Undervoltage - Page 1
Forum section: electronics / beginners

author: olsenn
author_rank: Frequent Contributor
timestamp: July 20, 2013, 09:57:58 pm
content: I recently purchased an Instapark Mercury 10 (watt) solar charger, but when I hooked it up to my DC electronic load, I noticed that the voltage isn't what I would consider "regulated". When no current is being sinked, the charger's USB ports give roughly 5 volts (as to be expected), but if I try to sink more current than what the device can output at 5 volts, it drops the voltage instead of shutting the output off.For example, if the sun is bright, and I try to sink 100mA, it will have no problem outputting 100mA @ 5V, If I increase the current to 200mA, it will stay at 5 volts, but source 200mA. However, if I try to source 1 amp to my load, it may actually source 1A @ 2.5V. Shouldn't it put out either 1A @ 5V or nothing at all?So I guess my question is, can this behavior hurt my phone/ipad/camera/etc? Is the only thing important in charging Li-Ion batteries that the voltage never exceed 5V (but go as low as it wants)?Thanks!

author: Rerouter
author_rank: Super Contributor
timestamp: July 20, 2013, 10:41:12 pm
content: under-voltage is not so much a problem, as a whole bunch of shitty cheap chargers do the same,the fact your device is trying to pull 1A implies there are some resistors between D+ and D- telling it to draw more than 1A (Assuming phone)as for it drawing 1A at 2.5V that implies whatever its connected to may not be a very smart device but rather one with a boost converter off the usb power pins,

author: David_AVD
author_rank: Super Contributor
timestamp: July 20, 2013, 11:38:20 pm
content: Is the USB output rated to supply a specific current?

author: IanB
author_rank: Super Contributor
timestamp: July 21, 2013, 12:29:32 am
content: Quote from: olsenn on July 20, 2013, 09:57:58 pmWhen you say "DC electronic load", do you mean an actual device, or a testing device of some kind?As far as the voltage dropping when you overload a supply, this is a typical response to an overload. Every supply is going to behave this way unless it has a specific overload cutout (which usually exists to protect the supply, not the device being powered).I am not sure what you are expecting here?

author: ptricks
author_rank: Frequent Contributor
timestamp: July 21, 2013, 12:42:44 am
content: According to their site:Technical Specifications:Power input: 10-watt Mono-crystalline solar panelPower output: 5V Standard USB port x 2Maximum output current: 2 ampThis is a solar panel connected directly to a device with a voltage regulator in the path.A bad idea because you have to have 100% output from the panel to get 5V@2A.If the regulator doesn't have the power input then you would see the voltage decrease as the load increases.

author: David_AVD
author_rank: Super Contributor
timestamp: July 21, 2013, 02:16:25 am
content: A solar panel is only going to give the rated output under optimum conditions.There are regulator losses, etc so a 10W panel is not going to give you 2A @ 12V anyway.Sounds like the claimed output is dodgy to start with as well.

author: Simon
author_rank: Unknown
timestamp: July 21, 2013, 06:16:36 am
content: 10W = 5W or less. the 10W is the power delivered on the equator at noon, unless you are there you won't get 10W. the panel has a fixed efficiency at converting sunlight to power so the less sun power the less electric power. the equator is rated at 1KW/sqm. check the % of sunlight expected in your area. for example here in the UK my 90W of solar panels make just 40W in reality.Solar panels are funny devices, the more current you try to draw the lower the voltage dips, that is why solar inverters and chargers use MPPT circuitry to find the sweet spot of maximum power and then convert it to the desired voltage. Basically you have been scammed. usually these things come with built in batteries that provide peaks of power and recharge more slowly off the panel.

author: olsenn
author_rank: Frequent Contributor
timestamp: July 21, 2013, 01:27:21 pm
content: QuoteThis is what I am trying to figure out. I understand that I will not get the full 10 watts even under the best circumstances... I am not worried about that. But what does bother me is that whatever power the device IS delivering seems to not always be at 5 volts. Will it hurt my expensive phone if I plug it into this solar charger which sometimes outputs lots of current at varying voltages below 5 volts? I am asking this because I was under the impression that these devices were supposed to ALWAYS be at either 0 volts (no output power) or 5 volts, regardless of how much current is going through them

author: Simon
author_rank: Unknown
timestamp: July 21, 2013, 01:38:18 pm
content: your panel needs a regulator and it might be a good idea that it cuts out if it can't deliver. If you use a switchmode regulator it will make best use of the power.

author: IanB
author_rank: Super Contributor
timestamp: July 21, 2013, 05:33:20 pm
content: Quote from: olsenn on July 21, 2013, 01:27:21 pmIf you think logically about what would be required to construct a device that behaves that way, you will see that such behavior is impossible to achieve.Consider:Your device starts out at 5 V and supplying 100 mA to the load. Good so far.Now the load increases and the current exceeds the capability of the supply.In response the supply switches off. It is now providing no output (0 V), and consequently no current and no power. This is what you have asked for.Later, the bad load is replaced with a good load that only desires 100 mA again.The supply does not know this, and cannot know this (it has no current to measure at 0 V, 0 A and no way to know what you have done outside its terminals). So the supply remains switched off.One way out of this problem is for the supply to automatically try switching on again and see what happens (but how often, at what frequency?). If it does this, the heavy load is likely to see ON...OFF...ON...OFF...ON...OFF... The load may like this far less than a low voltage situation.The other way out of this problem is for the supply to have a manual reset button. After it switches off a red light comes on, and you can press a button to switch it on again. If you try switching it on with the heavy load attached it will switch right off again, and now a typical user may be puzzled and confused--why won't the supply stay switched on?So ultimately, one does not design things this way unless there is a safety concern (as with a GFCI or an RCD). The normal approach is to provide a graceful decline in voltage as the load gets too much. No funny business, no cutting out, just a smooth response. This is how your supply behaves, and how 99% of similar supplies on the planet behave.