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Stanley
Joined: 09 Jan 2004
Posts: 4
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| Using PIC as a switch |
 Posted: Sun Apr 25, 2004 1:25 pm |
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Hi.
I have 2 AA battery 1.2V 1800mAh Ni-MH connected in serial giving me 2.4V. They are supplying a motor using 800mA and I am going to use a PIC to turn it on and off. I cant loose any voltage over the switch so I was thinking of using a relay or does anyone have a better solution? Is it possible to use some other componets than the relay? If not then does anyone know about a suitable relay to use?
Stanley |
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Ttelmah
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| Re: Using PIC as a switch |
| Posted: Sun Apr 25, 2004 2:21 pm |
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| Stanley wrote: | Hi.
I have 2 AA battery 1.2V 1800mAh Ni-MH connected in serial giving me 2.4V. They are supplying a motor using 800mA and I am going to use a PIC to turn it on and off. I cant loose any voltage over the switch so I was thinking of using a relay or does anyone have a better solution? Is it possible to use some other componets than the relay? If not then does anyone know about a suitable relay to use?
Stanley |
You are going to loose _some_ voltage in any switch. Even a relay has resistance. The 'downside' of a relay, is that the coil will draw significant current (you will need a driver transistor), and there will be a lot of back-emf when the coil is switched off. You will also need to protect the contacts from arcing when the load is switched. The big problem you will have is that this will need to be a relatively special 'low voltage' relay. The first alternative to a relay, is a power MOSFET, but your low voltage will then mean you have to use a seperate circuit to generate the gate drive. The 'on' resistance of a power MOSFET, can be tiny (these are what are being used to control motors drawing thousands of amps now...). The second alternative, is an 'all in one' solution, the 'solid state relay'. These comprise the FET, protections circuitry, and the drive system, built into a single module (for DC versions - the AC versions use triacs, or thyristors). Unfortunately all require that there is enough voltage to drive the input side of an optocoupler, and your supply is 'borderline'. Have a look at the Crydom website, to get an idea of what these modules can do. Most need about 3v.
Tyco, used to do some specialist low voltage relays, but most need at least 3.5v.
Your 'best bet', will probably be to use a charge pump, to give a few mA, at 5v, which will then allow a much larger choice of switching system.
Best Wishes |
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Rob
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| Posted: Sun Apr 25, 2004 3:16 pm |
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If you're not tied down to having only two batteries you could use four (giving you 4.8 V) and then pulse width modulate through a FET or H-Bridge or most any other power switching technology. You could then use these battereis to power your PIC too. just a thought.
-Rob |
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Guest 999
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| Posted: Mon Apr 26, 2004 12:49 am |
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| Use a latching relay |
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edi
Joined: 22 Dec 2003
Posts: 82
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| Posted: Mon Apr 26, 2004 4:32 am |
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You can use a new technologic MOSFET that need very low Vgs and has a Rdson of ~100mohm.
If you need 800mA to your load, you will lose ~80mV from the 2.4V source.
One MOSFET for example can be the Fairchild FDN340P (http://www.fairchildsemi.com/pf/FD%2FFDN340P.html)
Good luck |
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Ttelmah
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| Posted: Mon Apr 26, 2004 2:47 pm |
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| edi wrote: | You can use a new technologic MOSFET that need very low Vgs and has a Rdson of ~100mohm.
If you need 800mA to your load, you will lose ~80mV from the 2.4V source.
One MOSFET for example can be the Fairchild FDN340P (http://www.fairchildsemi.com/pf/FD%2FFDN340P.html)
Good luck |
It'll perform slightly worse than this. The Rds, gets higher with lower gate drive. At 2.4v, it'll be nearer to 150mohm (and you should really assume that the drive will be a little below the rail). Still great, and well worth considering.
Best Wishes |
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Stanley
Joined: 09 Jan 2004
Posts: 4
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| RE: Using PIC as a switch |
| Posted: Tue Apr 27, 2004 1:56 am |
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Thanks for all the help. I think I am going to try the new teck. MOSFET. hope it'l work 
Tnx. you'r the best
Stanley
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edi
Joined: 22 Dec 2003
Posts: 82
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| Posted: Wed Apr 28, 2004 9:27 am |
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The best FET option I found is the Si6475DQ from Vishay.
It's only 17mOhm at Vgs=-1.8V.
So at 800mA LOAD you lose only 13.6mV. |
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