relays

aj_ax24

Ok i have a logic output of 5 vdc running to a transistor and relay network.· The relay is one from a car.· That works great but i want to know how to switch a 24vac relay for 120v power.· I know how it works with normally but I don't want to damage anything in my circuit. If i need a power transistor or something that would be no problem.·

Martin Hebel

Depending on the current, and the hFE of the transistor, you may need 2 stages.

The BASIC Stamp can output 20mA. With a gain of 100, a transistor could drive a 2A relay (coil). This is quite high, but larger transistors have smaller values of hFE, such as 50 or 25 (or 5). You need to ensure the load current / minimum hFE value < 20mA.

If it's > 20mA, you need to drive a smaller transistor to drive the larger one. This is called a Darlington Pair. You can check out the Process Control text pdf for more info on this, though can't quote a page or chapter off the top of my head.

-Martin

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Martin Hebel
Southern Illinois University Carbondale - Electronic Systems Technologies

Personal Links with plenty of BASIC Stamp info
StampPlot - Graphical Data Acquisition and Control

Martin Hebel

OH
I just noticed your 120VAC comment.

Best bet is a to use a Solid-State Relay. The BASIC Stamp essentially drives an LED, that gates a Triac to allow 120V to flow. So easy and pretty inexpensive.

Search Digikey for them, many styles to choose from, starting at around $12.

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Martin Hebel
Southern Illinois University Carbondale - Electronic Systems Technologies

Personal Links with plenty of BASIC Stamp info
StampPlot - Graphical Data Acquisition and Control

Martin Hebel

* sigh *
Last note. On your relay, you really want a diode across the coil that is reverse biased, so that when on, current can't flow to it, but when the coil deenergizes, the induced current from the collapse of the coil's magnetic field has somewhere to go instead of damaging the transistor and possibly the Stamp ( applies to electro-magnetic relay, not solid state).

-out!

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Martin Hebel
Southern Illinois University Carbondale - Electronic Systems Technologies

Personal Links with plenty of BASIC Stamp info
StampPlot - Graphical Data Acquisition and Control

KatyBri

If you decide to use solid state relays, be aware they may "stick on" when you expect them to shut off. Sometimes a shunting resistor is required on the input- do your homework and check the manufacture's application notes.