Switching 24 Amps

Bill Chennault

All--

Gee. I guess my idea about using a 74HC595 and a 5vdc relay to switch 12vdc at 24 amps is not too realistic. I cannot seem to find any relays that even come close.

Any ideas?

Thanks!

--Bill

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You are what you write.

Mike Green

Here's one commercial solid state relay you could use: rocky.digikey.com/WebLib/Crydom/Web%20Data/1-DC%20Series%20(D1D,%20D2D,%20D4D,%20D5D).pdf.

Bill Chennault

Mike--

If I am searching for the right part at Digikey, then the version I want runs about $55.00. Sound right?

--Bill

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You are what you write.

Mike Green

I suspect the price is right. This part is rated at 100VDC and 40A. That's more than what you need, but I didn't see anything in lower ratings that would still fit your needs. You might do your own websearch for something more like 20VDC and 25A and see if you can find something cheaper.

Bill Chennault

Mike--

Ha! I already have! Nada!

Unless this first 'bot (well, the first in 30 years . . . things have CHANGED!) is a stunning success, I am beginning to think a good old fashioned manual switch might be in order.

--Bill

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You are what you write.

metron9

Can you switch the low side? RFP30N06LE-ND $0.89 60V 30 amp

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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

Mike Green

Futurlec has a 100VDC 40A SSR for $20. That's better than $55

Phil Pilgrim (PhiPi)

If you need an optoisolated high-side driver, consider using a photovoltaic output optoisolator (as opposed to a phototransistor type) coupled with a cheap N-channel MOSFET. In this configuration, the opto output would connect between gate and source, with the drain connected to your supply voltage, and the source connected to your load (whose other end is the negative supply). As always, a diode across any inductive load is de rigeur.

-Phil

Beau Schwabe

Bill Chennault,

First thing that comes to mind something like that would be an automotive relay... If you have 12V @ 24 Amps, why not use some of that to energize a 12V relay with an NPN transistor suitable for the Relay Coil. You could still use a 74HC595 to drive the base of your transistor which in turn would drive the relay. According to the datasheet below, at 12V your transistor would need to provide 145mA to drive the relay coil with a nominal coil resistance of 83 Ohms. If you were to use the 6V coil, your transistor would need to supply 289mA to drive the relay coil with a nominal coil resistance of 21 Ohms.

This company offers a 6V, 12V, and 24V coil capable of driving 40A
http://www.citrelay.com/Relay Products.htm
http://www.citrelay.com/Catalog pages/Relays/A2 Catalog.pdf

The datasheet says that the pickup voltage is about 75% of the coil rating so a 6V coil would pickup at about 4.5V ... After the diode drop of a transistor you’re cutting it close. I would go with a 12V coil if I could.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Bill Chennault

metron9, Mike, Phil, and Beau--

Thank you for the suggestions . . . I will have to study them, for sure. It definitely looks like there is an economical solution that can be automated.

--Bill

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You are what you write.

Bill Chennault

Mike--

I am now a Futurlec customer. Not a very big one, but a customer none the less!

I wonder where I can get that heat conductive silicone grease the data sheet talks about?

Thank you!

--Bill

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You are what you write.

Bill Chennault

Mike--

Duh. JAMECO had the grease.

Thanks.

--Bill

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You are what you write.

allanlane5

Shoot, Radio Shack has that grease.

Bill Chennault

allanlane5--

I had to order some stuff from Jameco, anyway. I went to RS yesterday and the day before. Pretty soon they are going to think I am stalking them.

--Bill

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You are what you write.