Power supplies

[glider]

This afternoon I've been experimenting with using a DC power supply rather than using a battery to power my grab and go setup.

 

I have a box of tricks that sits under the mount. It contains a 6 port hub, 12Vdc to 5Vdc (5A) converter for the hub, a serial to TTL converter to connect to the mount. The box takes in 12Vdc and also gives out to power the mount and the fan on the 314L imaging camera.

 

My imaging and guiding cameras connect to the hub.

 

Then I have a 10m powered USB lead back to the house.

 

Normally I sit a battery next to the mount and connect power to the box of tricks.

 

So................

 

Today I simply tried having a 0-30V 2.5A power supply in the house and a lead out to the box of tricks.

 

When I say lead I mean 10m of mains extension lead with the plug and sockets cut off the ends.

 

Now I know that I loose down that length of cable. I have to ramp the volts at the supply up to about 14.2Vdc before the mount light stops flashing and interestingly the fan on the 314L starts to 'sound right'. Also, until the supply is high enough that over 12.2Vdc appears on the 314L there is a noticeable pattern on flats!

 

Now, a fully charged car battery, that I'm happy to connect straight to the box of tricks can be reading getting on for 14.8Vdc.

 

Am I safe doing what I'm doing?

 

Can I just set the DC power supply to 14.8Vdc and assume it's nothing more than a battery?

Edited October 4, 2015 by glider

[BAZ]

The 14.8v you get on the battery may just be a float voltage, if it's that when everything is running, then the battery may be overcharging. Most 12v stuff is rated to run at 13.8v, which is the nominal charging voltage on a car.

I would check what the equipments spec is, but I would have thought for the dollar you pay, they would have a voltage regulator built in to protect them. I could be wrong, you don't want the white smoke of death to appear.

[Smithysteve]

Hi Glider, in addition to Martyn's good advice, I have two concerns;

1) The power supply unit seems to have a low current output of only 2.5 amps, which may struggle to feed all your hardware. You may be over ramping up the voltage to compensate for the lack of current available etc.

2) if you have everything running and you set the voltage at 14.8v, (which I think is too high!) and then after your session, you start switching things off, respectively decreasing the current flow, you may find the voltage level will go up even higher, as you switch things off! This voltage may be higher than some items can tolerate. (This could lead to failure of PCB's etc)

So....

If the power supply can be set to the correct voltage before plugging anything in to it, (e.g. With no load), and the output voltage level doesn't vary (I.e. Go down) with increases in load, then that is good, the controlled limited voltage will not harm anything.

Also, the power supply unit should be able to comfortably output enough current without overloading it - if so, then you should be ok.

Add up the current loads and make sure your unit can cope. Check your voltage level is kept within equipment tolerances.

Hope it all works out ok.

[glider]

If I use a short lead from the power supply I can set 13.8Vdc and every thing works.

 

If I use the 10m lead the mount end only gets about 12Vdc with the mount light flashing and noise on the 314L images.

 

If I up the supply to get 12.2Vdc at the mount then the supply end is at around 14.5Vdc, with and without load.

 

Would a heavier duty lead be better, less resistance?

[Smithysteve]

In short, a heavier duty lead would be much better, but there are other things coming in to play. Also, because you PSU (power supply unit) is capable of outputting 30v then it has potential, literally, to blow your equipment...

Regards your voltage test;

The volts will be the same at both ends when there is no load (I.e. No current flowing).

But this test result is deceptive... with a similar cable say 500m long etc the volts would be the same at both ends if there was no current flowing. Add current to the circuit and things change... As current (load) increases, volts drop increases per metre length. Again the longer the cable the more the volts drop. But the larger the cores of the cable the less volts drop will occur over its length...

Because it all works ok on a short cable, It looks like your load is not too much for your power supply, so that's good. I am just worried about the possibility of your voltage going too high. that setting your PSU end volts to 14+ volts could result in 14 + volts across sensitive equipment, if you routinely decrease the main load, (un plug the fans and dew etc.). Switching load off will increase voltage at the load end! a sensitive voltage dependant load, e.g. A Synscan etc, will only tolerate a certain voltage reduction... Or increase.

The trick is to minimise volts drop by minimising cable length and increasing cable CSA size... And keeping your load to a sensible minimum, Then you should not have a problem using say, 2.5mm cable of less than 10 metres with a load of less than 5amps etc. If you cannot reduce length, then increase CSA. Using 4mm csa would be better...

I am trying not to get too technical, but for your information, I will try and explain what's happening... I am trying to explain why you have the same voltage at both ends with no load and the relationship between volts current and resistance :-)

Volts drop depends on certain factors; e.g.

The Cable: It has a certain resistivity dependant on the conductor material, the conductor thickness and the length of its conductors. E.g. Copper conductors have less resistance than aluminium etc. ( the point here is resistance of the cable affects the following formula... See below)

The load. The Load takes power from the supply. In a dc circuit, power = volts x amps ( the point here is the higher the load, the higher the amps the more it effects the following formula... See below)

There are other factors, but let's keep it simple:-)

V = I * R

Or,

(Volts drop) V(d) = I (amps or current) * R (resistance in ohms)

This volts drop, say for example it's 2.7v, means you have 13.7vdc at the power supply and 11 volt at the load end.

Proceed with some caution and try to control your voltage from going too high... :-)

If you limit the PSU voltage at the supply end to less than the critical threshold of your equipment, then it cannot do any harm if you reduce loads. but to get everything running ok you will probably have to use cable with bigger cores!

I hope this lot helps 😄⚡ï¸ðŸ˜°ðŸ’¥