Heat Treatment Oven

Here is a link for thermocouple boards:

Reference Designs: Thermocouple Temperature Measurement | Analog Devices

This is a better start for a relative novice.  Unless phrases like 'Seebeck Effect' and 'Cold-Junction-Compensation' come trippingly off of your tongue, it would be good to start here.

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Little typo in my prior post:  You can put a small capacitor in shunt with the solenoid.  This makes a nice first responder before the diode changes state.  If you keep the energy of this capacitor down to a few percent of that stored in the solenoid, or watch what happens on a triggerable scope, you're good.  Use MMLC

What I was trying to say is that one needs to put bulk capacitance across the solenoid supply.  Use Al e-lytic.  This should have 10-100X the energy of the solenoid.  It needs to be physically and electrically proximate to the anti-spike diode and the coil.  We do not want to create an un-intended flyback supply!  Electrically proximate means low-Z.

Wide, flat, short trace, use two-ounce Cu material if possible.  Prototyping, returns made of solder braid can be pretty fast, the weave minimizes skin-effect, the Cu is thick and pure.

IIRC energy stored in a capacitor (condenser, localized concentration of the charge field)  =  VVC/2

       energy stored in an inductor (coil, localized concentration of the magnetic field)         = IIL/2

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where V is the potential drop across the device in Volts.

          I is the current through the device in Amperes.

Here is a link for thermocouple boards:

Reference Designs: Thermocouple Temperature Measurement | Analog Devices

This is a better start for a relative novice.  Unless phrases like 'Seebeck Effect' and 'Cold-Junction-Compensation' come trippingly off of your tongue, it would be good to start here.

==========

Little typo in my prior post:  You can put a small capacitor in shunt with the solenoid.  This makes a nice first responder before the diode changes state.  If you keep the energy of this capacitor down to a few percent of that stored in the solenoid, or watch what happens on a triggerable scope, you're good.  Use MMLC

What I was trying to say is that one needs to put bulk capacitance across the solenoid supply.  Use Al e-lytic.  This should have 10-100X the energy of the solenoid.  It needs to be physically and electrically proximate to the anti-spike diode and the coil.  We do not want to create an un-intended flyback supply!  Electrically proximate means low-Z.

Wide, flat, short trace, use two-ounce Cu material if possible.  Prototyping, returns made of solder braid can be pretty fast, the weave minimizes skin-effect, the Cu is thick and pure.

IIRC energy stored in a capacitor (condenser, localized concentration of the charge field)  =  VVC/2

       energy stored in an inductor (coil, localized concentration of the magnetic field)         = IIL/2

==========

where V is the potential drop across the device in Volts.

          I is the current through the device in Amperes.

IMHO, I would NOT put a capacitor in parallel with the coil of a solenoid.

You normally switch the solenoid with a transistor (or mosfet) rated for say 2-10 times the current the solenoid uses. Adding the capacitor will increase the current at the instant of turn-on by enormous amounts.

So you're using the transistor outside its recommended operating range.

If the diode is too slow, the voltage will rise until the transistor starts to conduct again.  Nowadays that is an allowed operating mode (for mosfets) that is allowed under circumstances. When the amount of energy stored in the coil is less than what the transistor is allowed to dissipate in this mode, you could forget the diode completely. Using the diode will probably, even if it is slow, reduce the amount of energy the transistor has to cope with by a huge amount.

So... My advice is: Use a diode, don't worry about the possible spike that occurs before the diode kicks in. (I did some measurements last Friday and the scope couldn't see the spikes due to the slowness of the diode. )

(I tried blowing up the transistor by giving small pulses to the solenoid, that someone claimed blew up his transistor. No such luck... The transistor held up.)

Don, the formulas, although written awkwardly here on the forum (couldn't get the forum to do it better myself), are correct. But how do you figure out the inductance (L) of a relay. I could add a current-sensing resistor and watch the current increase over time when suddenly turning it on. But from the datasheet?

I'm with Roger on this - if for some reason you MUST tune out the coil inductance with a capacitor than put  a resistor in series with it (not going into calcs here but its value should be the same as the DC resistance of the coil.) If you get the capacitor value right the complete network has a purely resistive impedance (well it would if all the parts were perfect).

Diodes are easier and cheaper, you don't (normally) need anything fast or fancy, a 1N400X (X 1 - 8 denotes max operating voltage) works well in most cases.

MK

Roger Wolff wrote:

 

IMHO, I would NOT put a capacitor in parallel with the coil of a solenoid.

 

So... My advice is: Use a diode, don't worry about the possible spike that occurs before the diode kicks in. (I did some measurements last Friday and the scope couldn't see the spikes due to the slowness of the diode. )

A reverse-biased diode is a variable capacitor.  The initial current from the inductor discharges this capacitor until the diode forward-biases and the capacitance vanishes.  At least that's how I understand it.

For details on the phenomenon: Varicap - Wikipedia, the free encyclopedia

So I agree with Roger and Michael: you should not need more capacitance.

More on flyback diodes: Flyback diode - Wikipedia, the free encyclopedia

Sorry, I was speaking in shorthand late at night.  If a Zener or MOV snubber works for you go with it.  If it doesn't we have to go to something like a QuenchArc, which is a metalized capacitor plus a resistor.  I was just thinking about the capacitor part.  You want to use the smallest quenchark (big R, small C) that reliably works.  If your load is usually on, put it across the switch.  If the load is usually off, put it across the load.

As far as the coil that drives a relay or valve goes, I wasn't talking about a capacitor large enough to change the net inductive character of the coil under our normal frequency of operation.  I was thinking of an RF short to try and keep things quiet.  A bead in series never hurts here, neither.

The thing to do is model your scheme in Spice using a good model and good data.