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