12-22vdc to 5vdc converter

It sounds like the 7805 will use a lot of the energy the panel is providing.

I can buy a pack of 10 0.5v solar panels on eBay - they each provide 250ma in strong sunlight. If I got them, and strung them all together, I could easily power the motor at 5v. But if the sun got really strong and the volatage went higher, could I kill the motor? My guess is yes (even though its underwater and wont get hot).

Is there a way of turning the extra energy into heat if the sun gets too strong?

I just want a pretty constant 5v for as long as the sun will allow. If there is an energy shortage I dont want a 7805 sucking it all up, and if there is an energy surplus I dont want the motor burning out!

To recap - I can get/have got:

10x 0.5v 250ma solar cells (monocrystalline)
pond pump 4.8v 200ma (ish)

Thanks

James

j_mchattie said:

It sounds like the 7805 will use a lot of the energy the panel is providing.
I just want a pretty constant 5v for as long as the sun will allow. If there is an energy shortage I dont want a 7805 sucking it all up,
The 7805 reg dosn't suck any energy, it drops the voltage over a range of inputs from 8V coming from the panel thru to 22V.
The lower the voltage from the panel [down to about 8V] the lower the energy dissipated as heat in the 7805.

and if there is an energy surplus I dont want the motor burning out!

To recap - I can get/have got:

10x 0.5v 250ma solar cells (monocrystalline)
pond pump 4.8v 200ma (ish)

Thanks

James

Click to expand...

Hi,
The 7805 dosn't use energy it dissipates the excess voltage/current as heat.

If you are in doubt, try it, the power levels you are talking of are not going to cause a fire.!

One of the posts said that it would only be 40 odd percent efficient. So I thought the panel might not provide enough power to run the motor. I wll give it a go though and see.

Thanks for your help Eric

The efficiency of an ideal linear regulator in percents is (V-out/V-in)*100 for example V-in=12V V-out=5V gives little over 41%. The bigger is the difference (V-in - V-out) and lower V-out the worse is the efficiency. As Eric said in the previous post, the excess energy gets dissipated by heat in the linear regulator.
The input current is always the same as the output current (I am ignoring here thq quiescent current of the regulator as it is usually very small) so 0.2A at 5V is your 1W output but the same 0.2A at 12V is 2.4W (the excess 1.4W gets dissipated as heat on the linear regulator). So if your solar cell is able to supply the needed current (0.2A) at the voltage it outputs it should be OK.

I have had a bit of an idea. I went to Maplin and they guy was pretty unhelpful. He said I should buy a £25 12v SLA battery, and a £7.99 circuit kit to make a 12v to 5v transformer.

He then said I should get a charge controller for the battery (£11.99) and power the pump from that.

So I went!

Anywaym on a walk round Tesco (its air-conditioned) I saw an in-car mobile charger which runs between 12 and 24v.

I have an incar charger which I dont use - so I put it across the solar panel (21.3v at the time) - and the charger gave 5.5v

I then did the same across a 12v battery and the charger gave 5.5v again.

I think the problem is solved!

Thanks for all your suggestions - I think I will pull apart my in-car charger and put the bits in a project box.

Thanks

Oh bum.

I juat had an email from the people who make the original solar panel and motor - they say the motor actually runs between 4.8 and 8.5v.

Obviously I want to run at around the maximum output of the pump because it's so small anyway - so I am aiming for about 8v here.

I can buy a solar panel on eBay which can supply 9-11v 3-4w
It is actually listed as a 6v 3-4w panel - but says in the description that the actual output is 9-11v

Is there a way of getting this nearer to 8v?

Sorry to start it all up again - blame SmartSolar!

Thanks

j_mchattie said:

Oh bum.

I juat had an email from the people who make the original solar panel and motor - they say the motor actually runs between 4.8 and 8.5v.

Obviously I want to run at around the maximum output of the pump because it's so small anyway - so I am aiming for about 8v here.

I can buy a solar panel on eBay which can supply 9-11v 3-4w
It is actually listed as a 6v 3-4w panel - but says in the description that the actual output is 9-11v

Is there a way of getting this nearer to 8v?

Sorry to start it all up again - blame SmartSolar!

Thanks

Click to expand...

hi,
Instead of a 7805 you can buy a 7808 thats a +8V reg.
Or if you have already bought a 7805, you can make it output +8V by adding two resistors... what would you like to do.?

I was blinded by looking too hard!

Thanks for that - 7808 will do nicely

Again - thanks!