I need to build a pulse width modulated power supply for electroplating. I have built a prototype, using a H-bridge that delivers a variable width pulse with an approx ratio of positive :negative of 10:1. This was pretty easy since the amplitude of both pulses was the same. All I had to do is write a sketch.
Now I find that I need to vary the relative amplitudes of the pos and neg pulses. I had already modified the H-bridge circuit to give me a variable current by using a NPN TO3 in series with the common return of the H-bridge. No problem, but when I try to use two transistors to control the amplitude from each leg... no luck... the circuit unbalances and goes into runaway.
Anyone out there tried something like this?
Are your desired amplitudes fixed? If so, how about putting a diode/resister pair in each direction to give you the voltage/current you want?
Now I find that I need to vary the relative amplitudes of the pos and neg pulses.
Why?
It is a H-bridge therefore is gives the same voltage in both directions.
One way might be to use two half H-bridges and feed each half with a different voltage, but you don't actually generate negative voltages with a H-bridge anyway.
the H-bridge allows a reversal of the polarity at the anode and cathode of the plating bath.
In theory... if you produce a short pulse of reversed polarity to the plating bath, the result is a smoother plate. This probably has to do with manipulation of the depletion layer around the cathode, as well as selectively de-plating high spots where the delta V potential is the greatest.
Anyway, it kind of works. with test plates there is a slight gain in smoothness of the plate... but not as much as I would like. So I am thinking, if I increase the amplitude of the short negative pulse, I might do better. I don't want to increase the interval of the negative pulse, because this increases the length of time it takes to plate the object. A 2 X higher negative pulse would (probably) not be as bad as doubling the time of the negative pulse. Also it would increase the voltage differential between the high and low spots and ( i hope) result in a more uniform plate.
What is the current you are trying to achieve?
It's impossible to give any advice on the circuitry w/o actually seen drawings.
In case different voltage for pos and neg, H-bridge is, probably, not the best solution. SCR controllable rectifier maybe, but again a lot depends on current, 4 A and 400 A make a huge difference
The current will not be more then 3 amps at the start and will be adjusted to about 500 ma toward the end of the plating cycle. The voltage across the plating solution seldom exceeds 3 volts.
at this point, I am using a simple 5 amp H-bridge witch I purchased and which is the usual implementation of two sets of PNP/NPN tip122 and tip127 bipolar transitors. I run the grounds of the two sides through another transistor (that I control with a pot to vary the current) then to true ground.
My hope was that I could,like Grumpy_Mike says, convert to two half bridges and get what I want. So far my muddling around with the existing circuit has allowed me to shut one side or the other down, but not to attenuate the output of either.
I have no good means of posting a schematic, but I think what I have is pretty much like all of the basic bipolar H-bridge circuits I have seem.
I was hoping that someone might have done something similar. It sounds pretty simple in theory I can't seem to get there.
I have no good means of posting a schematic,
Darw it on paper, photograph it. Then in the reply box the Additional options triangle will allow you to attach the photo to the post.
An SCR will not work because we are talking about DC here, so it is very hard to turn them off.
here is what I have so far
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There is quite a bit wrong with that it is hard to know where to start.
If you want different voltages each side of the bridge must be powered separately.
That big transistor in the ground of the bridge is not going to limit the current because the gain is only being fixed by the physical gain of the transistors.
You need something like an adjustable voltage regulator powering each side of the bridge and the overall current limit being controlled by a resistor in the emitter to give some current related feedback.
With such large currents you might be be better off using FETs.
That big transistor in the ground of the bridge is not going to limit the current because the gain is only being fixed by the physical gain of the transistors.
But it works... with the pot I can adjust output current from approx 0.2 to 3 amps and still maintain the correct pulsed wave form... at least according to my O-scope and panel meters.
If you want different voltages each side of the bridge must be powered separately.
I think this is probably a better approach than I am using. I think I am trying to keep things too simple by having the mind set of just modifying an existing H-bridge that I purchased. I thought that if I used a pot to control the output of the small transistors feeding into the base of the power transistors of the bridge, I could lower their gain and keep one side from going into saturation. Didn't work... I could get a slight sag in the output of one leg, but when I tried to reduce it more, then that side immediately went into cutoff.
But it works...
Yes it might but it is only working because of the specific gain of that transistor. You get another transistor of the same type and it might not work. Some one else tries to make it and it won't work. This is because the gain of a transistor is not a reliable thing. Look at the data sheet and you will see a range for the gain. This gain can change with temperature, aging or from device to device. It is bad practice to make a design that relies on a device parameter because that design is then not reproducible.
