From: John Larkin
Subject: Re: What makes coils AC or DC??
Date: Sun, 12 Jan 2003 13:50:17 -0800
Organization: Posted via Supernews, http://www.supernews.com
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On Sun, 12 Jan 2003 16:25:58 -0500, "Neil"
>Ok, I should have paid more attention in electronics class, but I'm 42 now,
>and that was a long time ago.
>What exactly makes a coil AC., or DC ?.
>Can a 24VDC solenoid be used with 24VAC?. The same question goes for a relay
>with a DC. coil. Can the relay be driven with the same voltage, only AC..
Generally, no. A solenoid or relay designed for DC drive will have a
higher impedance at AC, so won't pull as hard. Plus, it will buzz as
the AC periodically crosses through zero, and may overheat because of
eddy-current losses in their (usually) solid cores.
AC coils have laminated cores, the proper number of coil turns, and
often have a 'shading ring' phase shifter to reduce buzzing. AC
solenids are often more reliable than DC; this is because they can be
designed to have a very high pull-in current but a low seated current,
so they yank hard but idle at lower current, thus don't get as hot as
an equivalent DC device.
>If I needed to drive a 24VDC. solenoid on 24VAC., could I do so just by
>putting in a high power bridge rectifier before the coil, to change the AC.
That usually works. Keep in mind that 24 volts AC, passed through a
full-wave bridge rectifier, gives an average DC equivalent voltage of
about 19 volts. If a big filter capacitor is also used, the equivalent
DC will generally be above 24, typically in the high 20's.
>The reason I asked is that we purchase original 24VAC
>solenoid coils from our OEM, for a pretty high price. But today I was down
>at Ye Olde Surplus Shoppe, and they had the same coils, only in 24VDC. for
>only a buck.
>So can I use them??.
Maybe; experiment to make sure that they pull adequately and don't
overheat. I'd try a bridge rectifier and play with the filter
capacitor value if necessary to get the results you need.