Car Relay Wiring: Questions & Answers

I'm finally going to connect my climate control system to a Webasto auxiliary heater, but it's not that simple with a Peugeot.

Today, I got a car relay from the dealer, but unfortunately, he didn't have any with a freewheeling diode. In the place where the freewheeling diode would normally be, there is a resistor .

What is the difference here? I couldn't find anything about it on Google either.

Hi fricki.

There are relays where the "run-in" time is implemented using an RC circuit, meaning a capacitor in series with a resistor. This type of "run-in" mechanism can also be used with alternating current (AC) because it is bipolar.

So, please take a closer look to see if there isn't another capacitor (perhaps inside the casing?) in series with the resistor.

If not, then it's a bad design. Depending on the size, the resistor will cause permanent power loss, and, more importantly, it won't provide a free-running path for the voltage spikes from the relay coil when it's switched off!

So, if there's really only a single 'R' on the relay, I wouldn't use it.

Best regards,
Henning.

Hi,

Objection, dear Henning ( ).
A single resistor is perfectly sufficient. If it is sized appropriately, it does not represent a significant load (e.g., 10 kOhms --> 1.4 mA --> 20 mW power dissipation). The resistor effectively prevents the magnetic field from collapsing after the excitation voltage is switched off, thus avoiding voltage spikes.

A series connection of a capacitor and a resistor would be worse than a resistor alone.

Regards,
Sure, here is the translation of the text from German to English:

"Micha"

Hi Micha,

Mephisto wrote:

The inductance effectively prevents the magnetic field from collapsing after the excitation voltage is switched off, thus preventing voltage spikes.

How is this supposed to work? The magnetic field will inevitably collapse because the energy of the field will either be dissipated through this "free flow," or – without this free flow – the energy could result in an extremely high voltage spike.

Mephisto wrote:

A series connection of a capacitor and a resistor would be worse than a resistor alone.

I see it differently: A capacitor blocks direct current; there is no leakage current or anything like that. However, if the voltage across the capacitor changes, it acts – depending on the rate of voltage change – like a short circuit, and the energy of the magnetic field can be dissipated as a current through the ("short-circuiting") capacitor and the (not necessarily high-resistance) resistor.

In industrial 24V DC control relays, RC circuits are often used as freewheeling circuits, which are preferred over simple diode freewheeling.

Best regards,
Henning.

Thank you very much for the information.

I'm going to return the relay and get one with a freewheeling diode.

These are probably not installed by Webasto for free. When I wrote my master's thesis, I always used flyback diodes, and everything worked as it should.
I'm not interested in damaging the control unit of the auxiliary heater just because something isn't working correctly.
It was already enough for me that I had to adapt a relay and a voltage divider into the original wiring harness just to be able to control the Climatronic (long live France ). 2 hours just for the wiring harness .

Mephisto wrote:

A single resistor is perfectly sufficient. If it is sized appropriately, it does not represent a significant load (e.g., 10kOhm --> 1.4mA --> 20mW power loss).

In the factory-installed relays that I've examined so far (including the 109 series), there's a small 680 Ohm resistor connected in parallel with the coil. There's no capacitor to be found.

It's probably cheaper than a diode, and the polarity of the winding doesn't matter either.

Hi,

When using industrial contactors, I hope you are using the free-running modules from the contactor manufacturer (By the way, there is usually a diode inside...).

Yes, the magnetic field will definitely disappear eventually. The question is only how quickly that happens. The coil opposes the collapse of the magnetic field through self-induction (Ui = -L * (dI/dt); Ui: induced voltage, L: inductance, (dI/dt): time derivative of the excitation current).
Without a resistor or diode, (dI/dt) is very large, which results in a very high Ui (compare to a spark coil). --> The magnetic field collapses.
A resistor or diode, on the other hand, absorbs the magnetic field.
For me, there's a difference between fading away and collapsing. Maybe I should have written that earlier, so we wouldn't be talking past each other.

Regarding the relay mentioned by the thread starter: Sometimes, the flyback diodes are also integrated inside the relay. This can be determined by measuring, if they are not directly printed on the casing.

Regards,
Sure, here is the translation of the text from German to English:

"Micha"