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Forum Index : Microcontroller and PC projects : Hardware design issue
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| Malibu Senior Member  Joined: 07/07/2018 Location: AustraliaPosts: 237 |
G'day all, I've just got my first order from JLCPCB (great service, top-notch assembly; I'm very impressed!) for my 6-channel GP Controller. A simple idea- Webmite controls 6 outputs, depending on the time settings sent through a web browser. After checking all the relevent volts from the (2x) USB connection and the rectified AC supply, I soldered in the Webmite to do some further tests. All good until I fired up the AC supply and found that the 7805 reg is getting pretty hot, plus no output is actually changing state on the ULN output. The input is changing state as expected, but not on the output drivers for the relays. Here's the (cut-down) schematic...  The AC comes in, gets rectified and regulated to 12VDC. That's supplied to the ULN2003 which SHOULD drive the relays accordingly (along with an LED to show the status) For some reason, the reg heats up; the LED's don't change state; nor does the relays. There's no shorts that I can find on the PCB, so I'm pretty sure the PCB is correct. Removing the relays makes no change. Do any of the hardware wizards spot whatever the dumb mistake is that I've done? It's got me a bit baffled and I'm now in head-scratch mode.......  (PS: RLY2 to RLY6 is just a repeat of the RLY1 circuit) Edited 2024-12-04 16:18 by Malibu John |
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TassyJim Guru  Joined: 07/08/2011 Location: AustraliaPosts: 6110 |
12V goes to relays and common pin UNL2003 switches other relay leads to ground.  VK7JH MMedit MMBasic Help |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 886 |
 Yeah, that's how I remember it  |
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| Malibu Senior Member Joined: 07/07/2018 Location: AustraliaPosts: 237 |
OK, so that's the dumb mistake, it seems... Shoulda pulled the relay commons high instead of low...  Live and learn, I'll make some changes tomorrow and get back to you  Thanks for the input! John |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 886 |
There is also the not-as-popular high-side driver, ULN2981 or MIC2981. MIC2981.pdf Edited 2024-12-04 17:08 by PhenixRising |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6839 |
If you have AC in then rectify it as shown in the circuit the DC is *very, very* rough and will drop to zero on every half cycle as there is no reservoir capacitor from pin 1 of the 7812 to GND. I'd be very surprised if this power supply will work. Also, the output capacitor from the 7812 is high for those chips. Move the 220uF to the input of the regulator. That will give you about (12*1.4)-1.2= 15.5V DC. Dropout voltage on these is about 2V, so you should be ok for a 12V regulator if the current isn't too high. Normally you'd want to see around 18-19V here. Now use about 22-47uF capacitor for the output. It only really needs 0.1uF for stability, but I like a bit more for pulling relays in. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| lizby Guru Joined: 17/05/2016 Location: United StatesPosts: 3159 |
And also, why rectify 12V AC when 12V DC power supplies are readily available with all sorts of current capacities? Where I am (North America), it's a lot easier to get a nominal 12V DC power supply than an AC supply. PicoMite, Armmite F4, SensorKits, MMBasic Hardware, Games, etc. on fruitoftheshed |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4276 |
Hi lizby, Maybe to avoid the "wrong polarity" of the DC barrel connectors. It may not be intended for AC at all. Volhout PicomiteVGA PETSCII ROBOTS |
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