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Rectifier question

biglouieuk

biglouieuk

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Kent
evening, I wonder if I can ask an electrical question, mostly for my own interest...

I have a lamp/beacon on HS2. I have seen LED topper mods which add flasher LEDs like a police car.

Most of these are 12v DC (for use in cars of course) its 12VAC up at the beacon lamp.

Is it correct to say that you could use a 12v rectifier (https://tinyurl.com/t9dfpjw) to drive the DC LEDs, off the AC source?

Just interested if thats how it works... :)
 
biglouieuk said:
evening, I wonder if I can ask an electrical question, mostly for my own interest...

I have a lamp/beacon on HS2. I have seen LED topper mods which add flasher LEDs like a police car.

Most of these are 12v DC (for use in cars of course) its 12VAC up at the beacon lamp.

Is it correct to say that you could use a 12v rectifier (https://tinyurl.com/t9dfpjw) to drive the DC LEDs, off the AC source?

Just interested if thats how it works... :)
Click to expand...

more or less correct. you get a slight voltage drop across a bride. but only 1-2v typically. a bridge puts out a bit a a crappy dc voltage though. needs smoothing capacitors etc. but you’ll probably be ok with the bridge alone


Sent from my iPhone using Tapatalk
 
Got it, so that's why they have a capacity on the little board in the link...to smooth that drop out.. cool..

Thx @AlanJ 👍
 
biglouieuk said:
evening, I wonder if I can ask an electrical question, mostly for my own interest...

I have a lamp/beacon on HS2. I have seen LED topper mods which add flasher LEDs like a police car.

Most of these are 12v DC (for use in cars of course) its 12VAC up at the beacon lamp.

Is it correct to say that you could use a 12v rectifier (https://tinyurl.com/t9dfpjw) to drive the DC LEDs, off the AC source?

Just interested if thats how it works... :)
Click to expand...

if you want to drive a 12v dc load then just use the mod power board you have. It has 8 12v dc outputs for just this purpose

in answer to your 12vac rectifier question. To calc the rectified output voltage from an ac input you use

vdc = vac*1.41

so the rectified output is higher than the measured ac. This is because the ac measurement Is rms not peak and the ac is a sine wave

now this assumes a perfect world and no load so youd then deduct 0.7+0.7 (roughly) for the rectifier Diodes off this calc so

(12*1.41)-1.4=15.52v

if you then add a smoothing cap the dc voltage will raise again depending on what you add
 
Thx @myPinballs

Sticking with the initial thought for a minute. Given the info above from @Alan and @myPinballs , if I understand it correctly, I would now have 15.52 vdc presented to the topper. This is needed to drive 2 x 12vdc led flasher units.

The little card I linked to in the OP, has 4 diodes, a cap and a resistor. I presume the diodes stop the alternating current returning (and therefore not alternating and so now direct?), the resistor prevents it going over 12v and the cap smooths it out to give a constant 12v? I'm no doubt simplifying this way way to much.

The led units I see online, are from Amazon, so don't come with a spec sheet, so I'm not sure what they draw, low presumably, as they are 12 LEDs? Is there a way to understand if 2 of the (alarmingly cheap) flasher units would be safe to use?

Thinking about using the mod power board, I hadn't considered that. :) I would need to take 12vdc power from the board up to the lights and somehow signal the lights to run at the same time as the beacon. At the moment (with my incredibly limited knowledge) the Triac board sends power to resistor board to power the 12vac bulb and motor and thats how it knows to come on. So we would need a relay/switch (?) board that accepts the 12vdc power and then switches on/off from a signal, from somewhere.

This is why I was thinking about using the 12vac power up thats up there in the topper already, as that currently handles the "turning on" of the led and motor, by powering it.

I need to look at the schematic and understand that signal piece a bit more.

Fun stuff
 
Last edited:
@Solar Sailer fitted a nifty strobing light to his LW3 (check out his shop log), if memory serves that was a straight replacement to 18Vdc, it wasn't well marked so I tested it on my desktop PSU through a wide range of voltages.
 
myPinballs said:
if you want to drive a 12v dc load then just use the mod power board you have. It has 8 12v dc outputs for just this purpose

in answer to your 12vac rectifier question. To calc the rectified output voltage from an ac input you use

vdc = vac*1.41

so the rectified output is higher than the measured ac. This is because the ac measurement Is rms not peak and the ac is a sine wave

now this assumes a perfect world and no load so youd then deduct 0.7+0.7 (roughly) for the rectifier Diodes off this calc so

(12*1.41)-1.4=15.52v

if you then add a smoothing cap the dc voltage will raise again depending on what you add
Click to expand...
OOh yes I stand corrected, apologies. (I knew all this stuff when I first did electronics aged 10-13, somehow girls and alcohol took over and my memory hasn't been the same ever since! )
 
AlanJ said:
OOh yes I stand corrected, apologies. (I knew all this stuff when I first did electronics aged 10-13, somehow girls and alcohol took over and my memory hasn't been the same ever since! )
Click to expand...
At least it went in once, you always have that 🙂
 
astyy said:
@Solar Sailer fitted a nifty strobing light to his LW3 (check out his shop log), if memory serves that was a straight replacement to 18Vdc, it wasn't well marked so I tested it on my desktop PSU through a wide range of voltages.
Click to expand...

I'll search it out.. 👍
 
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