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help with tl431 & relay

Some days ago, a member suggested this following circuit(only a part shown) for switching between a battery and an extrnal pwr supply. However as soon as the battery voltage starts to drop to the trip value, the relay coil starts ringing. I think whats happening is that since both the relay coil and load are being powered from the battery, as soon as the trip value is reached, the relay de-energizes disconnecting the load, but in doing so, the voltage on the lead-acid battery drifts back up, so the relay energizes again. Can you guys suggest someway to get around this. I was thinking of using a latching relay, but that wont stop the ringing of the coil. Ry1-1 | |---+ +-----+----+-------+ | D1 | | | | | | [D2] [Ry1] | | | a| | [LOAD] +| | +----+ | --- / | | - 50K \\ --- | --- /

Reply to
mookiewookie
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...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

In my opinion you are going about this the wrong way. In your previous posts on this subject others have replied that you don't really need a battery changeover circuit at all.

What you need is a 12V battery charger for a lead acid battery which FLOAT CHARGES the battery (at 12V plus margin), which is permanently connected to the load. When the ac mains supply fails the battery simply continues to supply the load.

This is exactly the way that the Telco's do it in their switching centers.

You may also want a battery protection facility to prevent the lead acid battery from being fully discharged during long periods of mains outage so that you don't destroy your battery.

Given your apparent lack of electronics knowledge I would suggest you buy something off the shelf (approx $100).

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Reply to
Ross Herbert

That is not possible. Once the relay disconnects the load, it also disconnects the battery, so that even if battery voltage rises, the TL431 can't "see" it. But your schematic is a bit scrambled, so that might not be obvious. The 50K in your drawing is connected to the wrong side of the relay contact. I'll re-draw it for you: Ry1-1 | |---+ +-----+----+-------+ | D1 | | | | | | [D2] [Ry1] | | | a| | [LOAD] +| | +----+ | --- / | | - 50K \\ --- | --- /

Reply to
ehsjr

Hmmm - my diagram was more messed up than the original. Let me try again:

Ry1-1 | |---+ +-----+----+-------+ | D1 | | | | | | [D2] [Ry1] | | | a| | [LOAD] +| | +----+ | --- / | | - 50K \\ --- | --- /

Reply to
ehsjr

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