Author Topic: H-Bridges , Relays, and MOSFETs need input  (Read 2576 times)

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Offline silo_xtremeTopic starter

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H-Bridges , Relays, and MOSFETs need input
« on: July 18, 2007, 05:09:15 PM »
Hi Everyone,

I have read some great write ups about H-Bridges found here:

http://www.societyofrobots.com/schematics_h-bridgedes.shtml
http://www.dprg.org/tutorials/1998-04a/

Now these tutorials talk about MOSFETs so i have a good understanding about them.  The question is what about Solid State Relays, and H-Bridge Chips?  What is the difference (or is it possible) to use SS Relays instead of MOSFETs for an H-Bridge?  Can you use multiple H-Bridge chips to handle higher current?

My motors are going to operate at 24V @ 20A and stall is 70A. So needless to say, i don't want to spend $100's on a speed controller if we can build one.  ::)
« Last Edit: July 18, 2007, 06:52:54 PM by silo_xtreme »

Offline silo_xtremeTopic starter

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Re: H-Bridges , Relays, and MOSFETs need input
« Reply #1 on: July 18, 2007, 06:23:22 PM »
Ok, Now people have been taking too long to respond  ;) and I found one of my old college books "Principles and Applications of Electrical Engineering" where i found some useful information.

MOSFET is a voltage controlled device
RELAY is a device for controlling large current between or voltage accross two terminals.

Ok - we got this far.  Now, for a basic H-Bridge (forward/stop/reverse) of the motor i would imagine that a MOSFET would be faster, and a RELAY can make good clicking sounds?
 ???

What about H-Bridge Relays?  I wonder if i can get any more confused....  ;D

Offline paulstreats

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Re: H-Bridges , Relays, and MOSFETs need input
« Reply #2 on: July 18, 2007, 07:22:31 PM »
MOSFET's will allow you to control speed through pwm, and relay would only be used to switch hard on/off(ie set speed).
Although you can apply a pwm type effect using a relay, this is really noisy, inefficient and also likely to damage your relay as they have mechanical componemts which eventually suffer from wear.
So if you want to control speed through mcu use a mosfet.

Offline silo_xtremeTopic starter

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Re: H-Bridges , Relays, and MOSFETs need input
« Reply #3 on: July 18, 2007, 07:29:55 PM »
Great information, how does this apply to a Solid State Realy?

Offline paulstreats

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Re: H-Bridges , Relays, and MOSFETs need input
« Reply #4 on: July 18, 2007, 07:37:04 PM »
Yes, in theory, you could send a pwm signal to a solid state relay, as this signal will usually be interpreted (inside the relay) with an optically isolated (using led and photo diode) mosfet or thyristor.

Offline Robotboy86

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Re: H-Bridges , Relays, and MOSFETs need input
« Reply #5 on: July 19, 2007, 03:05:45 AM »
you CAN send a pwm to an SSR..

but don't.  For the love of god don't :P
Optically isolted relays require time to do SEVERAL things:

1.  Hit LED on.

2.  Hit LED off.

3.  Photoresistors take time to hit on as well.

4.  Take time to hit off.


MoSFET's??

1.  Gate Current increases electron flows.

2.  Gate current decreases electron flow. 

Both of those are done to a point of either hitting it on/off at a RAPID fashion.  We are talking thousands of times a SECOND.  *Very* rapid.  SSR's can probaly do the same, but its not the same.
a
MOSFET's  are the standard for a reason.

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Re: H-Bridges , Relays, and MOSFETs need input
« Reply #6 on: July 22, 2007, 11:40:45 AM »
FYI, a MOSFET is a solid state relay ;)

Solid state basically means no moving or lose parts - the advantage is that it is hard to brake when treated roughly (like throwing your robot out the window).

Offline elixier

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Re: H-Bridges , Relays, and MOSFETs need input
« Reply #7 on: July 27, 2007, 03:30:51 PM »
Using semiconductor devices instead of really in Hbridge may be summerised as under

1. Now inside the relay there is a coil so during start up relay will draw a large amount of current during start up.

2. During startup it will look for the search current.

3. it may be suitable for lage current in driving the motor.

4. Having a movable part and comparatively larger size use of semiconductor devices is must for the motor control.
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Offline zamboniman60

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Re: H-Bridges , Relays, and MOSFETs need input
« Reply #8 on: July 29, 2007, 06:12:49 PM »
You CAN use a solid state relay with a pulse width modulated signal. The time that it takes for the LED to turn on is minimal, and the receiver is a phototransistor -- not a photoresistor. The internal device is an optocoupler, which has a maximum response time of about 1 microsecond.

 


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