNP is saturated so coil -Vdiode at turn off is applied to output of 1117. Y

 

[josephkk]

As a rule you can assume the relay pull-in voltage is just equal to the
nominal coil rating at the maximum of the spec'd operating temperature
range. This will be due to increases in magnetic circuit reluctance as
well as winding resistance.

Pretty close. And very good design advice. In reality it will be about
10% lower, and no better than that over full rated temperature range.

?-)

 

[[email protected]]

Ditto insane. It's a simple voltage regulator followed by a switch. When the

switch disconnects the coil from the regulator, they are disconnected.

That's not how that works and you know it.

 

[Spehro Pefhany]

Pretty close. And very good design advice. In reality it will be about
10% lower, and no better than that over full rated temperature range.

?-)

Yeah, my impression is that they set the original design specs based
on unregulated power from the mains plus temperature. Those
hairy-eared relay designers didn't really add vast unnecessary margin.


Best regards,
Spehro Pefhany

 

[John S]

If you disagree, post an serious explanation or a Spice file.

I'd like to see it, too!

John S

 

[[email protected]]

Yeah, my impression is that they set the original design specs based

on unregulated power from the mains plus temperature. Those

hairy-eared relay designers didn't really add vast unnecessary margin.





Best regards,

Spehro Pefhany

--

"it's the network..." "The Journey is the reward"

[email protected] Info for manufacturers: http://www.trexon.com

Embedded software/hardware/analog Info for designers: http://www.speff.com

I know, and it's real easy to blow a coil open with over-voltage, +15% usually does it, sometimes less.

 

[Jon Lark]

I am connecting five 12V relay coils in parallel to switch five differentloads. Is it necessary to have fly-back diode for each relay coil or just one 1N400x diode in parallel is sufficient? Each coil is 300 ohms.



-bhavj

Just a general comment on the topic of relay coil transient suppression: While connecting a diode across the coil is a common practice, the techniquehas its drawbacks. Robert Baer alluded to this in his earlier post. The diode increases the release time of the relay. Even if you are not concerned with switching time per se, a slow release can cause excessive contact arcing and even welding. My favorite technique is to connect a diode in series with a Zener and place the combination across the coil. This allows the voltage across the coil to rise, thus minimizing the increase in release time, while keeping the voltage across the driver transistor within safe limits. This topic is discussed at length in the “Engineers; Relay Handbook” published by the National Association of Relay Manufacturers. In my copy (Fifth Edition) it’s covered in paragraph 15.3.9 under the topic “Suppressing Relay-Coil Transients.”

 

[Spehro Pefhany]

On Sat, 17 Aug 2013 06:18:25 -0700 (PDT), the renowned

No. Its an industrial device.

In that case, here's another idea:-


\
+12~18V _ _ o o
| )|
1N4003 - )|
^_)|
|
|
|
100K ||-+
___ ||<- SiA408DJ
o--|___|--------+---||-+
| |
0/3.3V \| |
2N4401 |----+

| |

| .-.
| | |
| | | 2.5R
| '-'
| |
+----- +
|
===
GND

It's even possible this is short-circuit resistant if you treat the
MOSFET well thermally.


Best regards,
Spehro Pefhany