Low voltage, sometimes

I have an issue with modules on a bus leg ending with 3X VMB2BLE, 1X VMB4RYLD, 1X VMB4DC.
When the lights on the VMB4RYLD and the VMB4DC are on, if I take all the inside blinds down (i.e. 3 blinds on 2 modules), the lights turn off and I have the signal on the VMB4RYLD that there is low voltage on the module.
But if I take the blinds down one by one, with let’s say 3 seconds of delay between them, it works fine.

Is the voltage decreasing significantly on the bus when a VMB2BEL is starting an action? My power supply is 15V.

Thank you

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Is the section of the 15V power supply cables big enough? You can have a voltage drop! Try to measure the 15V directly on the last module when actuating the 3 BLE’s together. If the 15V cables are in series, put them in parralel.

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You are both absolutely correct.

A simple solution is to put an extra 15v PSU in your “distant” cabinet.

Just remove the 15V+ connection, external of the cabinet.

As in

Link with just

  • 0V
  • Low data
  • High Data

The issue is the current draw is too great and drops the bus voltage when so the relays are closed.
I saw this on a project 11 years ago :wink:

Thank you both. I can try what Virago suggests (no cost) and if it still does not work, I’ll add another power supply in my distant cabinet indeed.
For the bus cable, I use an EIB cable.

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Don’t forget that you can get cheap 15V 1amp 1 din space rail supplies

MDAR, did you ever try a capacitor in the remote cabinet? One could use a resistor and two diodes to limit charge current and still provide peak current to the loads.

What I did not do is compare cost… these days PS are dirt cheap…


I think I’d rather just fit another PSU

What kind of cable is being used to carry the velbus power and signal?

Is your 15V power supply delivers 15V? If it’s possible, you can adjust it until 18V on the output of the power supply. Power supply range on Velbus devices go from 12V to 18V.

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in my case, an EIB cable

I’ll try doing that as well.

That is a bit odd. You can have quite a long run of EIB cable before dropping over 3V with normal velbus loads.

Is the cable run huge? Or are there any big loads?
Is main power supply undersized?
Or might there be a poor connection somewhere on + or - ?

I run my bus at almost 18v and had to use extra power connections but then my bus run is very long so I do see significant voltage drops. I did see a VMB4RLYNO dropping out for low voltage before I added another power connections to that bus section.

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No specific big loads.
I replaced in the remote cabinet the EIB cables by the velbus connection bar (VMBRAIL-R). It’s actually since then that I have the problem. I’ll check the connections again.
The length of the bus up to the remove cabinet is about 30 meters or so.
Thanks for your suggestions, I’ll update my post when the problem is found in case it would be of interest for others.

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30m of EIB should be no trouble.

60m of EIB (30m + and 30m -, at 0.8mm diam) is 1.38 Ohms.
Would need to draw 2.2 amps from the velbus cable at the far end to drop 3V.

Got to be a poor or loose connection somewhere.

I really like VMBRAIL-R. I have a lot of it in my setup here.
I confess I have occasionally forgotten to fully tighten a screw on a module or end-terminal though :-/

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The voltage is 12,05V at the entrance of my remote cabinet.
Coming from 15V where the power supply is, I already have to figure out why it is dropping so much.

But: in the remote cabinet, a VMB2BLE when activated makes dropping the voltage of 0,25V per channel. A VMB4RYLD, the same, 0,25V per channel… So, when I activate the 6 channels of the VMB4RYLD and he VMB2BLE, it drops to 10,5V…
Is it normal that activating a channel in one of these modules consume so much?

Thank you

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You did not specify at what exact moment you’re measuring the 0.25V drop. It can be quite normal indeed if we’re talking about the few moments when the relay is actuating, especially if its 3 of them. Activating an electromechanical relay, such as those the are used in VELBUS modules (you can tell them by the distinctive clicky sound they make when they engage) can result in fairly significant initial current loads. So I wouldn’t be surprised if your normally 12.05V leg drops another volt or two momentarily while the relays are actuating. On the other hand, these relays should not be consuming much more than zero-point-something of a watt to maintain a contact that’s already established (caveat: I haven’t pried modules open to check what exact model of relays is being used, talking from a general experience here.) and if that’s causing a 0.25V drop, then you have either a humongous amount of resistance in your cable (this is easy to check – just measure across the two ends of the cable – use a >1.5mm² cable connected to one of the ends to extend it if you can’t reach the two ends at the same time easily) or your power supply is underpowered.

You can also measure the current draw of an arbitrary module with just a multimeter connected in series between the module terminal and the corresponding wire if you’d like to rule out a potential (partial?) short.


@nagisa is absolutely correct. :+1:
The relays inside all modern modules are industrial grade, so it doesn’t surprise me that there is a bit of a high initial current draw when they close.

I won’t repeat his excellent summary and advice.

I have seen the behaviour you were reporting.

Your options are

  • Wind up the supply voltage in your main cabinet to <18v, which would allow for the volt drop

  • Remove (only) the 15V+ at the remote cabinet from the Velbus cable and install a small local 15V (1amp ?) PSU in your remote cabinet

  • Completely replace your Velbus cable with a much thicker version

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You can also use the actions 0108 and 0112 with a different delay for each blind

Is it normal that activating a channel in one of these modules consume so much?

No, and I don’t think they are consuming much. For the voltage to drop so much there must be a high resistance in the power supply somewhere.

The standby current consumption for a 4x relay module is only 30 to 50ma and max current is given as 250ma in the spec.

Looking more closely… the particular relay in those modules has a DC coil resistance of 272 ohms. So the max current is only 44ma per relay (and a little bit more if you take supply much over 12V). Allowing for the rest of the circuitry still only gets me to 230-ish ma if all four relays are ON.

Call it 50ma extra per relay channel switched ON. With 1.38 ohms from 30+30m of 0.8mm EIB that would drop about 0.07 volts per channel switched on.

So no, the voltage drops you see are definitely not normal. There is no sensible way your relay module could be drawing more current so you must have some extra resistance in the supply somewhere that is dropping this voltage… like a poor connection perhaps… or a faulty power supply.

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I’m with EvAndy on this one. While MDAR’s advice certainly will solve the problem. He is the practical man. I’m more the scientist and would like to know what is the cause of the issue :slight_smile: Also, though less likely, it may be extra work to get mains power to the remote cabinet.

Another way to test EvAndy’s assumption is to

  1. Remake all the connections in the remote cabinet and see if you still have the issue. The observation you may have the issue since the Velbus Rail was installed is not insignificant.
  2. Remake the connections in the main cabinet that move the PSU power to the start of the EIB cable.
  3. Less likely, but you must rule it out: measure the cable resistance in the EIB cable run. The easiest way is to disconnect +V and -V in the main cabinet, do the same in the remote cabinet and short them there. Measure the resistance between +V and -V in the main cabinet. You don’t need to know if +V or -V is the culprit, you need to pull a new cable anyway.

#3, if showing a high resistance, may be damaged by pulling the cable and +V or -V is just holding on. This is less likely as the EIB cable is solid core and may have a stretch limiter with it.

If #3 shows to be the case and you like to be lazy dirty harry, you may repurpose a good lead as PSU power carrier. And cause a nice short circuit the day you forgot you did :slight_smile:

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