The New Generation Of 12-Volt DC Relays

You mentioned the Opto-isolator

For my digital adapter for my ham radio I wished to completely DC isolate the Computer and radio side.

Thus both transmit and receive audio pass through transformres

But what of the keying line (PTT)? Opto isolator (home made) consisting of an I/R LED and an I/R Photo Transistor and a LED indicaator and a switch

Circuit as follows

Computer RS-232 out (Via FTDI USB Adapter) to LED assembley and I/R diode (This way I have a visual indication of when the computer wants to talk)

Switch in series with Opto-transistor to Transmitter KEY line and ground.

This way I can DISABLE the computers ability to key the rig (Safety/testing feature).

Works great.

dclark1946 wrote:

Salvo wrote:
They do have some not-so-great specs:

On resistance: as high as 25 mohm

Off leakage current: 2 mA

Given time, the leakage current can run down the battery.

The on resistance of a mechanical relay may be 10x better.

Can you still get latching relays that have all the low contact resistance advantages and only draw power when changing state?

Dick

Yes, one common RV application is for the salesman switch. ie A small switch by the door connects/disconnects some/most/all house battery loads. The latching solenoid itself which handles the heavy loads is usually located elsewhere.

Salvo wrote:
They do have some not-so-great specs:

On resistance: as high as 25 mohm

Off leakage current: 2 mA

Given time, the leakage current can run down the battery.

The on resistance of a mechanical relay may be 10x better.

Can you still get latching relays that have all the low contact resistance advantages and only draw power when changing state?

Dick

Here's what I use in our motorhome to automatically connect the atternator to the coach batteries. I replaced the El Cheapo one Winnebago built in with one of these 12 volt continuos duty models with silver alloy contacts for long term life with very little contact corrosion or pitting:

http://shopping.murcal.com/core/media/media.nl?id=24366&c=650659&h=e837048320f960d2122e&_xt=.pdf

They do have some not-so-great specs:

On resistance: as high as 25 mohm

Off leakage current: 2 mA

Given time, the leakage current can run down the battery.

The on resistance of a mechanical relay may be 10x better.

Checked what Crydom has to offer? Been using those for years.

HTElectrical wrote:
100a

That one is for switching AC. Maybe a DC one will become available?

A proper size heat sink bonded by heatsink thermal paste is absolutely necessary. One of the links is to a $2.95 heatsink from China.

The solid state relays contain a optocoupler for ABSOLUTE isolation between load and switch. A good size MOSFET transistor, some small capacitors, resistors, and a freewheeling diode. Unlike an NPN MOSFET device, the solid state DC relays are connected to the POSITIVE or NEGATIVE side of a load to be switched.

If I get a chance I'll do a write-up with links how to do a suppressor circuit for these relays that protects against transient voltage spike damage.

There are rumbles coming from the China manufacturers that a 100-amp version is in the works. It'll need quite the heat sink.

I used to use rubber encapsulated mercury filled relays. They were shockingly expensive but resisted burning of the contacts. The arcing takes place when the contacts part not when they engage.

These relays switch so fast I am using them for 90 Hz LED strobe feeds.

I leave black anodized aluminum heatsink mounting surfaces alone but with shiny aluminum heatsinks and mirror-like bottoms of solid state relays I scuff them with 400 sandpaper. Arctic Silver SILVER heatsink compound is another very useful thing to employ to get the heat away from the device and into the heatsink.

I would tend to discourage the use of a solid state relay in an engine starter motor solenoid circuit. Too many transients and too little benefit over a stout Ford starter motor fender mounted solenoid. Actually a relay but no one calls them that.

I wish I new about these last year, I had to replace battery relays for over $20 apiece. When I opened them up the contacts were burned. I assume there are diodes inside that do the switching?