# Reversing polarity with 2 SPDT relays to control an actuator

#### jhend1john

Joined Apr 21, 2022
5
Found this diagram on another chat years back on this forum. So I bought two Bosch 0332209158 relays. However, when I measure resistance between 85 and 86 I always see like 100 ohms no matter which direction I measure. What am I missing. The diagram on the part shows a diode.

#### LowQCab

Joined Nov 6, 2012
2,182
You're missing the fact that your Meter has an extremely High-Impedance,
and uses a very low Voltage to measure Resistance.
The Diode inside the Relay may have as much as ~1.5V "Forward-Voltage-Drop",
( 0.7-Volts is the minimum Forward-Voltage You can expect from a "generic" Diode ),
so your Meter never puts out enough Voltage to cause the Diode to start conducting.

Your Meter "may" have a "Diode-Test" function.
When all else fails, read the instructions !!!
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#### MrSalts

Joined Apr 2, 2020
1,846
The diode is in parallel with the coil, your meter will not see the diode. The diode is only there to suppress any back emf click when the pedal switches are opened.

#### MrSalts

Joined Apr 2, 2020
1,846
You should apply + coil voltage to terminal +86 to prevent applying a constant forward voltage to the diode.

Joined Jul 18, 2013
25,469
Found this diagram on another chat years back on this forum. So I bought two Bosch 0332209158 relays.
If you have a automotive wrecker near you, you can usually get a large handful of those relays for a couple of $$BTW, the relay is marked with EU style symbols, coil and contacts. Thread Starter #### jhend1john Joined Apr 21, 2022 5 The diode is in parallel with the coil, your meter will not see the diode. The diode is only there to suppress any back emf click when the pedal switches are opened. That explains it then. WRT suppression, I read that somewhere. Would Back emf cause current to flow to ground through 85 when a pedal switch is opened? Thread Starter #### jhend1john Joined Apr 21, 2022 5 If you have a automotive wrecker near you, you can usually get a large handful of those relays for a couple of$$
BTW, the relay is marked with EU style symbols, coil and contacts.
Good to know!

#### jhend1john

Joined Apr 21, 2022
5
Note that I cobbled together the circuit on my dinner table and was able to see that it will likely work. I’m just trying to install a power antenna in my old truck using an up/down window switch.

#### MrSalts

Joined Apr 2, 2020
1,846
That explains it then. WRT suppression,
I read that somewhere. Would Back emf cause current to flow to ground through 85 when a pedal switch is opened?
A large reverse voltage can be created (making +86 very negative vs pin 85 when your pedal switch is opened and the current stored in the coil dissipates. The diode doesn't conduct at all while the switch is closed and the coils is energized. The diode very briefly conducts as the switch is opened and any high voltage (EMF) that could develop across the coil causes a brief current flow through the diode to keep the emf across the diode within 0.7v of ground. Actually, within -0.7v vs. ground reference.
In this circuit it would protect a small battery (I'm not sure a big lead acid battery would be too bothered) and it protects the contacts on your little pedal switch from a high voltage arc.