Posted by twillmon on May 8, 2005, 6:51 pm
On 2005-05-08 loren@pacific.net said:
>Newsgroups: alt.energy.homepower
[snip]
>>The counter EMF back diodes or "snubber" diodes can cause the DC
>>relay to hold on much longer as the diode turns the inductive
>>current attempt back into the windings of the coil. You should
>>find very slow drop-out times with this technique. A resistance to
>>absorb some energy in series with the diode can help this
>phenomenum. >
I looked at the effect of various snubbing schemes on drop-out times
about 20 years ago and decided I liked a small MOV best. IIRC, the
drop-out times were extended by very small fractions of a second,
like a few milli-.
[snip]
>>>Typically with a DC relay you want to
>>>install a "diode" or power rectifier across the relay terminals,
>>>oriented so that your control signal is not shorted to ground
>>>but the inductive kickback (with the opposite polarity) is
>>>absorbed. Without such protection, whatever switch controls the
>>>relay coil will get hit by the kickback current as it is trying
>>>to open the circuit, and its life will be shortened.
Make that "kickback _voltage_".
"Long life" indicator lamps across the relay coils were burning out
from the inductive kick-back. After installing the MOV snubbers, I
started getting proper lamp lifetimes.
Tom Willmon
near Mountainair, (mid) New Mexico, USA
.... Be nice to your children. They select the old folks home.
Net-Tamer V 1.12.0 - Registered
Posted by John P@ Bengi on May 8, 2005, 8:45 pm
A MOV has a calibrated voltage drop and with a current dissapates power.
This is unlike a lone diode which, with it's only 0.5-1.0 V drop across
itself, disapates a very small portion of the inductive kick power from the
coil flux collapse. All the power has to disapated in the coil and thus the
armature stays picked up for quite some time. The MOV circuit armature
dropout is thus shorter because much of the power is absorbed by the MOV and
not turned back into the coil.. I have seen diode snubbers that keep
armatures in for over 1 second. Can be annoying and baffling at times.
> On 2005-05-08 loren@pacific.net said:
> >Newsgroups: alt.energy.homepower
> [snip]
> >>The counter EMF back diodes or "snubber" diodes can cause the DC
> >>relay to hold on much longer as the diode turns the inductive
> >>current attempt back into the windings of the coil. You should
> >>find very slow drop-out times with this technique. A resistance to
> >>absorb some energy in series with the diode can help this
> >phenomenum. >
> I looked at the effect of various snubbing schemes on drop-out times
> about 20 years ago and decided I liked a small MOV best. IIRC, the
> drop-out times were extended by very small fractions of a second,
> like a few milli-.
> [snip]
> >>>Typically with a DC relay you want to
> >>>install a "diode" or power rectifier across the relay terminals,
> >>>oriented so that your control signal is not shorted to ground
> >>>but the inductive kickback (with the opposite polarity) is
> >>>absorbed. Without such protection, whatever switch controls the
> >>>relay coil will get hit by the kickback current as it is trying
> >>>to open the circuit, and its life will be shortened.
> Make that "kickback _voltage_".
> "Long life" indicator lamps across the relay coils were burning out
> from the inductive kick-back. After installing the MOV snubbers, I
> started getting proper lamp lifetimes.
> Tom Willmon
> near Mountainair, (mid) New Mexico, USA
> .... Be nice to your children. They select the old folks home.
> Net-Tamer V 1.12.0 - Registered
Posted by Morten on May 8, 2005, 5:59 am
> There is no inrush to a DC relay. the coil is inductive and the current
> rises slowly. The exact opposite of inrush.
Correct, but remember to put a reverse diode accros the coil to quench the
back emf when the field breaks down again when the power is removed from the
coil, otherwise this back emf WILL kill your power mosfets very quickly...
/Morten
Posted by Me on May 6, 2005, 5:42 pm
wrote:
> > Hi I just recieved an Isuzu 3lb 10kw diesel with a keyed start. I'd
> > like to put the remote start in the house which is about 150' away. The
> > guys at the place where I bought it were skeptical because of the dc
> > voltage drop, and referred to putting in a relay. But, surprise he
> > wasn't clear on how to do that. Could you all advise me on how to do
> > that?
>
> I have run dc controls some really long distances ~300 feet @ 24v. I was
> pulling in relays so the current was low. But the inrush to close the relay
> was up there, ~6-8 amps. Run the hot to your relay, locally, and the ground
> to your remote switch and back. I always use stranded wire.
>
>
>
If the guy was smart, he would use a PowerMosFet to drive the 24Vdc
relay, and trigger that with +5vdc at a few miliamps on the gate
of the PowerMosFet. You can do that, with up to 4K o4 5K feet of 22Ga
Teleco wire on the remote side. Now that is a No-Brainer......
Me
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> >Newsgroups: alt.energy.homepower
> [snip]
> >>The counter EMF back diodes or "snubber" diodes can cause the DC
> >>relay to hold on much longer as the diode turns the inductive
> >>current attempt back into the windings of the coil. You should
> >>find very slow drop-out times with this technique. A resistance to
> >>absorb some energy in series with the diode can help this
> >phenomenum. >
> I looked at the effect of various snubbing schemes on drop-out times
> about 20 years ago and decided I liked a small MOV best. IIRC, the
> drop-out times were extended by very small fractions of a second,
> like a few milli-.
> [snip]
> >>>Typically with a DC relay you want to
> >>>install a "diode" or power rectifier across the relay terminals,
> >>>oriented so that your control signal is not shorted to ground
> >>>but the inductive kickback (with the opposite polarity) is
> >>>absorbed. Without such protection, whatever switch controls the
> >>>relay coil will get hit by the kickback current as it is trying
> >>>to open the circuit, and its life will be shortened.
> Make that "kickback _voltage_".
> "Long life" indicator lamps across the relay coils were burning out
> from the inductive kick-back. After installing the MOV snubbers, I
> started getting proper lamp lifetimes.
> Tom Willmon
> near Mountainair, (mid) New Mexico, USA
> .... Be nice to your children. They select the old folks home.
> Net-Tamer V 1.12.0 - Registered