Power supply swap (gate opener)

Thread Starter

xcr

Joined Sep 13, 2021
5
I have a quick question that hopefully can be answered easily.

I am working on a residential Liftmaster gate opener that appears to have a bad 120vac to 24vdc power supply. The power supply is a linear type with a toroidal transformer that appears to be bad (secondaries measure dead short and no vac output). The power supply is separate from the main control board and the last component appears to be a bridge rectifier right before external leads feed the resulting 24v into the control board.

The control board has two large electrolytic capacitors, right next to the power connector, so it appears filtering is done there.

My question is, finding a replacement toroidal transformer not only appears difficult, but fairly costly. Would it be acceptable, especially considering that the filtering seems to be being done on-board, to replace the entire linear module, up to and including the rectifier with a switching 24v DC power supply of similar amperage rating?
 

Reloadron

Joined Jan 15, 2015
6,206
I would just replace with a 24 VDC supply having adequate current. The additional filtration shouldn't matter. I would also fuse it at the start as possibly the dead shorted transformer is only a symptom and something else caused the transformer's untimely death.

Ron
 

Thread Starter

xcr

Joined Sep 13, 2021
5
Thanks for the feedback and that's what I was hoping. Yeah seems odd the secondaries were shorted on such a large transformer, seems like the least likely point of failure. The AGM Batteries were bloated and showed signs of overheating, so maybe they went dead short & caused the issue. They have since been removed and are to be replaced.
 

LowQCab

Joined Nov 6, 2012
1,329
The Secondary Resistance on your Transformer will normally be extremely low,
so low that it may be difficult to measure accurately.
Are you sure that You have proper Input-Voltage, with zero Output-Voltage ?
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ElectricSpidey

Joined Dec 2, 2017
1,965
I would be concerned that the existing filter caps might be too much of a startup current for a SMPS, so f you see any issues with startup I would consider removing the existing caps.

Also be sure of your voltage, a 24 volt rms AC output from a transformer results in a higher DC voltage when rectified and filtered.
 

Thread Starter

xcr

Joined Sep 13, 2021
5
If too difficult to measure on the secondaries & get an accurate measurement (I had an OK quality meter, but not professional level), they actually may be good but I just remembered that I did measure input voltage good, then I pulled the leads and measured the primaries and they were open. It seemed like the transformer should be the least likely point of failure, so I double and triple checked everything. I have open primaries (on both sets - it's 2x120 and 2x13), I have good input voltage, and 0vac output with the secondary leads disconnected from any potential load --- so unless I'm missing something really basic, it seems like the transformer is the issue.

The system charges two 12v AGM batteries wired in series and operates on 24v, that said, I'm assuming the system is tolerant of a little greater voltage than 24v. The transformer is labelled 2x120v primaries (paralleled) and 2x13v secondaries (wired in series). I assumed the 24v power supply would work, but maybe I should get an adjustable power supply that I can set a tad higher?

And thanks for the heads up on the caps, I'll keep that in mind if I run into issues.

I've attached a photo of the transformerIMG_3395.jpg
 

Thread Starter

xcr

Joined Sep 13, 2021
5
Are open primaries a typical failure mode for this type of transformer? If I had to guess, I would think shorting would be more typical, but that's just me guessing...
 

LowQCab

Joined Nov 6, 2012
1,329
It's possible that the Transformer has a built-in Thermal-Fuse on each of the Primaries.
If the Transformer is definitely the problem,
then peeling off the outside wrapping to inspect for a Fuse or broken Wire
would be the next step.
At this point you have nothing to loose,
and there's a chance that You might be able to fix it.
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jiggermole

Joined Jul 29, 2016
48
all of the industrial power supplies I have in machines have an adjustment on them, usually 22vdc to 28vdc via a trim pot accessible without dismantling the power supply. From the internets a 24v battery is fully charged at 28.7v so the ready made power supply may or may not be enough for your application. If you go for the industrial lego, din rail mounted power supply.
 
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