why is everyone so bias againsed relays o.0

[mashersmasher]

i know, relays are a bit retro for most people but they have a few advantages for high voltage/current applications. i like using relays for drive motor control since it lets me use pretty much whatever power sources i have on hand whithout the need to regulate it (except for the electronics which you would probably need to do anyway) and the kind you stick in cars often take pretty kindly to being flicked on and off in pulses. the only downside is that they have a lower reaction speed (seing as they're mechanical) and can't pulse as fast as a transistor. i use car relays with 110 ac with no problem all the time and they work great.

 

[Krumlink]

You posted this in the wrong forum.

Most relays cannot go above 50 hz

 

[mashersmasher]

well i was more meaning in regards to robots. i'm sure people use relays for other things but as soon as i mention relays on some robot forums people go nuts.

 

[Sceadwian]

Two words, "Mechanical failure"
Solid state relays and h-bridges while making circuits a little more complicated enable you to do a lot more as far as controlling the current your load is getting. For high voltage/current applications I'd go with a solid state circuit any time as the chances of contact bouncing or hot switching a relay can weld the contacts together, modern solid state power handling parts are cheaper and have very wide operating conditions.

 

[kchriste]

I guess because they are clunky and can't be PWM'd very well for motor drive control circuits. They tend to wear out quickly compared to a transistor/FET solution. But they do have their place. They are very simple to understand and use for the hobbyist so if that's what floats your boat, go ahead and use them without shame.

 

[dknguyen]

No speed control and the contact current tends to be proportional to coil current. And they wear out. That is why I don't use them.

 

[Nigel Goodwin]

Well I love relays - for robotics it's fairly common to use a DPDT relay for reversing the motor, and a single transistor or FET for doing PWM - this is a LOT cheaper than an H-bridge, and likely to be more reliable as well.

 

[GonzoEngineer]

I call them RMD's....repulsive mechanical devices!

 

[mvs sarma]

Well I love relays - for robotics it's fairly common to use a DPDT relay for reversing the motor, and a single transistor or FET for doing PWM - this is a LOT cheaper than an H-bridge, and likely to be more reliable as well.

I feel, relays problems if their contacts are not protected properly agaist sprks created during change over times. Thus they need periodic replacements.

 

[Nigel Goodwin]

I feel, relays problems if their contacts are not protected properly agaist sprks created during change over times. Thus they need periodic replacements.

Cars are full of relays, yet I've never occasion to replace one - relays are really pretty reliable devices. I would expect a relay in a decent size robot to be more reliable than an H-Bridge, and cost a lot less as well.

 

[ericgibbs]

I feel, relays problems if their contacts are not protected properly agaist sprks created during change over times. Thus they need periodic replacements.

Hi Sarma,
If the correct relay type is used for the application and the contacts have 'snubbers', they can give reliable service for years.

 

[mvs sarma]

Hi Sarma,
If the correct relay type is used for the application and the contacts have 'snubbers', they can give reliable service for years.

Totally agreed Eric. After having worked and played with relays of kinds in telecom, I can't say that I am avert for relays. Incidentally, relays used in low signal paths like speech signals, needed wetting currents for their contacts to behave. But as you rightly put it, the snubbers are the main issue. The snubbers too need periodical check and replacement, may be after few years.
The proper relay would have been chosen for the job except when a bargain had to be made due to economy.
Now solid state relays (call them H bridges in some cases) have come and they help reducing the size and these offer better performance, except when they face spikes of kinds. Perhaps one has to adopt and benefit from the modernization.

 

[b_reagle]

No speed control and the contact current tends to be proportional to coil current. And they wear out. That is why I don't use them.

Why can't you have speed control? If you use a DPDT with the negative(common) to a transistor you could have it pulse anyway you want.

 

[Nigel Goodwin]

Why can't you have speed control? If you use a DPDT with the negative(common) to a transistor you could have it pulse anyway you want.

Just like I said above, it's common practice in decent size robots to do that, saving cost, size, and making it more reliable.

Now solid state relays (call them H bridges in some cases) have come and they help reducing the size

Not at all, a decent size H-bridge is MUCH larger than a relay, it's only tiny sized ones where an h-bridge is smaller.

 

[b_reagle]

What do you suggest for non"decent sized robots"?

 

[Nigel Goodwin]

What do you suggest for non"decent sized robots"?

Depends how small it is, if it can be fed from a single IC then an h-bridge is obviously a good idea - but only very small motors can be fed from them.

 

[b_reagle]

So the better way to do it small is h-bridge, if motor current is small? What is the limit (mA) of an h-bridge with single IC?

 

[mvs sarma]

..............
Not at all, a decent size H-bridge is MUCH larger than a relay, it's only tiny sized ones where an h-bridge is smaller.

Thanks Nigel I got the H-Bridge vs h-bridge.