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Author Topic: Do I need a fly back diode?  (Read 1980 times)

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Offline ConscriptedTopic starter

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Do I need a fly back diode?
« on: January 06, 2011, 07:12:38 AM »
Good morning all.

I'm working on a small line follower using a pair of pager motors. They are rated to have a stall current of 260mA @3vdc. I don't think they will ever stall since it will be running on a table top. I'll be using a transistor to turn them on and off. Do I need to put in protective diodes? I have them if I need but I'm trying to keep the part count down as much as possible.

Thanks.

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Offline hopslink

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Re: Do I need a fly back diode?
« Reply #1 on: January 06, 2011, 08:56:32 AM »
Yes you need them, your current may be small but it is the short switching time which is mostly responsible for high back-emf.

You could look at using a darlington transistor, they often have protection diodes included and should be available as a drop in replacement.

Offline Soeren

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Re: Do I need a fly back diode?
« Reply #2 on: January 06, 2011, 05:56:23 PM »
Hi,

They are rated to have a stall current of 260mA @3vdc. I don't think they will ever stall since it will be running on a table top. [...] Do I need to put in protective diodes?
Yes, you need the diodes.
Diodes have nothing to do with the stall current. It's the flyback voltage spikes that they're protecting against.
Use fast diodes if at all possible, a 1N400x is too slow and can let short spikes through.
Regards,
Søren

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Offline ConscriptedTopic starter

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Re: Do I need a fly back diode?
« Reply #3 on: January 06, 2011, 09:45:19 PM »
Thank you for the replies. I've got a couple of SD103B Schottky Diodes that I purchased for this reason. I was hoping not to use them.

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Offline TrickyNekro

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Re: Do I need a fly back diode?
« Reply #4 on: January 08, 2011, 09:28:20 AM »
Sorry to say but I wouldn't got with a Schottky diode, just for batteries shake... They tend to have ugly reverse current drain ratings... which current would be loaded on the transistor/mosfet and deprived from your motor...

Go p-n junction style... I just don't know a model to suggest...
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Offline Soeren

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Re: Do I need a fly back diode?
« Reply #5 on: January 08, 2011, 05:21:14 PM »
Hi,

[...] They tend to have ugly reverse current drain ratings... which current would be loaded on the transistor/mosfet and deprived from your motor...
Datasheet says:
Maximum Reverse Current   5.0 uA
SD103A @ VR= 30V
SD103B @ VR= 20V
SD103C @ VR= 10V
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline madsci1016

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Re: Do I need a fly back diode?
« Reply #6 on: January 08, 2011, 06:29:29 PM »
Use fast diodes if at all possible, a 1N400x is too slow and can let short spikes through.

What are your favorite model numbers?

Offline Soeren

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Re: Do I need a fly back diode?
« Reply #7 on: January 10, 2011, 12:32:37 PM »
Hi,

Use fast diodes if at all possible, a 1N400x is too slow and can let short spikes through.

What are your favorite model numbers?
That depends on current and the application in general. I always keep a stock of BYV27 (3A and 25ns max. RRT), for fairly low currents, signal diodes like 1N4148 and 1N914 are aces (both about 4ns RRT).
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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