24 vdc relay on 24 vac
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bensonelectric
Senior Member
Re: 24 vdc relay on 24 vac
Yes, your relay wont function correctly, as it will continually open and close with the cycliing of the AC current. Get a 24vac relay or build a rectifier for the DC relay if you must use the one you have.
Yes, your relay wont function correctly, as it will continually open and close with the cycliing of the AC current. Get a 24vac relay or build a rectifier for the DC relay if you must use the one you have.
Re: 24 vdc relay on 24 vac
Remember that the core of an AC and DC relay are different. It's not just the coil. With DC you simply need to magnetize the core in one magnetic direction to pull the armature. No hystereses, no circulating current within the core.
With AC, those laminations that the core consists of deal with the alternating magnetic field which results in a constantly reversing magnetic field, hystereses, which result in heating which is not an issue with DC. Applying an relay with an AC coil on DC is much more feasible. Also, don't forget that with DC the current fow in the coil is determined by the coil resistance. With ac AC coin the current flow on the AC current is determined by the counter EMF developed by the coil and the core and not just simply the coil resistance.
Remember that the core of an AC and DC relay are different. It's not just the coil. With DC you simply need to magnetize the core in one magnetic direction to pull the armature. No hystereses, no circulating current within the core.
With AC, those laminations that the core consists of deal with the alternating magnetic field which results in a constantly reversing magnetic field, hystereses, which result in heating which is not an issue with DC. Applying an relay with an AC coil on DC is much more feasible. Also, don't forget that with DC the current fow in the coil is determined by the coil resistance. With ac AC coin the current flow on the AC current is determined by the counter EMF developed by the coil and the core and not just simply the coil resistance.
physis
Senior Member
Re: 24 vdc relay on 24 vac
But that would be a bad idea in this case because the voltage wouldn't be RMS anymore. It would be peak.
24 x 1.414 = right about 34 volts. Hot coil!
Frank,a rectifier on an AC source is a dc supply. Someone's gonna probably want to correct me because it wont make "real" DC. If you add a capacitor (they're usually called filter capacitors when used like this) in parallel with the DC you can get much closer to a flat line (DC).
But that would be a bad idea in this case because the voltage wouldn't be RMS anymore. It would be peak.
24 x 1.414 = right about 34 volts. Hot coil!
Re: 24 vdc relay on 24 vac
I had an application challenge with and MCC where the customer wanted DC power for a brake. I had to go back to the basics and figure what the unfiltered DC voltage would be from a given AC input source. I selected a properly size transformer with a given secondary voltage when rectified would provide the DC voltage that I wanted. I used one of those puck shaped bridge rectifiers with a high P-P voltage rating and plenty of capacity so it would never fail. It was mounted directly to the back of the bucket which acted as a great heat sink.
It worked excellent as well as being reliable.
I had an application challenge with and MCC where the customer wanted DC power for a brake. I had to go back to the basics and figure what the unfiltered DC voltage would be from a given AC input source. I selected a properly size transformer with a given secondary voltage when rectified would provide the DC voltage that I wanted. I used one of those puck shaped bridge rectifiers with a high P-P voltage rating and plenty of capacity so it would never fail. It was mounted directly to the back of the bucket which acted as a great heat sink.
It worked excellent as well as being reliable.
physis
Senior Member
Re: 24 vdc relay on 24 vac
What usually kills rectifiers is voltage in the opposite direction that it conducts in. The PIV or peak inverse voltage rating is how much of that it can take. The forward direction you're more concerned with current.
What people don't usually realize is that there's more to be aware than operating voltage. A really fast little voltage spike at five or seven hundred volts can take out a 400 v. PIV rectifier real easy. And those spikes aren't uncommon. For a 120 v. application I would usually use a 1000v. PIV. That number might sound big but it's only a little high for a lot of rectifiers.
What usually kills rectifiers is voltage in the opposite direction that it conducts in. The PIV or peak inverse voltage rating is how much of that it can take. The forward direction you're more concerned with current.
What people don't usually realize is that there's more to be aware than operating voltage. A really fast little voltage spike at five or seven hundred volts can take out a 400 v. PIV rectifier real easy. And those spikes aren't uncommon. For a 120 v. application I would usually use a 1000v. PIV. That number might sound big but it's only a little high for a lot of rectifiers.
bensonelectric
Senior Member
Re: 24 vdc relay on 24 vac
[ July 24, 2005, 12:43 AM: Message edited by: bensonelectric ]
True, however you could still build a small recifier (single inline case full-wave), voltage regulator (24v) and filter circut for under 2 bucks if you really had to use that relay. All those parts including a small wirewrap board can be purchased from a local electronics warehouse, or if you must, a local Ripoff Shack, I mean Radio Shack should have all those parts in stock.
[ July 24, 2005, 12:43 AM: Message edited by: bensonelectric ]
bensonelectric
Senior Member
Re: 24 vdc relay on 24 vac
Wow, I clicked the quote button instead of the edit button and quoted my own post. Sorry guys about the double.
[ July 24, 2005, 12:42 AM: Message edited by: bensonelectric ]
Wow, I clicked the quote button instead of the edit button and quoted my own post. Sorry guys about the double.
[ July 24, 2005, 12:42 AM: Message edited by: bensonelectric ]
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
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- Electrical Contractor
Re: 24 vdc relay on 24 vac
I've done that! :roll:
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Re: 24 vdc relay on 24 vac
Originally posted by LarryFine:
I've done that!
physis
Senior Member
Re: 24 vdc relay on 24 vac
You can build a minimal regulated supply with only three parts and it'll work just fine, for a lot of things.
The trouble is though that semiconductors and inductive loads really don't get along very well. The back emf you get when you disengage an inductive load is bad news for transistor type stuff. They hate backwards voltage. That being the case, if you want to drive a relay with a regulator you'll want to take some steps to protect the thing. So much for the three component regulated supply.
But it's not that bad either. A few capacitors and a diode and it it'll have a good chance of working forever.
The most important thing if you're driving a coil directly with a semiconductor is a clamping diode. It goes across the output reverse biased. The supply doesn't see it but when the coil starts spitting backwards emf into the supply the diode is forward biased and the back emf is shorted to ground. The diode really doesn't care because for it it's forward current.
You'll also want some small value capacitors to shunt the higher frequency garbage the coil's gonna make, the diode's a little slow and doesn't get everything.
[ July 25, 2005, 06:17 PM: Message edited by: physis ]
I'm not so sure about under two bucks. Most of the consumer type electronics joints I know of everything is packaged for $2.50 and up.
You can build a minimal regulated supply with only three parts and it'll work just fine, for a lot of things.
The trouble is though that semiconductors and inductive loads really don't get along very well. The back emf you get when you disengage an inductive load is bad news for transistor type stuff. They hate backwards voltage. That being the case, if you want to drive a relay with a regulator you'll want to take some steps to protect the thing. So much for the three component regulated supply.
But it's not that bad either. A few capacitors and a diode and it it'll have a good chance of working forever.
The most important thing if you're driving a coil directly with a semiconductor is a clamping diode. It goes across the output reverse biased. The supply doesn't see it but when the coil starts spitting backwards emf into the supply the diode is forward biased and the back emf is shorted to ground. The diode really doesn't care because for it it's forward current.
You'll also want some small value capacitors to shunt the higher frequency garbage the coil's gonna make, the diode's a little slow and doesn't get everything.
[ July 25, 2005, 06:17 PM: Message edited by: physis ]
Re: 24 vdc relay on 24 vac
I think you will find that most (if not all) AC relay's are simply a DC relay with a diode or diode bridge built in. I think there is a good chance if you put a AC relay on DC, it would work OK. But it wouldn't be a good idea because of the diffenences in DC an rectified AC voltages that Sam mentioned.
Steve
I think you will find that most (if not all) AC relay's are simply a DC relay with a diode or diode bridge built in. I think there is a good chance if you put a AC relay on DC, it would work OK. But it wouldn't be a good idea because of the diffenences in DC an rectified AC voltages that Sam mentioned.
Steve
Re: 24 vdc relay on 24 vac
For large AC contactors and starters, usually size #5 and larger, it is common to have a DC coil with a rectifier. The design even goes in so far is to use a economizing circuit to go from full to half wave DC after the armature is pulled in.
However, while assigned as an application engineer for a major electrcal manufacturer non of the smaller size contactors and starters as well as control relays never employed a rectifier in the coil circuit.
For large AC contactors and starters, usually size #5 and larger, it is common to have a DC coil with a rectifier. The design even goes in so far is to use a economizing circuit to go from full to half wave DC after the armature is pulled in.
However, while assigned as an application engineer for a major electrcal manufacturer non of the smaller size contactors and starters as well as control relays never employed a rectifier in the coil circuit.
physis
Senior Member
Re: 24 vdc relay on 24 vac
I agree with templdl about the core difference. I don't know a lot about the magnetic behaviour of relay coils (something else I'm learning here) but my experience has been that a DC relay will work with tha same AC voltage but it's
likely to buzz.
If what I gather is right the armeture wont be attracted as efficiently and the coil will actually operate hotter.
I'm just making an underinformed guess.
Something else that might be of interest, I've seen a whole lot of relays fail mechanically rather than a melted or open coil. Wear at the area where the armeture pivots is one reason. If a relay's going to stop working because of that it'll happen sooner with an AC driven DC coil.
There's two reasons. The buzzing from the AC could all by itself be responsible for the wear. The other is that the reduced effeffiency leaves the coil unable to attract the worn armeture sooner.
I'm guessing you could cut a relay's life expectancy in half. That depends a lot on design too of course though.
Edit: Error B
[ July 25, 2005, 04:29 PM: Message edited by: physis ]
I agree with templdl about the core difference. I don't know a lot about the magnetic behaviour of relay coils (something else I'm learning here) but my experience has been that a DC relay will work with tha same AC voltage but it's
likely to buzz.
If what I gather is right the armeture wont be attracted as efficiently and the coil will actually operate hotter.
I'm just making an underinformed guess.
Something else that might be of interest, I've seen a whole lot of relays fail mechanically rather than a melted or open coil. Wear at the area where the armeture pivots is one reason. If a relay's going to stop working because of that it'll happen sooner with an AC driven DC coil.
There's two reasons. The buzzing from the AC could all by itself be responsible for the wear. The other is that the reduced effeffiency leaves the coil unable to attract the worn armeture sooner.
I'm guessing you could cut a relay's life expectancy in half. That depends a lot on design too of course though.
Edit: Error B
[ July 25, 2005, 04:29 PM: Message edited by: physis ]
bensonelectric
Senior Member
Re: 24 vdc relay on 24 vac
A local electronics shop here, PACELEC, +24v 1amp regulators for $.50, 1200?f caps for $.05, and full wave rectifiers for $.60. Add on a prototyping board for $.75 and you got yourself a simple AC-DC regulated power supply. I realize that shops like this, that will sell individual parts for that cheap are rare nowdays. Too bad radio shack can't be like it was in the 70's...
A local electronics shop here, PACELEC, +24v 1amp regulators for $.50, 1200?f caps for $.05, and full wave rectifiers for $.60. Add on a prototyping board for $.75 and you got yourself a simple AC-DC regulated power supply. I realize that shops like this, that will sell individual parts for that cheap are rare nowdays. Too bad radio shack can't be like it was in the 70's...
physis
Senior Member
Re: 24 vdc relay on 24 vac
That's really cool Matt. That's what that stuff should cost. The seven dollar board I have posted is out of line but all the stuff I listed comes from one place that way. I don't know of a place around me where a regular guy can get this stuff without paying through the nose.
Edit: 1200 uf, a nickle. That's cheap!
Edit: That's not normal cheap.
[ July 26, 2005, 03:49 AM: Message edited by: physis ]
That's really cool Matt. That's what that stuff should cost. The seven dollar board I have posted is out of line but all the stuff I listed comes from one place that way. I don't know of a place around me where a regular guy can get this stuff without paying through the nose.
Edit: 1200 uf, a nickle. That's cheap!
Edit: That's not normal cheap.
[ July 26, 2005, 03:49 AM: Message edited by: physis ]
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