Any electronic wizards out there? How to regulate a 12v supply

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Ammonite

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I want to run a 12V DC supply to a masthead aerial amplifier that was originally designed to be fed by a 240v / 12v transformer. I'm led to believe by the manufacturers that the amplifier may not take kindly to a fluctuating DC voltage which could be anything up to 14.4v with the engine running.

I was wondering whether there was anything readily available on eBay, RS, Maplin etc but not being an electronics expert don't really know what to look for. Whenever I search for regulated 12v supply or some such I seem to end up with another 240 / 12V transformer. Failing that what sort of circuit would I need to have built assuming it's cost effective to do so?

Thanks
 
Iasked the same question in a local branch of Maplins some time ago.

One of the assistants said they get asked for that sort of thing all the time, but they don't stock an off the shelf product so he promptly drew out a wiring diagram schematic for a 12v regulator, then went and sourced the bits from the back office!

All I had to do was wire it up and solder together.

So try your local branch of Maplins, I was impressed by their knowledge and helpfulness.
 
Normal regulators like the 7812 need to drop about 1.2 vcd across them to regulate correctly so with and input of 12.7 volts (fully charged battery) you will only get
about 11.5 VDC out max.
 
Hoolie said:
For use on board you'd be better with this http://www.ebay.co.uk/itm/DC-DC-Con...029?pt=LH_DefaultDomain_0&hash=item3a813f9d25 as it's much more robust and (semi) sealed.
Click to expand...

That particular DC DC converter is a "step down" converter so you will get only get a lower voltage than is input to the device.

I take it that the OP has a 12v electrical system on his boat so you will not get 12 out from that device.

What you need is a step down/up converter which allows for the output voltage to be equal of greater than the input voltage.

This is the kind of device I used

http://www.ebay.co.uk/itm/DC-DC-Ste..._Measurement_Equipment_ET&hash=item5af19ff8be

which is the first one on my posting #4.

Also pls read my post #7 about the volt drop you get on the most common chip regulator 78** series. (last 2 digits indicate the regulated output voltage providing the input id higher than the min required as specified in the chip datasheet.

Most DC DC converters on ebay do not specify the minimum input voltage for step down devices so beware
 
Rogershaw said:
That particular DC DC converter is a "step down" converter so you will get only get a lower voltage than is input to the device.

I take it that the OP has a 12v electrical system on his boat so you will not get 12 out from that device.

What you need is a step down/up converter which allows for the output voltage to be equal of greater than the input voltage.

This is the kind of device I used

http://www.ebay.co.uk/itm/DC-DC-Ste..._Measurement_Equipment_ET&hash=item5af19ff8be

which is the first one on my posting #4.

Also pls read my post #7 about the volt drop you get on the most common chip regulator 78** series. (last 2 digits indicate the regulated output voltage providing the input id higher than the min required as specified in the chip datasheet.

Most DC DC converters on ebay do not specify the minimum input voltage for step down devices so beware
Click to expand...

Just to confirm - will this bit of kit ensure I get a steady 12V output +/- 0.5V assuming a fluctuating input range of 12 - 14.4V, as well as dealing with any spikes above 14.4v should they happen. I'm not sure I need the step up function as in practice I never let the batteries get below about 12.4v or are you saying it is needed because of the drop induced by the regulator itself (post #7)
 
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chaktoura said:
I want to run a 12V DC supply to a masthead aerial amplifier that was originally designed to be fed by a 240v / 12v transformer. I'm led to believe by the manufacturers that the amplifier may not take kindly to a fluctuating DC voltage which could be anything up to 14.4v with the engine running.

I was wondering whether there was anything readily available on eBay, RS, Maplin etc but not being an electronics expert don't really know what to look for. Whenever I search for regulated 12v supply or some such I seem to end up with another 240 / 12V transformer. Failing that what sort of circuit would I need to have built assuming it's cost effective to do so?

Thanks
Click to expand...

If it's just like a TV or FM aerial booster, many of them were constant current devices taking from a few mA to a few tens of mA.
The old Avantek/HP/Agilent MSA series were typical. Often biased with just a resistor anyway.
A simple resistor in the supply line would do the job in that case.
But a low drop out linear regulator IC might be better, and less risk of injecting noise than a buck/boost switcher.
However, off the shelf and built for under a tenner is a strong case....
 
It is unlikely that a simple TV amplifier would be damaged by 15v in. It may well be that you have come up against the attitude we better just make sure by telling em it may be damaged. Much depends on the value of the amplifier but i would be inclined to try it on straight battery voltage if the cost a of a new amp is comparable to that of a regulator.
Having said that a simple regulator for the very low current involved would be a series resistor and a 1 watt zener diode of 13 v rating. The resistor would be about 47 ohms fitted in series with the amplifier. With a 10 ma drain this will lose only .47 volt probably less in practice. The zener diode is wired from the input of the amp downstream of the resistor to negative. NB the cathode banded end goes to pos. A zener diode is a diode which when presented with a voltage in excess of its rating in the reverse direction will pass a lot of current ie .08 amp for a 1w 13v zener. This current which only flows when the battery voltage is high, passes through the resistor so pulling down the voltage to the amp.
An interesting alternative is a IC called TL431 which is in effect an adjustable zener diode. By connecting a variable resistor you can set the limiting voltage of the circuit.
good luck olewill
 
William_H said:
It is unlikely that a simple TV amplifier would be damaged by 15v in. It may well be that you have come up against the attitude we better just make sure by telling em it may be damaged. Much depends on the value of the amplifier but i would be inclined to try it on straight battery voltage if the cost a of a new amp is comparable to that of a regulator.
Having said that a simple regulator for the very low current involved would be a series resistor and a 1 watt zener diode of 13 v rating. The resistor would be about 47 ohms fitted in series with the amplifier. With a 10 ma drain this will lose only .47 volt probably less in practice. The zener diode is wired from the input of the amp downstream of the resistor to negative. NB the cathode banded end goes to pos. A zener diode is a diode which when presented with a voltage in excess of its rating in the reverse direction will pass a lot of current ie .08 amp for a 1w 13v zener. This current which only flows when the battery voltage is high, passes through the resistor so pulling down the voltage to the amp.
An interesting alternative is a IC called TL431 which is in effect an adjustable zener diode. By connecting a variable resistor you can set the limiting voltage of the circuit.
good luck olewill
Click to expand...

Thanks olewill. Sounds like a very simple and cheap solution. With hindsight the step up /down device does sound OTT but I've bought it now. Would the zener diode route work for LED spot interior lights or would I be better off with something else - the eBay device I've just bought for instance. Cheers
 
In the world of remote control models they have gadgets called UBEC's (also known as SBEC's) like this...

http://www.hobbyking.com/hobbyking/store/__18788__12V_4_5A_UBEC_2_5S_Lipoly_7_2_21V_.html

Designed for RC models to output a specific voltage whatever battery is used as the supply (1s=3.7v, 2s=7.4v, 3s=11.1v, 4s=14.8v...). However the reviews say this is also just a step down device and requires at least 12.5 volt to achieve 12v output, maybe more. Being designed for RC the noise of the output should be quite minimal, and being intended for flying models it is very small and light. This is the 4.5A version, there is a 2.5A version as well apparently.

Don't forget even if you have 12.5v at the base of the mast, volt drop along the cable in the mast will reduce that further at the top.
 
chaktoura said:
Thanks olewill. Sounds like a very simple and cheap solution. With hindsight the step up /down device does sound OTT but I've bought it now. Would the zener diode route work for LED spot interior lights or would I be better off with something else - the eBay device I've just bought for instance. Cheers
Click to expand...

This simple regulator circuit is really only suitable for low current devices. It requires that the series resistor be high enough in resistance to drop the voltage when the zener starts conducting witha zener current low enough that it does not exceed the ratings of the zener. Yet under normal low input voltage conditions the resistor will reduce the voltage to the device so needs to be a low ressitance. Obviously the less current the device draws the less voltage drop in the resistor. Further the zener at high input voltages will waste power and possibly get hot.
I suggested a 1watt zener of 13v rating. This means it can conduct 1/13 amps or about 77ma. Now if our input voltage is 15v then we need to lose 2 volts in the resistor at 77ma 2000/77 about 26 ohms. Now if the amp draws 10 ma you are going to lose .01x 26 ohms or .26 volts at all times. Hopefully your amp won't draw so much maybe half of that so even less volts dropped or you can use an even larger resesitor.
Now you can see that if you are regulating say 200 ma of LED (about 2.5 watts) you need a lot lower series resistor to avert losing too much power to LEDs but a lower ressitance means you have to be able to suck away more current at high input voltage (15v ) so you need a much higher power rated zener.
This assumes that you are using LEDs with a series resistor current limited for 12 to 14volts. If you can get into the LED fitting a reduce the series resistance then you can have it running at full brightness perhaps at 10v. Now you can have a larger resistor in series with the zener regulator without needing to bypass such large currents to get the needed volt drop when battery goes to 15v.
However for your situation. I suggest you find out exactly how much current the amp draws at 12v. If it is around 5ma then a zener regulator would be fine. If it is much more than say 10ma then the switch mode buck boost regulator you have bought will be needed. If mnot then the buck boost regulator will be ideal for LEDs. Although for me I would risk the cheap LEDs on occasional 15v from charger. good luck olewill PS apologies if i go the math wrong anywhere.
 
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