AFAIK to serialize 2 or more power supplies you must ensure they have diodes of appropriate power connected with reverse polarity in parallel with the outputs.
And the weakest cell often becomes reverse charged, - bad news if they are were rechargeable.
Neither of those apply to what paulca is doing, so he is quite safe without any diodes.
The diode protection on a power supply is needed when you have two supplies working against each other, say, when you're changing a battery. If the PSU is inadvertently switched off without being disconnected from the battery, the battery will try to feed power into the power supply, which might damage it. The problem only arises when you have the negative rail of one PSU connected to the negative rail of the other PSU (or battery), and the positive of one to the positive of the other.
The battery voltage inversion only happens when you have a good amount of current running through an empty battery. It happens if the voltage drop over the internal resistance is larger than the voltage created by the chemistry. This typically happens when you have several batteries in series, and most of them are still good enough to supply a decent amount of current; the current is simply pushed through the bad battery, and its internal resistance creates a voltage potential that is inverse to its normal voltage. This always happens with batteries of course, but as long as they're good the internal resistance is small enough for it not to overcome the voltage potential created by the chemistry.
Now paulca is using power supplies, not batteries, with internal resistance that is negligible and unchanging.
Or is it? What happens when we change paulca's setup (and to be clear paulca: this is not what you're doing, so you don't need to worry about it) and say, we set a current limit on the weaker PSU of 1A, but not the other, and then short the output of the combined serialized PSUs? The current-limited PSU will reduce the voltage of its output as much as it can, ideally to 0V, against the other supply, who'll try to force some voltage onto there until it hits its maximum current of 20A. Whether the current-limited PSU survives this depends on its design I reckon. I'd expect it to be ok as long as you're under the limited PSU's max current capability (so under 5A), but above that, damage will occur eventually.
This is where having OCP (Over-Current Protection) will save you. If the PSU sees more current than the OCP level, it won't just reduce its output voltage (as it does when you hit the current limit), it will disable the output thus breaking the circuit. OCP in the weaker PSU should be able to do that on its own, but to be on the safe side, I'd lower the OCP in the stronger PSU to match the capabilities of the weaker one.
Home
Help
Search
Login
Register
