Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
Hi,
I need to control a motor and I am thinking what to use 2 relays or h bridge
The motor( DC) would go back and forward many times . Guys, what do you think is better ?
Thanks
I think relays will be the cheapest - Any electronical solution will probably cost more then a relay for the same voltage/current setup...
But a well designed electronical solution should outlive the motor (and even you for that matter). While a relay WILL fail for sure, the question is not will it fail, but when will it fail.
Relays are simple and cheap, and if you choose plug-in ones they are VERY easily replaceable. By careful design, and spending a LOT more money, you could probably make a more reliable design using transistors or FET's.
Personally I'd stick to the relays - easy maintenance, and very low cost.
I will use cheap relay to change direction under zero current, then one big fat juicy transistor/MOSFET to power the motor circuit ON/OFF. Without any arcing/sparking your cheap relay will practically last forever.
You can get 5A power fets much cheaper than the mechanical equivalent.
Just for kicks...
assume the mechanical switches 300 times / day. Also assume that switching under load, you can expect 10^6 operations before a serious degradation or complete failure (this is not unrealistic).
At that rate, you will be on borrowed time after 9.16 years. This is one relay only, so if there are two or more in the system, one of them will fail well before 9 years. Also if the environment is adverse (hot, humid, dirty or whatever) halving that life estimate is not unrealistic.
what are you building? how many times they need to switch and what is enviroment? what is life expectancy for your project?
relays aren't that bad. while they do wear out while solid state device don't, people are usually underestimating relays since they are mechanical devices. unlike transistors, relays are way more robust when it comes to short circuits (what can you do, cables brake too...).
we use relays to run motors on car seat testers etc. they switch on and off about 10 times per minute or about 2700 per shift. running up to three shifts for five years didn't need to change one yet (installed over 300 of them). that's about 2000000 cycles per relay. they are switching currents of 5-20Amp and occasional short circuit, all day long (and night)...
There is your problem. It's in the way you see things.
1 relay= around 9$
Let's say 3 spares or more=27$ +
Allready 27$ has your FET's or transistors paid for.
You need 4 x9$ =36$ + your spares, + downtime to replace your wonderful relays.
I know that if relays need to be replaced so often, that I have to stock spares, then automatically I would look for an alternative to remedy that problem, especially since you are designing a new system, and also controlling DC power.
I mostly use relays to isolate a circuit when controlling AC power(motors ect...)
Recently I have used triacs to get away from relays.
I know , that sooner or later when power, cost and apllications demand,I'll be back to the relays.
FETs will outlast any low amp relay and create very little or no noise.
Transistors are reliable and affordable. Much less then 9$ each, and could last a life time if properly setup.Add to this a couple diodes, and resistors ect...
If you have many boards to make, I would strongly suggest full electronic control for the simple fact of "reliability".
Furthermore, power consumption is minimal when you go full electronic.
Is PWM in near future???
In anycase, it's up to you.
I don't mean to minimize anything that has been suggested to you by the other members, but in the grand scheme of things, 24V----5amps---DC---, it's a moderate amount of power.
Just to add my two cents to the relay option, high reliability relays made just for this purpose are used in the millions in cars everyday. There are relays made just for H-Bridge operation in very small robust packages, with all the H-bridge wiring done internally.
These relays operate the electric seats, windows, mirrors, vents, etc. Bosch, Fujitsu, NEC, Panasonic, Aromat, etc, all make these models. Even in these days of complex electronics and the engineering resources of the car manufacturers, they still choose relays for reliability and cost.
Yes, it can do without PWM, it ALLOWS for pwm if you want...
You can control it with a PLC, just use a resistor network to Lower the PLC's output voltage from 24V to 5V.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
