12V DC Keypad to feed an 16-24V AC electric door strike?

johncritterJuly 22, 2009

Hi folks... I just realized my little project has a small snag -- two different voltages involved...

I bought a RFID Access control unit (a keypad-style device where you can swipe an Radio-Frequency ID tag near the reader to allow access into the building), and I hoped to run an electric door strike from it. The door strike normally sits in the locked position, and when juice hits it, it moves a piece of metal out of the way (I assume via magnets) to permit the door's latch to clear the door strike so people can enter.

I think normally the door strikes would be operated directly by a doorbell transformer, getting the electric only when someone inside pushes the button to "buzz" someone in. Since this is for an unattended building, I needed the Access Control unit to decide when to "push the buzzer", when a proper RFID is presented to it.

The Access Control unit runs on 12VDC, and it says its Lock Relay is 12VDC, but the electric door strike said 16-24V, and I think it is AC.

How would I hook up the two? Seems like I'd need a second transformer??

Pic of Keypad unit

Pic of door strike

Keypad specs

Keypad wiring diagrams

Any instruction on what wires on the keypad need to be connected to the strike or the strike transformer, etc would be a help... I'm getting very confused, partially because of all the extra terminals in the diagram (for bell, ground, etc), and the butchered-English the pamphlet is written in isn't helping.

Thanks for any help or suggestions!

John

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pharkus

You're right, that pamphlet's engrish is horrible, and I can barely make any sense of it.

It APPEARS that the output is an SPDT relay with its common terminal internally connected to ground. I haven't personally seen a real-life electromechanical relay whose contacts are rated at a mere 12VDC, so, if I were feeling really daring, I might tell you that it'll work fine simply using a seperate transformer and tying the grounds together.

In any case, yes, you need a seperate transformer.

Assuming the built-in relay really, truly, can't handle more than 12V (impossible, imho, unless it's not a real relay, but rather some semiconductor-based output), you also need a seperate relay.

Perhaps if you give us the manufacturer and model number of the RFID unit, we can look up more informative data.

As it stands, I'd say the best bet is to obtain a 12V-coil relay, attach its coil between "+12V" and "PUSH/NO". Then wire the external relay's COM and NO terminals in series with the door latch and its 24VAC transformer.

I don't know what's going on with the second row of terminals in "build in connection two special electrical source for access control"...

mfr/model of rfid unit...

    Bookmark   July 22, 2009 at 9:30PM
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steve_fl

Easiest way is to get an electric strike that is rated 12/24VDC (field selectable)

    Bookmark   July 24, 2009 at 6:11PM
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pharkus

I'm not convinced this "john" critter is really too dedicated to this project if he can't even give us the model number!

    Bookmark   July 24, 2009 at 7:32PM
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johncritter

Sorry guys, for the week's delay. I was on a cruise and didn't have access to a computer (at least one that didn't cost a lot per-minute). For some reason I didn't see the post from 7/22 before I left, so sorry...

It's an off-brand access control unit (although it did have most of the features I was looking for), and seems to be made by KAWA, model #MG236C.

I can buy a 12V DC door strike, since that seems like it would simplify the setup. Voltage: DC 12V, Current: 450mA (NO function -- Normally Opened -- Fail Secure).

KAWA does say about this door strike: "Please note that this electric strike can connect directly to access control unit, but we do not suggest buyer to do so because it will shorten the lifespan of your access control unit due to high current passing through the electric strike. Instead, you should connect the strike directly to an intelligent uninterruptible power supply that has NC/NO output. Make sure your UPS is "intelligent" meaning that it has NC or NO power output and can receive "door open" signal input from your access control."

Would a relay work in place of this "intelligent" UPS? (I don't need a UPS functionality in this application.)
So would the components be:
AC-DC TRANSFORMER, feeding:
ACCESS CONTROL UNIT (12V DC), feeding (via "+12V" and "PUSH/NO" terminals):
RELAY (12VDC), feeding (via "COM" and "NO" terminals):
DOOR STRIKE (12V DC, 450mA)

Would this relay be the right one: http://www.radioshack.com/product/index.jsp?productId=2062481

Thanks again for your help! I'll be sure to be checking this thread reliably now that I'm back.
Take care,
John

    Bookmark   August 3, 2009 at 1:48AM
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kensnh

No matter what the strike and reader voltages, etc are, I would cascade it through a relay. Buy a relay with coil voltage that matches the reader, I guess it's 12VDC so you must already have 12VDC available, right?

Have the relay trip whatever you want, at any allowable voltage, etc. Just be careful ,many people screw the DC/AC , you'll wreck everything. It's very easy if you go slow and check all the voltages. The readers and strikes can be expensive, so GO SLOW.

The reader can be supplied with the same transfomer a sthe stike. But be careful. You can get multi-ttap transformerhat will give you 12 and 24 VAC make sure you rectify the DC taht you need.

    Bookmark   August 4, 2009 at 9:08PM
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johncritter

Thanks for the advice...
You said "so you must already have 12VDC available, right?" -- Are you just asking if the reader is 12VDC?

I'm a little confused on the capabilities of a relay -- I'd match the coil voltage to the reader (12VDC in this case); but would the relay I'd want for an AC strike be any different than if I were using a DC strike? (i.e., can any relay handle both AC or DC, and I only have to specify the coil voltage and Amps?)
Where's a good place to get relays? Radio Shack online had some, but mostly pretty low Amps.

Thanks!

    Bookmark   August 4, 2009 at 11:18PM
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pharkus

Electromechanical (normal) relays don't care whether their contacts are used for AC or DC, to an extent. For reasons related to arc quenching, you will find the contacts rated for lower voltage DC than AC, but I haven't seen one rated any less than 24VDC yet.

    Bookmark   August 5, 2009 at 8:12PM
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johncritter

Can any relay handle BOTH AC and DC at the same time? (i.e., the Access Unit completes the circuit to the magnetic coil (via DC), which then completes the AC circuit for the strike?
They're entirely separate circuits, and the DC from the Access Unit never conflicts with the AC? Any relay of appropriate voltage can feed an AC circuit with a DC feed to the relay?

Can you recommend a good place to purchase relays?
Thanks!

    Bookmark   August 6, 2009 at 11:59PM
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pharkus

Remember in fourth grade, in science class, you wrapped a wire around a nail and hooked it to a battery, and it made a magnet that you used to pick up paperclips and such?

Imagine taking a switch, putting a spring on the handle that pulls it toward "OFF". The switch has a metal handle.

Now put that magnet you made on the "ON" end of the switch.

When you turn on the magnet, it pulls the switch "ON". When you turn off the magnet, the spring pulls the switch back to "OFF".

That's _EXACTLY_ how a normal relay works.

Stop over-analyzing things. We said it'd work. It'll work.

Try Radio Shack.

    Bookmark   August 8, 2009 at 9:17PM
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kensnh

YES,

Generally, the CONTACTS of a relay can, and usually do, control different voltages in different circuits. Almost any relay you can find will probably work in this application, IF IT HAS THE CORRECT COIL VOLTAGE. The coil is the smart side, it will be supplied, I think, 12VDC when your reader wants to. The contacts are independent and can switch any loads you want up to that relays rating, if it's only a 24VAC strike, almost any CORRECT COIL VOLTAGE electromechanical relay that is bigger than your ENTER key will work.

So, yes, Radio Shack, and get one with a socket, it's easier to rewire and troubleshoot when it gets screwed up.

    Bookmark   August 12, 2009 at 8:23PM
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