EMF suppression relay-driven automotive refrig clutch 12V about 1.8A draw help please.

[PaulNZ2]

Good evening. I came across this thread with google and it is some help but I have a question. https://www.electro-tech-online.com/threads/diode-across-a-relay-1n4148-or-1n4001.20794/

The Omron timer-driven Omron relay always arcs across the contacts driving an Ogura automotive clutch and as it recently failed (had to sand the contacts) decided to instal IN4004 diodes as planned for ages ago. Initially placed the diode across the contacts, still got an arc, then realised had to place across the clutch coil, cathode to +ve coin lead, anode to circuit earth.

Arc has disappeared, but the clutch now takes maybe half a second to drop out, and the above conversation explains why. Shouldn't be a problem. However as its apparent that the back EMF is dissipating through the diode my question please is the IN4004 average forward 1A current and 50A peak rating sufficient for this setup? Clutch coil draws about 1.8A.

Thank you for your help. Also, am interest (if time permits) in building a 0 - 30V bench power supply can someone point me in the right direction please?

Paul

 

[Les Jones]

Hi Paul,
I think the 1N4004 should be OK The current through the diode will be 1.8 amps at the point when the relay contact opens but it will drop to near zero over the half second it takes for the clutch to drop out.

Les.

 

[ericgibbs]

hi Paul,
A zener diode in series with the 1N4007 will reduce the hold time of your clutch.
Use a 10V zener rated at 5 Watt should be 'beefy' enough.
Ref images showing single diode and diode/zener combination
E

 

[MikeMl]

For 12VDC, if the coil current is up to about 0.25A, then a 1N914 or 1N4148 is fine. Higher coil currents use a 1A rectifier, like a 1N4001.

Since the fan likely only turns on/off occasionally, the Zener shown by Eric is not needed.

 

[ericgibbs]

hi Mike,
I do not see any reference to a fan.?
The OP was complaining that the clutch release was slow , when he added a suppression diode across the clutch coil.

Eric

 

[MaxHeadRoom78]

If you want to eliminate the delay caused by the diode, you could look at a relay with magnetic arc blow-out feature. I have used the P&B PRD style in the past, although slightly large for your app.
In a pinch you could try customizing a standard relay with a small bar magnet close to the contact points.
Max.

 

[PaulNZ2]

Fellas, thank you for your interest and advice, much appreciated. The left hand configuration in Eric's diagram is pretty much the situation, although the coil/diode and S1 are interchanged.

It's part of the control setup for a PTO-driven mobile soft-serve churn in a vehicle. In fact there are two relay-driven automotive clutches. One through a bunch of pulleys and idlers drives the churn's beater blade, the condenser cooling fan, and an open-drive 1.5HP open-drive Blissfield compressor. The second clutch cuts the compressor in and out of this arrangement depending on viscosity of the soft serve in the churn's barrel. The compressor can only work when both clutches are energized, and the churn (upon product draw-off) initiates the timing sequences that result in one or both clutches turning on.

Eric, does your top graph show that in fact the max current I of L1 (the clutch coil) and therefore the diode is only 300mA? Well within the diode specs. The clutches come in and out once, twice, maybe thrice a minute but this can go on several hours if at an event where there are a lot of people present.

I was pleased to eliminate the arc but taken by surprise by the delay in the clutch(es) dropping out so it has been good learning more about back EMF.

The main thing is, can I use it with just the diode for the meantime and if the delay becomes a problem add the zener in as well.

Thank you for the suggestion.

By the way, just for clarity, it's In4004 not In4007.

Still interested to hear more discussion.

Paul

 

[ericgibbs]

Hi Paul,
If you give the specification of the clutch coil, I can give you an indication of the actual current.
The inductance of coil itself is important, if you want a precise answer.
I have had to 'guess' for the simulation.
E

 

[MikeMl]

hi Mike,
I do not see any reference to a fan.?
The OP was complaining that the clutch release was slow , when he added a suppression diode across the clutch coil.

Eric

I was thinking of the clutch like is used on an AC compressor. I said "fan" by mistake.

Here are three different versions of a snubber: 1. do nothing (red) , 2. 10Ω resistor added to diode(orange) , 3. 27Ω resistor added to diode (yel).

Top plot pane is voltage across the switch. Note that at 27Ω, the transient is shortest, but the switch must stand off ~70V. I used the 1/2sec hold time to come up with the estimate of 1.4H for the clutch inductance. I used the 1.8A to come up with an estimate of the clutch resistance.

Middle pane shows current through the diode. Note the times before the clutch releases... A 1Amp 1N4001 is fine for this...

Bottom pane shows power dissipation in the resistor. I would use a 2W or 5W resistor for either 10Ω or 27Ω.

 

[PaulNZ2]

It's an Ogura MA-6A but doesnt show up in their 139-page online pdf. I have a paper that says 3.4A max I at 12V and 72 ft lbs static torque. It's dawn now, been outside and measured 3.2A both clutches, temperature about 10 deg C. Sorry for my mis-apprehension about the current earlier. How does this change things?

 

[MikeMl]

Here is a new sim.
This time, no resistor, 5, 10, 22Ω.
Peak voltage 13, 28, 44, 86V.
Peak current 3.2A for all.
Peak res. power 0, 50, 100, 220W.

 

[PaulNZ2]

Thank you Mike. The diodes are in on their own for the moment. I wonder if they will handle the instantaneous current?

 

[MikeMl]

Should be ok: Note the (occasional) Peak Forward Surge current.

 

[PaulNZ2]

Before we get too far down the track, may I say thanks for your help and interest. We'll see how the diodes go on their own and if necessary, make adjustments. The dropout delay doesn't seem to be causing problems, in fact hardly noticeable in the running of the machinery.

 

[debe]

Theres plenty of automotive air con compressor clutches come factory fitted with a diode across the clutch coil.

 

[MaxHeadRoom78]

The dropout delay doesn't seem to be causing problems, in fact hardly noticeable in the running of the machinery.

It has never really been an issue for me, usually time intensive apps such as dispensing valves may have a problem.
Max.