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Old January 20th, 2013, 09:14 AM   #1
mikas_m
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Using coils with different voltages AC vs DC

Hello to all,

by mistake we ordered and got delivered el. magnetic valve with AC 24V coil but we have only 24 V DC voltage. I know that if we try to connect DC voltage to AC coil it is very probable that we'll burn out the windings. I know that using resistor is also a bit tricky because in that case we cannot be sure if coil will work always as intended.

I was thinking to try something like this:
I'll measure ohm resistance of this AC coil and use resistor of about the same resistance in series. This way I'll limit the current that can damage the coil. And then I would connect electrolytic capacitor in parallel with this resistance. My idea is that during initial loading higher current will pass to the coil causing it to react and then capacitor will block the DC current so all current will flow through resistor. When 24 VDC disappears, capacitor will burn its energy on resistor and AC coil will de-energize.

Will this work? Is here any danger to damage something?

What do you think about this improvisation?

I need to say that due to the limit time we cannot wait for DC coil to be delivered.

Thank you
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Old January 20th, 2013, 09:27 AM   #2
PeterW
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Personally I'd get the right solenoids.
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Old January 20th, 2013, 09:34 AM   #3
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sounds a lot easier to get the right stuff though.
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Last edited by randomFire; January 20th, 2013 at 10:12 AM.
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Old January 20th, 2013, 10:08 AM   #4
milldrone
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Getting the right solenoid coil would be the right thing to do, but if it's not possible. You can try this method.

1. Measure the amps the coil uses when connected to AC and the armature is sealed or seated.

2. Use Ohms law to determine the correct resistor (resistance and wattage) to put in series with the coil to get the current to the level it would draw when connected to a AC current supply.

3. Use two DC outputs. One with a resistor in series and one without.

4. When first energizing use both outputs, after 1/2 second use the output with the resistor only.

5. I should also add that this will only work if the outputs have the current capacity to energize the coil.
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Last edited by milldrone; January 20th, 2013 at 10:29 AM.
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Old January 20th, 2013, 10:55 AM   #5
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Hi,

You should be aware because resistor will be heated
Except your idea and colleagues,
If you can make one dc/dc steep-down 24Vdc/12Vdc it will work
Depending on the power of solenoid you can use one LM7812
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Old January 20th, 2013, 01:24 PM   #6
Mickey
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Get a cheap 24 volt AC power supply and a slave relay(24 volt DC coil) controlled by the PLC. Use contacts on the slave relay to control the 24 volts AC.

Surge protection on all loads.
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Last edited by Mickey; January 20th, 2013 at 02:16 PM.
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Old January 20th, 2013, 03:39 PM   #7
mikas_m
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Thank you all for the answers. I'll try this with 12 V.

This suggestion with two outputs is basically what I'm trying to achieve with capacitor.

Quote:
Originally Posted by Mickey View Post
Get a cheap 24 volt AC power supply and a slave relay(24 volt DC coil) controlled by the PLC. Use contacts on the slave relay to control the 24 volts AC.

Surge protection on all loads.
Mickey what do you think about this surge protection on all loads. Can you give me example?

Thank you
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Old January 20th, 2013, 04:35 PM   #8
Mickey
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Quote:
Originally Posted by mikas_m View Post

Mickey what do you think about this surge protection on all loads. Can you give me example?

Thank you
Sure, see PDF
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File Type: pdf surgeprotection.pdf (274.8 KB, 35 views)
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Old January 21st, 2013, 08:19 AM   #9
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mickey is best with relays (i have a few boxes full of them for sale.
another way is to measure the current on one coil when running on 24 AC.
now use a DC power supply to have same current.
that does the trick.
The power to the outputcontacts is hopefully isolated from the PLC so you can put another psu on it.
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Old January 21st, 2013, 10:21 PM   #10
DickDV
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There is no guarantee that 12VDC will be the correct voltage. I would use a series resistor starting with too much resistance and reducing it until you get a solid pickup. Then watch for excessive heat in the coil. If heating is ok, you've got it. If too much heat, your cap scheme or the two outputs would be good.

You are lucky that the mistake didn't go the other way---you have AC and the coil came in DC. That is not "fixable" without changing out the coil.
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Old January 22nd, 2013, 02:38 AM   #11
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dick wrong it is good possible with a diode bridge.very simple very cheap.
many coils do have this already inside so they can be used in AC and DC
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Old January 22nd, 2013, 08:00 AM   #12
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I should have said "not fixable without changing the coil or adding additional external equipment" such as a bridge rectifier.
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