HT Relay
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From: Ian Bell on 24 Dec 2009 09:44 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? By the way I prefer a 5V coil. Cheers Ian
From: Jan Panteltje on 24 Dec 2009 10:34 On a sunny day (Thu, 24 Dec 2009 14:44:00 +0000) it happened Ian Bell <ruffrecords(a)yahoo.com> wrote in <hgvunj$ovn$3(a)localhost.localdomain>: >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? > >By the way I prefer a 5V coil. > >Cheers > >Ian 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. Most of those relays have a much lower DC rating. A simple HV MOSFET perhaps? or transistor? Else a tube... Or a real HV relay.
From: John Fields on 24 Dec 2009 11:00 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
From: Ian Bell on 24 Dec 2009 11:07 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. Cheers ian > --- > >> By the way I prefer a 5V coil. > > --- > OK > > JF
From: John Fields on 24 Dec 2009 11:34
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 |