Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Making a mains powered tuner box run off car 12v supply.

Status
Not open for further replies.

john_s

New Member
Hi All

1st post so apologies that it is asking for advice!

I want to run a low power(>5w), mains (240vAC) powered tv tuner box off a car battery.

Rather than running a car power inverter up to 240VAC then back down it seems to make more sense to run it all off DC power.

I've opened it up and there's a smallish transformer which must have 2 outputs one at 12vDC and one at 5vDC.

I assume I can just supply 12v from the car to the part of the circuit that runs off 12v, but what is the easiest way of powering the 5v? A DC-DC converter?

In addition should i place any further fuses/filters in there, i dont want to set my car on fire....

Thanks in advance

John S
 
I've opened it up and there's a smallish transformer which must have 2 outputs one at 12vDC and one at 5vDC.

I do not understand the "must have" part. Did you measure its power supply secondary circuits ? It is highly likely that if there is both those voltages, 5V is derived from 12V. You must follow traces and draw a schematic of what is going on, and very likely the 12V car battery can be fed directly to the right point the right way, but you have to be sure of the existing circuitry.
 
Apologies, yes the tranformer has 2 secondary coils which i have measured with a dvm at 12 and 5 volts dc after a big diode on the 5v and a small diode on the 12v. Ill have a go at tracing the tracks when i get home tonight.

thanks
 
If the receiver box does use two voltage rails; 12 and 5 Volt; supplying 12 from the car plus adding an internal +5V regulator should make it work.
 
Car "12V" wanders between ~8V when cranking to over 14.7V with a fully charged battery running at highway speeds. Are you sure that such a wide variation will work for the formerly regulated 12V? If not, then you will have to use a "low dropout regulator" to make 12V out of the nominal battery voltage, and only turn on the box after the car is started.
 
thanks for all the tips!
Ive powered the box with 12v and 5v from a computer PSU and it works perfectly, so i just need to sort out some fuses and the suggested regulators for it to work off the cars 12v.

Ive had a look at the pcb and its a bit more complicated than i thought, theres 2 parts one i assume the high voltage and the other the low voltage. bridging them is the transformer, a cap (filter?) and a 4 legged IC of some sort (not sure what that does).

Anyway should i just tap in the required dc supplies as it is now and leave it or should i remove/isolate all the transformer and high voltage side of the board?

John
 
If 'works perfectly' with the PSU, applying the car battery and a regulator should perform well. The transformer section would be out of circuit by the rectifier diodes, but preferably to cut traces to isolate.
Adding a couple of diodes could bring the car 13.8V to a lower voltage within reasonable tolerance :

+Car13.8V---------|>|-------|>|--------+12.6V supply node---------------7805----------------+5V supply node
 
Thanks for the tips all, got it all working. Power in the car is automatically switched by the car and seems pretty well regulated.

John
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top