HT Relay
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From: Ian Bell on 24 Dec 2009 11:43 John Fields wrote: > On Thu, 24 Dec 2009 16:07:58 +0000, Ian Bell <ruffrecords(a)yahoo.com> > wrote: > >> John Fields wrote: >>> On Thu, 24 Dec 2009 14:44:00 +0000, Ian Bell <ruffrecords(a)yahoo.com> >>> wrote: >>> >>>> I am trying to select a relay for a delayed HT switch (which will also >>>> discharge the HT when off). Most relays I can find have contacts rated >>>> at 250VAC which translates into a peak of about 350V. However, data is >>>> scarce on what dc voltage these relays can switch. So far I have found >>>> only one that gives a dc current versus voltage curve and that stops at >>>> 210V dc (and 200mA) and I really want to be able to switch up to 350V at >>>> up to 200mA. The rest just give a dc voltage at max current value. >>>> >>>> So, I am guessing that this problem has been faced before and there are >>>> relays that are known to work fine in this application. Any recommendations? >>> --- >>> The problem with using relays on DC is that when the contacts open an >>> arc is established which is hard to put out. >>> >> I can understand that with high currents which is why many of the 250VAC >> 10 or 20 amp relays I have seen are only rated at about 30Vdc for the >> same amps but here I am only switching a small fraction of an amp. >> >>> With AC contacts the problem is minimized because as the AC waveform >>> goes through zero volts the arc is automatically extinguished. >>> >> Understood. >> >>> A trick I've seen before, but never used, is to wire two sets of >>> contacts in series, which will give you twice the effective gap, helping >>> to quench the arc. >>> >>> Another trick is to use magnets to "blow out" the arc when the contacts >>> open, and some larger relays/contactors are designed that way. >>> >>> Can you use high voltage transistors or solid-state relays instead of >>> mechanical relays? >> I would rather not. > > --- > 12V coil, and a little pricey, but perhaps something like this, then: > > http://www.jenningstech.com/pdf/rel/RF_1D_1E_1G.pdf > > JF LOL Cheers Ian
From: Martin Riddle on 24 Dec 2009 12:52 "John Fields" <jfields(a)austininstruments.com> wrote in message news:f827j5dotam4ic9jdn4avra557gn4mih7k(a)4ax.com... > On Thu, 24 Dec 2009 14:44:00 +0000, Ian Bell <ruffrecords(a)yahoo.com> > wrote: > >>I am trying to select a relay for a delayed HT switch (which will also >>discharge the HT when off). Most relays I can find have contacts rated >>at 250VAC which translates into a peak of about 350V. However, data is >>scarce on what dc voltage these relays can switch. So far I have found >>only one that gives a dc current versus voltage curve and that stops >>at >>210V dc (and 200mA) and I really want to be able to switch up to 350V >>at >>up to 200mA. The rest just give a dc voltage at max current value. >> >>So, I am guessing that this problem has been faced before and there >>are >>relays that are known to work fine in this application. Any >>recommendations? > > --- > The problem with using relays on DC is that when the contacts open an > arc is established which is hard to put out. > > With AC contacts the problem is minimized because as the AC waveform > goes through zero volts the arc is automatically extinguished. > > A trick I've seen before, but never used, is to wire two sets of > contacts in series, which will give you twice the effective gap, > helping > to quench the arc. > > Another trick is to use magnets to "blow out" the arc when the > contacts > open, and some larger relays/contactors are designed that way. > > Can you use high voltage transistors or solid-state relays instead of > mechanical relays? > --- > >>By the way I prefer a 5V coil. > > --- > OK > > JF Kilovac? <http://relays.tycoelectronics.com/kilovac/> Cheers
From: Phil Allison on 24 Dec 2009 19:27 "Jan Panteltje" Ian Bell >> So far I have found >>only one that gives a dc current versus voltage curve and that stops at >>210V dc (and 200mA) and I really want to be able to switch up to 350V at >>up to 200mA. > > > You canot use an AC relay rated at 250V for DC at 350V (the sqrt(2) > value). > This because, as recently discussed here, in a DC circuit you get arc > forming, > and the arc will not extinguish. ** Course you can, if you de-rate the breaking current figure sufficiently. The OP has, like YOU, made a blunder in his interpretation of the specs. .... Phil
From: Jan Panteltje on 25 Dec 2009 06:48 On a sunny day (Fri, 25 Dec 2009 11:27:57 +1100) it happened "Phil Allison" <phil_a(a)tpg.com.au> wrote in <7pif8hF3t3U1(a)mid.individual.net>: > >"Jan Panteltje" > Ian Bell > >>> So far I have found >>>only one that gives a dc current versus voltage curve and that stops at >>>210V dc (and 200mA) and I really want to be able to switch up to 350V at >>>up to 200mA. >> >> >> You canot use an AC relay rated at 250V for DC at 350V (the sqrt(2) >> value). >> This because, as recently discussed here, in a DC circuit you get arc >> forming, >> and the arc will not extinguish. > > >** Course you can, if you de-rate the breaking current figure sufficiently. Sigh, hey, zero current may work! How clever of you:-) But wanna bet he wants to switch some more current then zero? > The OP has, like YOU, made a blunder in his interpretation of the specs. Got nothing for Christmas Phil?
From: John Fields on 25 Dec 2009 13:34
On Thu, 24 Dec 2009 16:43:18 +0000, Ian Bell <ruffrecords(a)yahoo.com> wrote: >John Fields wrote: >> On Thu, 24 Dec 2009 16:07:58 +0000, Ian Bell <ruffrecords(a)yahoo.com> >> wrote: >> >>> John Fields wrote: >>>> On Thu, 24 Dec 2009 14:44:00 +0000, Ian Bell <ruffrecords(a)yahoo.com> >>>> wrote: >>>> >>>>> I am trying to select a relay for a delayed HT switch (which will also >>>>> discharge the HT when off). Most relays I can find have contacts rated >>>>> at 250VAC which translates into a peak of about 350V. However, data is >>>>> scarce on what dc voltage these relays can switch. So far I have found >>>>> only one that gives a dc current versus voltage curve and that stops at >>>>> 210V dc (and 200mA) and I really want to be able to switch up to 350V at >>>>> up to 200mA. The rest just give a dc voltage at max current value. >>>>> >>>>> So, I am guessing that this problem has been faced before and there are >>>>> relays that are known to work fine in this application. Any recommendations? >>>> --- >>>> The problem with using relays on DC is that when the contacts open an >>>> arc is established which is hard to put out. >>>> >>> I can understand that with high currents which is why many of the 250VAC >>> 10 or 20 amp relays I have seen are only rated at about 30Vdc for the >>> same amps but here I am only switching a small fraction of an amp. >>> >>>> With AC contacts the problem is minimized because as the AC waveform >>>> goes through zero volts the arc is automatically extinguished. >>>> >>> Understood. >>> >>>> A trick I've seen before, but never used, is to wire two sets of >>>> contacts in series, which will give you twice the effective gap, helping >>>> to quench the arc. >>>> >>>> Another trick is to use magnets to "blow out" the arc when the contacts >>>> open, and some larger relays/contactors are designed that way. >>>> >>>> Can you use high voltage transistors or solid-state relays instead of >>>> mechanical relays? >>> I would rather not. >> >> --- >> 12V coil, and a little pricey, but perhaps something like this, then: >> >> http://www.jenningstech.com/pdf/rel/RF_1D_1E_1G.pdf >> >> JF > > >LOL > >Cheers > >Ian --- I just found this, which might help: http://schrackrelays.com/appnotes/app_pdfs/13c3236.pdf JF |