12v regulated supplies

[Tusses]

I do agree with Gibbo that the zener solution is a non-starter for the reason he articulated. I don't think the people proposing zeners understand how they operate.

 

Chris

 

I was thinking of using the zener as a voltage reference to drive a power Transistor - rather than as a sink to any over voltage. see my post #26 - its would only come into use when the voltage went abouve a set amount.

[Gibbo]

I was thinking of using the zener as a voltage reference to drive a power Transistor - rather than as a sink to any over voltage. see my post #26 - its would only come into use when the voltage went abouve a set amount.

 

Well you can either use a very low value, very high wattage resistor and a very high wattage zener driving a transistor which will give terrible regulation, waste lots of power and dissipate tons of heat or you can use two transistors and suffer the large voltage drop of 1.5 to 2 volts.

 

Haven't we covered this already in this thread?

 

Gibbo

Edited April 1, 2008 by Gibbo

[Tusses]

Well you can either use a very low value, very high wattage resistor and a very high wattage zener driving a transistor which will give terrible regulation, waste lots of power and dissipate tons of heat or you can use two transistors and suffer the large voltage drop of 1.5 to 2 volts.

 

Haven't we covered this already in this thread?

 

Gibbo

 

I can see I'll have to figure out how to get drawings up on here !

[chris w]

If the batteries are at around 60% state of charge and a load is put on of around 25% the battery capacity the voltage will fall to WELL below 11 volts.

 

 

 

No he isn't. See below.

 

 

 

No it isn't. It's 2 volts. Recommended headroom to keep away from this is 2.5 volts.

 

http://www.fairchildsemi.com/ds/LM/LM7805.pdf

 

See page 8 - "Vdrop" spec.

 

I didn't write that. The people that make them wrote it. I think it's fair to assume it's correct.

 

Gibbo

 

Why do use use such exaggerated figures like "25% of the battery's capacity". Of course if you are going to use huge currents then yes I agree. If you put a spanner across the battery the terminal voltage will be pretty much zero and will also heat your boat at the same time.

 

With regard to the 12v regulator question, the 2v/2.5v headroom figure cited by Fairchild is the drop when the input voltage is ABOVE the nominal regulated voltage and I agree with this figure (in fact I quoted it myself in an earlier post). 1.5v is the drop if you take the input BELOW the nominal regulated voltage. In fact, if you take it well below the nominal regulated voltage the drop becomes even less than one volt.

 

Chris

[PaulG]

Paul

 

12.5v is the off-load battery voltage in your example. When running loads, there will be a voltage drop across the battery's internal resistance. So although the off-load voltage may be 12.5v the on-load voltage will be lower. If you are drawing 20A for example and the off-load voltage is 12.5v the on-load (20A drawn) voltage will drop to around 12.2v.

 

Gibbo is exaggerating the 78XX voltage drop to try to score points against me for some reason. If my battery bank ever went down to 10.5v on load, my fridge would have already cut-out and my inverter too. The lowest I ever see even under heavy load is about 11.5v. Normally they are running above 12v on a typical load.

 

So the lights could get down to 10.5v (the drop on a 78XX is 1.5v not 2v as Gibbo says). So they will be slightly dimmer, but this may be better than putting 15v through them. At the end of the day it's just another one of many possible solutions with some pros and some cons like all solutions.

 

I do agree with Gibbo that the zener solution is a non-starter for the reason he articulated. I don't think the people proposing zeners understand how they operate.

 

Chris

 

12.5 volts is the supply voltage to the device, and of course in the real world, you would need to take account of voltage drop on the supply cables etc. This applies equally to your beloved 78XX! However, what the OP actually asked about was overvoltage protection for sensitive devices.

[chris w]

12.5 volts is the supply voltage to the device, and of course in the real world, you would need to take account of voltage drop on the supply cables etc. This applies equally to your beloved 78XX! However, what the OP actually asked about was overvoltage protection for sensitive devices.

 

Battery internal resistance drop will usually be more significant than cable drop (assuming the correct cables were installed). My idea of using a 78XX reg was indeed to offer the OP an over-voltage protection idea.

 

Chris

[PaulG]

They would be ok but low drop regs all suffer from 2 problems which are relevant in this case.

 

They use a PNP pass transistor (I mentioned this earlier in the thread) which allows them to drastically reduce the voltage drop. The problem is that they are very susceptible to damage from high input voltage. That particular one has an absolute maximum input voltage of 26 volts. They also don't stop voltage spikes getting through.

 

One of those would last about 10 minutes in most 12 volt battery systems.

 

Gibbo

 

I wasn't really suggesting that it was the only component you would need, but with some transient protection I think it could be used to build a basic low-cost circuit to protect some LED lights, for example.

[PaulG]

Battery internal resistance drop will usually be more significant than cable drop (assuming the correct cables were installed).

Chris

 

Sweeping statement indeed! It depends on whether high load conditions are "usual". Cable drop is always present. Even using the correct cable, 0.2 volts would not be unusual, 0.5 volts on some circuits on older boats where the cables were less well specified.

[Dylan]

Well I wish you would make your minds up; two more halogens popped last night Always the ones at the back of the boat nearest to the dis.board.........

[chris w]

Sweeping statement indeed! It depends on whether high load conditions are "usual". Cable drop is always present. Even using the correct cable, 0.2 volts would not be unusual, 0.5 volts on some circuits on older boats where the cables were less well specified.

 

Well the voltage drop for 20A due to internal battery resistance will be about 0.4v. 20A is not a "high load". If the cable drop is 0.5v as in your example then clearly the right cables have not been specified. Of course cable drop is always present but its maximum is fixed for that partcular circuit by the selection of correct cable size.

 

Suppose you had 2 x 20A circuits running. If you select the right cable the volts drop due to cable resistance should not be more than 0.3v in EACH circuit. However, the volts drop AT THE BATTERY due to the battery's internal resistance will be around 0.8v which will affect BOTH circuits.

 

Hence my earlier statement that battery internal resistance drops are more significant than cable drops. For any individual circuit, the total volts drop at the end equipment is the cable drop for that particular run PLUS the battery internal resistance drop for ALL runs. So, for our 2 x 20A circuits above, the total volts drop at the end equipment will be 1.1v of which cable drop contributes 27% and the battery drop 73%.

 

Chris

[Ex- Member]

If one absolutely needs a constant 12v then you are correct and an SMPS is the solution.

 

However for stabilising LED voltage we are not really so concerned with undervoltage as ensuring there is no overvoltage. Afterall, if one has incandescent lamps in the boat (tungsten or halogen) the voltage on them is not constant either but we don't notice the subtle changes of brightness from moment to moment, as other equipment switches on or off and/or the batteries lose charge and the terminal voltage drops.

 

The point is that a 7812 type regulator WILL work at input voltages under the minimum specified and will still produce a healthy output voltage. So, for a typical 7812 regulator the minimum input is usually specified as being around 15v to ensure 12v out. However, the output will remain within a few tenths of a volt of 12v down to about 13.5v input. Thereafter, the output will drop very linearly against input voltage, with the output remaining at an almost constant 1.5v below the input voltage. This means that even at 12v INPUT, we will still get 10.5v OUTPUT which will still be perfectly bright for LEDs.

 

I use this system on my boat to run a string of LED lamps, viz: a simple 7812 regulator at about 60 pence each and a couple of caps for stability.

 

Chris

 

Cheers Chris, that sounds the soultion. How is this actually connected before the light. I'll also mention that I have 22 LEd cieling lights on board paired through the length of the boat. Each pair has it's onw separate fuse except the 4 lights nearest to the 12 volt panel. So 12 fuses.a bit ott i know, or maybe not Can this item be fitted colse to the 12v panel, o anywhere in line. If I'm being a pain just say

[chris w]

Cheers Chris, that sounds the soultion. How is this actually connected before the light. I'll also mention that I have 22 LEd cieling lights on board paired through the length of the boat. Each pair has it's onw separate fuse except the 4 lights nearest to the 12 volt panel. So 12 fuses.a bit ott i know, or maybe not Can this item be fitted colse to the 12v panel, o anywhere in line. If I'm being a pain just say

 

If you go down this route, you need to fit the regulator at the end of the line which is nearest the batteries. Put a small capacitor (100nF) between the input and ground pins (which aids stability) and another 100nF between the output and ground pins (which aids transient response). You can get 2A versions so that should run a lot of your LEDs.

 

Chris

[Ex- Member]

If you go down this route, you need to fit the regulator at the end of the line which is nearest the batteries. Put a small capacitor (100nF) between the input and ground pins (which aids stability) and another 100nF between the output and ground pins (which aids transient response). You can get 2A versions so that should run a lot of your LEDs.

 

Chris

 

Cheers Chris, as you're in the know, what's the best internet site for these parts. I live rurally mostly use internet for purchases.

 

Cheers

[chris w]

Cheers Chris, as you're in the know, what's the best internet site for these parts. I live rurally mostly use internet for purchases.

 

Cheers

 

I don't know which is best but I tend to get my bits from Maplin who will deliver next day if you order in the morning of the day before.

 

clicky

 

Chris

[Ex- Member]

I don't know which is best but I tend to get my bits from Maplin who will deliver next day if you order in the morning of the day before.

 

clicky

 

Chris

 

Cool Have used them before, cheers

[Chris Pink]

vvv

Edited April 1, 2008 by Chris Pink

[chris w]

Clip a heat sink on to each regulator as well (again Maplins). Sod's law - I've just seen Maplin's stock of 2A versions is on 14 day delivery. Farnell(another good supplier - usually cheaper but have a minimum order size) have 1.5A versions.

 

Chris

[Dylan]

If you go down this route, you need to fit the regulator at the end of the line which is nearest the batteries. Put a small capacitor (100nF) between the input and ground pins (which aids stability) and another 100nF between the output and ground pins (which aids transient response). You can get 2A versions so that should run a lot of your LEDs.

 

Chris

Is 2A the largest available?

[chris w]

Is 2A the largest available?

 

I believe 2A versions are the highest BUT it is easy to put a bypass transistor around the 7812 so that you can have it handle much higher currents.

 

Rather than describe in words how to do it, see the clicky here and go to Page 24 of the document where there is a simple circuit diagram.

 

Chris

[Dylan]

I believe 2A versions are the highest BUT it is easy to put a bypass transistor around the 7812 so that you can have it handle much higher currents.

 

Rather than describe in words how to do it, see the clicky here and go to Page 24 of the document where there is a simple circuit diagram.

 

Chris

Thanks Chris.

[smileypete]

Is 2A the largest available?

 

I came across these on Ebay the other day, 5A ajustable with typical 'dropout' of 1.3V

 

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?Vie...em=380010582423

 

cheers,

Pete.

Edited April 1, 2008 by smileypete

[chris w]

From the volts point of view, any 78XX type regulator can be made adjustable by inserting a resistor into the ground lead (ie: between the "ground" pin and actual ground). The maths is very simple and an example is also illustrated in the Fairchild document I referenced above. (page 23; Fig 12)

 

Chris

[Gibbo]

I believe 2A versions are the highest BUT it is easy to put a bypass transistor around the 7812 so that you can have it handle much higher currents.

 

 

Chris

 

And lose another 0.7 volts!

 

Gibbo

 

From the volts point of view, any 78XX type regulator can be made adjustable by inserting a resistor into the ground lead (ie: between the "ground" pin and actual ground). The maths is very simple and an example is also illustrated in the Fairchild document I referenced above. (page 23; Fig 12)

 

Chris

 

You need another one from the output to what is normally the ground terminal too.

 

Gibbo

[chris w]

And lose another 0.7 volts!

 

Gibbo

 

To which saturated V_CE are you referring to that's 0.7v?? Huh?

 

You need another one from the output to what is normally the ground terminal too.

 

Gibbo

 

I know...................... that's why I referenced the diagram which shows the other resistor.

 

Chris

[PaulG]

Well the voltage drop for 20A due to internal battery resistance will be about 0.4v. 20A is not a "high load".

On my boat I would only draw 20A intermittently (when a pump is running for example). So whether 20A is a high load or not depends on individual circumstances. In my case, 20A would be a high load.

If the cable drop is 0.5v as in your example then clearly the right cables have not been specified.

Of course cable drop is always present but its maximum is fixed for that partcular circuit by the selection of correct cable size.

 

Funnily enough, that's exactly what I said. So at least we agree on something!

 

There's a big difference between theory and practice, however, and I suspect that there are a lot of boats out there with under specified wiring. After all, it saves the builder a few quid and who's to know? Put a meter on your tunnel light and you'll probably see what I mean.

 

Suppose you had 2 x 20A circuits running. If you select the right cable the volts drop due to cable resistance should not be more than 0.3v in EACH circuit. However, the volts drop AT THE BATTERY due to the battery's internal resistance will be around 0.8v which will affect BOTH circuits.

 

Hence my earlier statement that battery internal resistance drops are more significant than cable drops. For any individual circuit, the total volts drop at the end equipment is the cable drop for that particular run PLUS the battery internal resistance drop for ALL runs. So, for our 2 x 20A circuits above, the total volts drop at the end equipment will be 1.1v of which cable drop contributes 27% and the battery drop 73%.

 

Chris

 

So you had to double the load to try to prove your point?

 

Just run the sums under a 5A load (which is just as likely a scenario as 2 X 20A) and see what you get. So you just can't say that drops due to internal battery resistance is "more significant". Sometimes it is, sometimes it isn't, so you should take both into account.

 

Have a nice day

 

Paul G.