# Question on 24v Mac Valves?



## mroct31 (Nov 10, 2007)

I have 2 for a prop that are MAC 24v 5.4watts and I purchased a couple of 24v 450mah wall transformers for them but when I connect it all up all I get is a red LED on the MAC valve but no valve operation going on. I'm wondering if these wall warts are not powerful enough?


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## bradbaum (Jul 26, 2008)

Your wall warts are rated in milliamp hours? Normally this is a battery rating saying that the battery can supply 450 miiliamps for 1 hour.

if they are indeed just milliamps then ohms law should serve:

P/E=I or 5.4W / 24Volts = .225 Amps (or 225 milliamps)

your wall warts are rated above that so they should be good to go.

I would double check that both the valves and the wall warts are DC (or AC) doesn't matter as long as they match.


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## mroct31 (Nov 10, 2007)

That's it! The valves are 24VDC and the wall warts are AC! DOH!


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## bradbaum (Jul 26, 2008)

You can use a full wave bridge rectifier to convert the AC to DC:

http://www.radioshack.com/product/index.jsp?productId=2062581


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## mroct31 (Nov 10, 2007)

So from what I can tell you run the 2 wires from the wall wart to 2 "legs"( I'm guessing there are 2 + and 2-'s ) on the rectifier then "continue" the wires from the other 2 "legs" to the the valve?


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## pshort (May 6, 2008)

The peak output voltage from using a 24VAC transformer and bridge rectifier is around 35V, which hopefully won't damage the valves.

In any case, run the two output leads from the transformer to the two legs of the bridge rectifier that are labelled '~' (or perhaps AC), and the connect the + and - leads from from the bridge to the + and - leads of the valves.


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## hpropman (Jul 27, 2008)

I would also solder a 47uF electrolytic capacitor across the + and the - lugs of the bridge. Note the cap has a negative side and a positive side make sure they match the bridge rectifier + to + and - to - (the cap will normally have an arrow with a - sign pointing to the neg lead) after soldering clip off any excess leads from the cap. This might help to identify:






Please PM me if you have any questions.


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## pshort (May 6, 2008)

@hpropman,

I somewhat disagree with you on the cap. There will still be ripple on the output of the rectifier with that cap in place, but the cap will serve to increase the RMS voltage to somewhat above 24V. While I'm not familiar with the MAC valve, that voltage is high enough that I fear that the valve will be damaged (or end up with a short life span).

His best bet, I think, is to get a DC supply with the correct ratings.


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## bradbaum (Jul 26, 2008)

I use unregulated DC wall warts on these valves all the time and have never had a problem.

An unregulated DC wall wart is just a transformer and a full wave bridge rectifier, no cap, or anything else.

you have to realize the valve is just a coil of wire, it really doesn't care if the voltage is a little high or low or has some ripple.


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## pshort (May 6, 2008)

In that case, I'd still skip the cap. The only halfway useful purpose that it serves is the decrease the ripple. It increases the effective (RMS) voltage to something quite a bit above 24V (not just a little), so leave it off and save yourself the trouble and expense.


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## 4nntt (Nov 28, 2006)

Without the cap though, wouldn't the voltage drop to near 0v, which might cause the valve to open/close rapidly? ...unless the coil in the solenoid smooths out the voltage variances.... ?


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## bradbaum (Jul 26, 2008)

Yes, it is a pulsating DC, but it is pulsating so rapidly the coil sees it as pure DC.

The PWM speed controls for motors work in a similar fashion - by varing the off time between the DC pulses the motor's speed is controlled - but the motor doesn't stop between pulses.

Another reason to leave the cap out is reverse EMF from the coil, If you leave the cap out of the circuit there is nothing to destroy with the reverse EMF generated when the valve de-energizes.


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## mroct31 (Nov 10, 2007)

I'm going to get the correct wall warts as I can return 2 of the 3 AC ones I purchased as they aren't cut up so I'm only out $5 with the one I did cut. Moral of the story for me is to pay more attention when shopping for electronics!


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