Linear power supples in burglar alarms - Page 1
Forum section: electronics / projects

author: 5U4GB
author_rank: Super Contributor
timestamp: January 17, 2025, 06:37:00 am
content: Not quite sure which category this would go under so I'll post it here: Why do so many burglar alarms have linear power supplies? I was replacing the crummy alarm this place has with an ESP32-based one and swapped out the linear power supply with a Meanwell SMPS, assuming the power supply just went with the crappiness of the rest of the alarm. Then I looked for photos of alarms online and most of them seemed to feature honkin' big transformers feeding AC into some sort of linear supply circuit.Why don't they use SMPS modules? I assumed this one was a leftover design from the 1980s (did I mention it's a crappy alarm?), but given how many others do it maybe there's some other reason.And for anyone else doing this, the first time I tried to power it up with a load connected it didn't actually power up. After quite a bit of head scratching it turns out there was an M205 fuse hidden inside a side panel in the screw terminal block where the mains came into the cabinet which was fine for the linear but not quite high enough capacity for the SMPS.And as an aside, what fuse should I replace it with? Meanwellrecommend one "rated slightly higher than [the PSU input current] value", with their example being a 4A fuse for a PSU with a 2.2A max current draw at 115V. In this case it's 0.42A at 230V, so a T1AL250V?

author: hli
author_rank: Frequent Contributor
timestamp: January 17, 2025, 08:27:51 am
content: (Somewhat) uneducated guess: it reduces the number of things which can break. SMPS need circuitry on the mains side, which is susceptible to break more easily / often than stuff on the low-voltage side. So using a linear supply makes it easier to get higher reliability. Probably a SMPS can more easily be interfered with by disturbing mains / injecting voltage pulses, so it also might be a security thing.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 17, 2025, 10:28:07 am
content: Possibly cost but also EMC issues particularly with all those long cables.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 17, 2025, 11:10:18 am
content: Linear supplies are more reliable, harder to tamper/influence from outside, probably cheaper, too.Saying this because linear power supplies do not need low ESR capacitors, do not produce high temperature, do not dry their electrolytic caps, do not produce EMI, do not waste a few W in idle, they are not influenced by nearby magnets or by nearby EMP tampering.All these look like possible advantages of the linear over SMPS, but I don't really know if any of these is considered, or a requirement when designing alarms.

author: inse
author_rank: Super Contributor
timestamp: January 17, 2025, 11:20:38 am
content: Quote from: RoGeorge on January 17, 2025, 11:10:18 amWhile I agree to the other statements, those are definitely not true

author: wasedadoc
author_rank: Super Contributor
timestamp: January 17, 2025, 11:22:21 am
content: Quote from: RoGeorge on January 17, 2025, 11:10:18 amMostly correct but certainly not about idle power. Linears waste more than switchers.

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2025, 11:55:11 am
content: Quote from: RoGeorge on January 17, 2025, 11:10:18 amFixed.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 17, 2025, 01:22:12 pm
content: Quote from: RoGeorge on January 17, 2025, 11:10:18 amThis must be a joke, right?

author: tszaboo
author_rank: Super Contributor
timestamp: January 17, 2025, 02:01:16 pm
content: I was working at a security systems company a long time ago. We said we had to build in some "gravity" into the system. It needed thick steel cabinet, large relays, a linear power supply with a transformer, and a lead acid battery backup. Customers (mis)identified weight as quality, so a heavier system was perceived as higher quality. Efficiency was zero concern, as it wouldn't result sales.In a way an oversized, but then this is supposed to work for years, so that's not an inherently bad thing.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 17, 2025, 05:30:42 pm
content: Quote from: Zero999 on January 17, 2025, 11:55:11 amNo, that's forgery with quotes. Straight to jail!I never said what you quoted as my words there.I believe linear is better than SMPS because I expect a burglar alarm to need very little DC current, probably not more than a few mA. At very low currents, a linear PSU has all the advantages I've enumerated already inreply #3.

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2025, 08:33:13 pm
content: Quote from: RoGeorge on January 17, 2025, 05:30:42 pmNo forgery. I simply corrected the part of your post I quoted. That's what I meant by adding the word fixed, under it.You're mistaken. Linear power supplies aren't better for low currents. Small transformers have large core losses, which are present, even when no power is being drawn. The standby power consumption of a linear power supply can be 5W or more. Modern switched more power supplies draw less than 0.5W in standby in order to comply with the most recent EU directive.

author: 5U4GB
author_rank: Super Contributor
timestamp: January 18, 2025, 10:51:43 am
content: Quote from: tszaboo on January 17, 2025, 02:01:16 pmBeen there :-). I once worked for a company where some of the guys got tasked with opening up some devices they were shipping and hot-gluing in wheel balance weights to add, uh, quality to the device.

author: Poroit
author_rank: Frequent Contributor
timestamp: January 19, 2025, 03:06:29 am
content: Been there :-). I once worked for a company where some of the guys got tasked with opening up some devices they were shipping and hot-gluing in wheel balance weights to add, uh, quality to the device.That sucks....can you tell us the product type?

author: ohren
author_rank: Contributor
timestamp: January 19, 2025, 04:23:54 am
content: Quote from: Poroit on January 19, 2025, 03:06:29 amWheels, amirite?

author: 5U4GB
author_rank: Super Contributor
timestamp: January 19, 2025, 08:27:31 am
content: Quote from: Poroit on January 19, 2025, 03:06:29 amOh, it wasn't a poor-quality product, it was just that the electronics in newer generations had been integrated and miniaturised to a point where it felt like half the device was missing, so they re-added the mass of the missing part. Imagine some industrial sensor where 30 years ago the case would have been full of discrete circuitry while now it's mostly full of fresh air.

author: Njk
author_rank: Frequent Contributor
timestamp: January 19, 2025, 11:45:28 pm
content: Quote from: 5U4GB on January 18, 2025, 10:51:43 amOld time ago, when a HI-Fi equipment was in short supply, my friend bought a couple of big speakers in a used equipment store. Later he found that in one of the cabinet the standard loudspeaker is substituted with a very cheap one. To compensate for weight difference, a big smoothing iron is fastened inside the cabinet.As for the PSU types, I think there may be some EMC concern. My car alarm system uses 433 MHz RF channel to communicate with the indoor unit. So I prefer to power it from old linear PSU, just to be sure.