If I wanted to know the voltage across the Peltier, I would measure it directly with a multimeter.
The Peltier datasheet you linked a says it's rated for a maximum current of 3 A DC.
If you push 3A DC through it, then the graph at the bottom of that one-page datasheet seems to indicate that the voltage across the Peltier should be about 0.8 V.
That's the voltage directly across the Peltier at 3 A DC no matter what other stuff you have in series with it -- diodes, resistors, inductors, whatever.
From the description, it sounds like one
Fairchild SB560 Schottky diodes b
is in-line in each of the two wires that come out of this Peltier cooler.
(I'm a little mystified what they are for, but I've already speculated on a few possibilities).
Like all diodes, it has a datasheet c.
On the first page of the datasheet, it says each SP560 has a forward voltage drop ("VF") of 0.67 V at 5 A.
But at the 3 A you're probably trying to use, the graph on page 2 of the datasheet
shows it has a forward voltage somewhat less than 0.6 V -- maybe 0.56 V?
So I expect the total voltage across the entire series chain when 3 A is going through the chain -- two diodes at 0.56 V each and a Peltier at 0.8 V -- to be roughly 1.92 V.
Honestly, that's an unusual power supply voltage.
The Peltiers I'm using this week are 12 V 5 A
(similar to d e ); and 12 VDC power is more convenient than 1.92 V or 0.8 V.
The forward voltage itself is not really the value that you need to worry about - using it in this circuit won't break it as you aren't going to be intentionally dropping large voltages across the diode. In fact it is the power rating of the diode you need to look at.
Why? Well, you will need something at the output to pull it down when both are low - e.g. a resistor.
With the diodes your signal output levels will e reduced - by the forward voltage drop of the diodes. So if you have say a 10k pull down resistor, this would mean when the 3V logic signal is pulsing you will get a 0V low level (pulled down by the resistor) and a 3-Vf=2V high level (sourced through the diode). In this case you would have Id=2/10k=0.2mA flowing through the diode which should be fine for most diodes (it is only ~0.2mW).
For the 5V logic signal you would have a 4V high level output, and in this case there would be 0.4mA flowing through the diode, which again should be fine.
The problem with this is that if the load on the PWM signal is quite highly capacitive then it will switch from high to low quite slowly as you only have the resistor doing the switching. If the PWM signal was driving a large power transistor which would have a fairly high gate capacitance, this slow switching could cause excessive heating in the transistor (but that is a whole other story).
If you want to switch faster you may find you need to reduce the resistor value to increase the speed in which the output is pulled low. When doing this you have to be mindful of the current that flows through the diode and on through the resistor during a logic 1 and hence the power dissipation in the diode (Vf * Id).
Given you have a 3V signal and a 5V signal, what you can do (and I have done in the past) is use a TLL level 2-input OR gate (e.g. 74HCT32). These tend to have high input voltage thresholds (Vih) which are quite low and can thus run at 5V but support a 3V input (e.g. this 74HCT32 has a Vih of 2V on a 5V power supply). By using an OR gate like this you eliminate the issue of voltage dropped across diodes and get a Push-Pull output - the output of the logic gate sources and sinks current, so you don't need the pull down resistor.
Best Answer
Many manufacturers that produce the same part or compatible one, use slightly different markings. For example the classic general purpose NPN transistor BC548 is often mark as C548 as well. This practice is very common.
Your 4148ST diode is for sure the same part as or compatible with the 1N4148.
For more detail about what the manufacturer meant by 'ST', you need to get the datasheet from that exact manufacturer and look for a section typically named 'Marking'. Some times, particularly in SMD stuff, markings in components tend to differ from the real model name they are known for, so this datasheet section becomes almost the only clue available for identifying the part and its characteristics.
In case the brand is unknown for being a cheap China fake generic clone... part,the only hope is trying to find the manufacturer they tried to clone and check its datasheet.