Help designing 24 VAC to DC, w/ relay & toggling LEDs? - Page 1
Forum section: electronics / projects

author: vv_eev
author_rank: Contributor
timestamp: June 18, 2023, 04:09:13 pm
content: There are a few parts of a project I'd like to run by people with much more experience than me. Everything should be small SMD and stay cool.1. First, toggling a SSR and alternating different LEDs off a timer. Using an NE555 seems most cost effective, and I'm reading 1.7V drop on the 555 output. 5V VCC with 2.xx Vf across the LEDs doesn't give good granularity for the LED current limiting resistors, so I'm thinking of 8 to 10V, which could allow 9V (9.6V) battery if there was any desire to later. NE555 should tolerate 15V, so that looks fine to me. Is there a better equally inexpensive timer?2. Running the "On" LED and SSR from the timer output is easy. For the "off" LED, I'm thinking of something like this with a BJT: (https://europe1.discourse-cdn.com/arduino/original/4X/4/6/4/4649e1814917a603361f965a091e54d7b960293d.png) with constant power to "2" and grounded through the LED, so when the timer switches on and powers "1", the LED will be bypassed and switch off. Is there a better way? I can't just use the timer output as the "ground" for the normally-on LED because the current might trigger the SSR.3. For power, I'm not seeing a great, cheap, 1-chip solution. I need about 80 mA so I'm shooting for 100 mA. I'm thinking bridge rectifier chip and bulk cap giving about 30-33V DC into a switching buck converter. A 34064 looks like the cheapest solution on Digikey that can handle at least 36V input for some headroom, and there are calculators available. Would you use something else? It needs to withstand at least 150°F ambient temperature, so I don't think linear regulation is feasible.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 18, 2023, 06:31:04 pm
content: A lot of mind reading needed here. Granularity, is that inconsistent led brightness or sand in your boxers? What current and /or voltage does the SSR need to switch on?An LED in series with an SSR on a 5V supply will probably not work correctly, there's just not enough headroom to set a predictable current. Dunno what a 34064 is. A power supervisor ic on digikey?See figure 4 of the attached. The output voltage range a 555 can manage depends on the load. This will not have a tight spec and may vary from chip to chip.The 555 is not great at driving loads directly.Use a transisor switch like the one in your link. The collector emitter volt drop will be a more consistent 250mV or so. An MMBT3904 or 2222 npn will work fine for modest LED currents.All the above applies to driving your SSR as well.That circuit is a simple inverter, you just put your LED in series with R2 as above.\150F (Fareneheit, is that like drachmas?) is only 65C, not too a big deal for a 1A linear regulator ticking over at 100mA. eg LM317 happy to 40V or the LM317HV if your nervous. You just need the right heat sink. Power dissipation is (Vin-Vout)/current draw. Not a lot for your application.

author: vv_eev
author_rank: Contributor
timestamp: June 19, 2023, 01:17:43 pm
content: Granularity means fine control or data. You have a lot more choice of resistors and less influence of tolerance in the kOhm range than 50 Ohm. It's a common term in any science dealing with data, or engineering, or computing.34064 is a very common DC-DC converter you should know of, which a simple Google search reveals. EEVBlog has made videos on it as well as a calculator.The reg/rect will be in a small case in a hot environment, so it will run much hotter than that d/t inability to dissipate heat, so a heatsink will only go so far. Linear regulator seems like a poor choice to me. Are you sure that would be ok? Heatsinks also add cost, complexity, space, and assembly time/cost.Edit: your formula removed

author: macboy
author_rank: Super Contributor
timestamp: June 19, 2023, 01:42:23 pm
content: vv_eev,You have a not so subtle calculation error there. Power is V*I not V/I.So a linear regulator would dissipate (33V-6.7V) * 0.080A = 2.1 W

author: vv_eev
author_rank: Contributor
timestamp: June 20, 2023, 02:27:49 am
content: "Power dissipation is (Vin-Vout)/current draw." -- You.