Protection diodes in relay applications

Professorwiz

Hi all,
I'm pretty new to electronics.· I've been told by many people (including the instructor of an electronics course I'm taking) to use a diode to protect my chips from the reverse voltage from a relay.· They also tell me to use a fast one, but I've not heard any specific recommendations.· I want to use my SX chip to run a relay, small pc board mount one, does anyone have any kind of an idea what one I should start with?· Or perhaps some specs?

Thanks,
Russ

Twisted Pair

Professorwiz,
If your relay is a mechanical one, a 1N4148 works well for me although others like the 1N4001 or 1N4004 for higher currents. Solid state relays normally dont require a diode....Radio Shack should have these....

Twisted Pair....

JonnyMac

Do use a diode, but also drive the relay through a simple transistor buffer; this will minimize the source/sink current on the pin and put another device between your relay coil and the SX.

Twisted Pair

Good point JonnyMac....I like the 2N2222 or the 2N3904....

Twisted Pair....

RobotWorkshop

Hello,

I think you'll find just what you're looking for in one of the documents on the Parallax website. It works great with the SX28 chip!

http://www.parallax.com/Portals/0/Downloads/docs/cols/nv/vol1/col/nv6.pdf

Hope this helps,

Robert

QuattroRS4

I use 1N4007 diodes in reverse bias - as its Vrrm - peak repetitive reverse voltage is 1000v -

Twisted Pair - a 1N4001 - is not really suitable for this purpose with a Vrrm of only 50v !!

I have attached a data sheet ...

Regards,
John Twomey

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'Necessity is the mother of invention'

kjennejohn

Hi.
Some other sugestions:
- Use a driver IC like the ULN2803. These are eight Darlingtons in a single chip. Each will handle a five hundred mA. They also have the protective diode built in.
www.st.com/stonline/books/pdf/docs/1536.pdf for the -2803.

- Get a relay with the diode built in.
Here's a page from Digi-Key: search.digikey.com/scripts/DkSearch/dksus.dll
This page is a list of small signal relays that contain the suppression diode. These are various contact/power combos.

Happy hunting! 'Luck on your project!
kenjj

Twisted Pair

Roger That QuattroRS4,
Just thought that a nudge in the right direction would help but it turns out that I learned something as well. Thanks for the help. Speaking of help QuattroRS4, can you take a look at my recent post regarding a source for servo pots ? I need Help. I've been pulling hair out trying to locate one of these....I'm almost bald now. Sorry for invading your post Professorwiz....

Twisted Pair....

kjennejohn

Nuts. That last link for a page of relays is a bust, it doesn't take you to a menu. It is a starting point, though. Find 'relays' farther down the list and try your luck with Digi-Keys selection system. You're looking for all small signal relays with the diode builtin. Perhaps your instructor can help.

Well, maybe next time. *sigh*
kenjj

Professorwiz

Thanks All for your help!

Johnnymac, that does simplify things for me, using a transistor.· I've just bought the incorrect relays - 12v but have a whole reel of 3904's so I'll just go 12v and use the transistors and diodes.·

Robotworkshop, thanks, I just never am able to find exactly what I'm looking for in the huge amount of info on the web site here!· Not that it's a bad thing.·

Thanks again.

Russ

StampNut2

Professorwiz: Check out my web site for tutorial robosapienv2-4mem8.page.tl/Components-.htm This may also help you, Has a relay section.

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Never give up when things go wrong.

robosapienv2-4mem8.page.tl/