# [EVDL] Snubber Circuit



## EVDL List (Jul 27, 2007)

Can anyone help me understand how to design a snubber circuit to go with a heater relay? 

My limited understanding (and please correct me if I'm wrong here) is that the purpose of the snubber circuit is to suppress transient voltage when power is removed and the contacts open. The desired results are reduced arcing of the contacts. So we put a resistor and capacitor in series but do they go in parallel with the load or across the relay contacts?

How are the values of the components selected? I figure that the voltage of the cap should be somewhat greater than the maximum voltage of the pack, but I'm not sure how the capacitance value is selected. And what about the resistance and what power?

Bruce
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## EVDL List (Jul 27, 2007)

The purpose of a snubber circuit is to dissipate the energy stored in the
magnetic field produced by the load. If the load is non-inductive you don't
really need a snubber. The load you have (the heater) may or may not
require a snubber.

Suppose your heater has some appreciable amount of inductance. The moment
the relay contacts open, there is energy stored in the magnetic field that
must be released - it is released in the form of a spark between the relay
contacts.

So the first thing to do is examine the spark. Is it larger when the relay
contacts close or open? If the spark is larger when the contacts close,
there is no point installing a snubber. If the spark is larger when the
contacts open, a snubber can increase the life of the relay.

The resistor and capacitor (in series) can be either in parallel with the
load or across the relay contacts - it doesn't matter much. If the snubber
is in parallel with the load, the capacitor has voltage across it when the
heater is on. If in series, the capacitor is has voltage across it when the
heater is off.

If I were to design it, I would size the resistor at roughly twice the
resistance of the heater (you can measure heater current and calculate its
hot resistance). The capacitor should probably be a few uF in size, rated
about 2x to 3x the pack voltage. I cannot tell you an exact uF figure
without knowing the inductance of the heater. It is best to avoid
electrolytic and use a "poly film" capacitor.

It is also possible to use a single diode as a snubber. Simply connect the
cathode to the positive side of the load (heater) and anode to the negative
side of the load. The diode should be rated at least 1.5X the pack voltage. 
The diode's "surge" or peak current should be greater than the heater
current.

Fran




> Bruce-64 wrote:
> >
> > Can anyone help me understand how to design a snubber circuit to go with a
> > heater relay?
> ...


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## EVDL List (Jul 27, 2007)

The purpose of a snubber circuit is to dissipate the energy stored in the
magnetic field produced by the load. If the load is non-inductive you don't
really need a snubber. The load you have (the heater) may or may not
require a snubber.

Suppose your heater has some appreciable amount of inductance. The moment
the relay contacts open, there is energy stored in the magnetic field that
must be released - it is released in the form of a spark between the relay
contacts.

So the first thing to do is examine the spark. Is it larger when the relay
contacts close or open? If the spark is larger when the contacts close,
there is no point installing a snubber. If the spark is larger when the
contacts open, a snubber can increase the life of the relay.

The resistor and capacitor (in series) can be either in parallel with the
load or across the relay contacts - it doesn't matter much. If the snubber
is in parallel with the load, the capacitor has voltage across it when the
heater is on. If in series, the capacitor is has voltage across it when the
heater is off.

If I were to design it, I would size the resistor at roughly twice the
resistance of the heater (you can measure heater current and calculate its
hot resistance). The capacitor should probably be a few uF in size, rated
about 2x to 3x the pack voltage. I cannot tell you an exact uF figure
without knowing the inductance of the heater. It is best to avoid
electrolytic and use a "poly film" capacitor.

It is also possible to use a single diode as a snubber. Simply connect the
cathode to the positive side of the load (heater) and anode to the negative
side of the load. The diode should be rated at least 1.5X the pack voltage. 
The diode's "surge" or peak current should be greater than the heater
current.

Fran




> Bruce-64 wrote:
> >
> > Can anyone help me understand how to design a snubber circuit to go with a
> > heater relay?
> ...


----------



## EVDL List (Jul 27, 2007)

The purpose of a snubber circuit is to dissipate the energy stored in the
magnetic field produced by the load. If the load is non-inductive you don't
really need a snubber. The load you have (the heater) may or may not
require a snubber.

Suppose your heater has some appreciable amount of inductance. The moment
the relay contacts open, there is energy stored in the magnetic field that
must be released - it is released in the form of a spark between the relay
contacts.

So the first thing to do is examine the spark. Is it larger when the relay
contacts close or open? If the spark is larger when the contacts close,
there is no point installing a snubber. If the spark is larger when the
contacts open, a snubber can increase the life of the relay.

The resistor and capacitor (in series) can be either in parallel with the
load or across the relay contacts - it doesn't matter much. If the snubber
is in parallel with the load, the capacitor has voltage across it when the
heater is on. If in series, the capacitor is has voltage across it when the
heater is off.

If I were to design it, I would size the resistor at roughly twice the
resistance of the heater (you can measure heater current and calculate its
hot resistance). The capacitor should probably be a few uF in size, rated
about 2x to 3x the pack voltage. I cannot tell you an exact uF figure
without knowing the inductance of the heater. It is best to avoid
electrolytic and use a "poly film" capacitor.

It is also possible to use a single diode as a snubber. Simply connect the
cathode to the positive side of the load (heater) and anode to the negative
side of the load. The diode should be rated at least 1.5X the pack voltage. 
The diode's "surge" or peak current should be greater than the heater
current.

Fran




> Bruce-64 wrote:
> >
> > Can anyone help me understand how to design a snubber circuit to go with a
> > heater relay?
> ...


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## EVDL List (Jul 27, 2007)

> Bruce wrote:
> > Can anyone help me understand how to design a snubber circuit to go
> > with a heater relay?
> 
> ...


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## EVDL List (Jul 27, 2007)

> fsabolich wrote:
> > The purpose of a snubber circuit is to dissipate the energy stored in
> > the magnetic field produced by the load. If the load is non-inductive
> > you don't really need a snubber. The load you have (the heater) may
> ...


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## EVDL List (Jul 27, 2007)

>Snubber circuits are used with any kind of load. They are the electrical
>equivalent of a mechanical shock absorber; they absorb unwanted bounces
>and oscillations.

Well, to have ringing you have to have some sort of inductive and capacitive
component. In this case the question was about a heater - which will
basically be inductive (plus resistive of course). With a diode snubber, it
may ring a tiny bit (some capacitance in the circuit), but that is not what
is causing the arcing across the relay - that is release of energy stored in
the magnetic field.

>The snubber circuit for an inductive load normally consists of a 
>freewheel diode across the inductive load. But even here, extra 
>resistors, capacitors, and other parts are sometimes used.

Sure, if you have some capacitance in there - you will have ringing (without
a snubber) and a resistor-capacitor snubber helps eliminate this ringing.

>> The resistor and capacitor (in series) can be either in parallel with
>> the load or across the relay contacts - it doesn't matter much.

>It makes a difference because the power source also has resistance, 
>capacitance, and inductance. For example, if you put the snubber across 
>the load, and the power source has significant inductance, when the 
>switch turns off, the supply instantly sees zero current, so it's own 
>inductance causes the voltage to spike way up. The snubber protects the 
>load, but not the switch.

True.

> It is also possible to use a single diode as a snubber.

>This is only effective for inductive loads. It also does not limit the 
>rate of rise in voltage across the switch, and so is inadequate for 
>certain situations.

Well, the heater would be an "inductive load". And as for limiting the rate
of rise, we are not talking about something that is being switched at a high
frequency - RF interference isn't really an issue here. We are basically
talking about dumping the energy in the magnetic field somewhere so that it
isn't released as a spark across the relay contacts.

Perhaps I should have made it more clear that I was talking about his case
in particular, and not snubbers in general.

Fran
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## EVDL List (Jul 27, 2007)

> fsabolich wrote:
> > Well, to have ringing you have to have some sort of inductive and capacitive
> > component. In this case the question was about a heater - which will
> > basically be inductive (plus resistive of course).
> ...


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## EVDL List (Jul 27, 2007)

>I measured samples of each type so you can see what I mean.
>
>1. ceramic heater (each of two elements)
>	450 watts at 120 vac
>	219 ohms cold
>	38,000 pF capacitance
>	<1 uH inductance

Wow, I see what you mean. I completely misunderstood the load.

Well, I'm learning something.

Thanks,

Fran

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