I am controlling the coil of a relay using a switching transistor.  To 
protect the transistor I intend to use a freewheeling diode in parallel 
with the coil.  I'm not sure of the issues in specifying the diode.

My impulse is to simply use a 1N4007
http://www.fairchildsemi.com/pf/1N/1N4007.html
(1000V peak repetitive reverse voltage, 1.0A average rectified forward 
current) because it is common, cheap, and seemingly the most heavy-duty 
of the 1N400x line.  Would that be a good choice for about any 
PCB-mounted relay?

"Matt" <t...@xxyyyzzzz.com> wrote in message 
news:Xt58i.9619$y...@news02.roc.ny...
>I am controlling the coil of a relay using a switching transistor.  To 
>protect the transistor I intend to use a freewheeling diode in parallel 
>with the coil.  I'm not sure of the issues in specifying the diode.
>
> My impulse is to simply use a 1N4007
> http://www.fairchildsemi.com/pf/1N/1N4007.html
> (1000V peak repetitive reverse voltage, 1.0A average rectified forward 
> current) because it is common, cheap, and seemingly the most heavy-duty of 
> the 1N400x line.  Would that be a good choice for about any PCB-mounted 
> relay?

I just finished a project that uses 12V, 30 ma relays. Used  1N914 s.

Tam 

"Matt"
>
>I am controlling the coil of a relay using a switching transistor.  To 
>protect the transistor I intend to use a freewheeling diode in parallel 
>with the coil.


**  Normally called a "kick back" diode   - IME.

Freewheeling diodes are associated with  DC motor drives and swiching 
regulators.


>  I'm not sure of the issues in specifying the diode.


**  Long as the diode can pass the same average current need to drive the 
coil and sustain the DC voltage across it   -  it should be OK.

The cheapest possible diode are commonly used  -  like  1N4148s or 1N4001s.




........   Phil

Phil Allison wrote:

> "Matt"
>
> >  I'm not sure of the issues in specifying the diode.
>
> **  Long as the diode can pass the same average current need to drive the
> coil and sustain the DC voltage across it   -  it should be OK.
>
> The cheapest possible diode are commonly used  -  like  1N4148s or 1N4001s.

I've known quite small relays to 'take out' 1N4148s, 914s etc.

Graham

Matt wrote:
> I am controlling the coil of a relay using a switching transistor.  To 
> protect the transistor I intend to use a freewheeling diode in parallel 
> with the coil.  I'm not sure of the issues in specifying the diode.
> 
> My impulse is to simply use a 1N4007
> http://www.fairchildsemi.com/pf/1N/1N4007.html
> (1000V peak repetitive reverse voltage, 1.0A average rectified forward 
> current) because it is common, cheap, and seemingly the most heavy-duty 
> of the 1N400x line.  Would that be a good choice for about any 
> PCB-mounted relay?

The absolute minimum repetitive surge current rating for the 
diode is the steady state current for the coil, under the 
highest supply voltage.  If the diode is rated for a 
continuous current equal to or greater than the coil 
current, the brief inductive quench can't possibly overheat 
the die.

The minimum reverse voltage rating for the diode is the 
highest possible supply voltage.  Some extra voltage 
capability seldom costs much.  But a lot of extra voltage 
capability may have a down side.  High voltage diodes 
generally turn on and off slower than lower voltage devices 
and may recover with a vicious snap that generates high 
frequencies.  So a 1000 volt diode is probably not quite as 
good as a 100 volt diode for a 24 volt coil.

Eeyore wrote:

> I've known quite small relays to 'take out' 1N4148s, 914s etc.

I hear people say this, but I have never seen a case of it. 
  I have used 1N4148 diodes for 200 mA coils for years.

In article <n...@comcast.com>,
   John Popelish <j...@rica.net> wrote:
> Eeyore wrote:

> > I've known quite small relays to 'take out' 1N4148s, 914s etc.

> I hear people say this, but I have never seen a case of it.
> I have used 1N4148 diodes for 200 mA coils for years.

 I've had it.  1N4148s gone to a short circuit.
 Never found out why.

-- 
Tony Williams.

On Jun 1, 9:24 pm, John Popelish <jpopel...@rica.net> wrote:
> Eeyore wrote:
> > I've known quite small relays to 'take out' 1N4148s, 914s etc.
>
> I hear people say this, but I have never seen a case of it.
>   I have used 1N4148 diodes for 200 mA coils for years.

It's the energy storage, proportional to inductance and square of
the current, that matters.  A 200 mA coil on a reed switch
has much lower stored energy than a 50 mA coil on a frame
relay.   When you're using more current than the (average)
rating on your diode, it only succeeds if the temperature
spike doesn't melt anything.

Tony Williams <t...@ledelec.demon.co.uk> writes:

> In article <n...@comcast.com>,
>    John Popelish <j...@rica.net> wrote:
>> Eeyore wrote:
>
>> > I've known quite small relays to 'take out' 1N4148s, 914s etc.
>
>> I hear people say this, but I have never seen a case of it.
>> I have used 1N4148 diodes for 200 mA coils for years.
>
>  I've had it.  1N4148s gone to a short circuit.
>  Never found out why.

ESD? Were the contacts connected to "external" signals?


-- 

John Devereux

whit3rd <w...@gmail.com> writes:

> On Jun 1, 9:24 pm, John Popelish <jpopel...@rica.net> wrote:
>> Eeyore wrote:
>> > I've known quite small relays to 'take out' 1N4148s, 914s etc.
>>
>> I hear people say this, but I have never seen a case of it.
>>   I have used 1N4148 diodes for 200 mA coils for years.
>
> It's the energy storage, proportional to inductance and square of
> the current, that matters.  A 200 mA coil on a reed switch
> has much lower stored energy than a 50 mA coil on a frame
> relay.   When you're using more current than the (average)
> rating on your diode, it only succeeds if the temperature
> spike doesn't melt anything.

But the diode will still only see the 50mA current, even if the stored
energy is *enormous*. It will just take a bit longer for that current
to decay when the driver is switched off (and this will be independent
of the diode rating).

-- 

John Devereux