Transformer voltage seems too high for Gecko

[bgolash]

Hi Group
I just purchased this transformer and have it setup with these
components. Notes: I have the secondaries at 55 volts wired
in parallel
http://cgi.ebay.com/ws/eBayISAPI.dll...e=STRK:MEWN:IT
Bridge Rectifier Vishay Semiconductors
GBPC35 35 Amp 600 Volt
-----------------------------------------
Resistor Xicon
PWR 10W 2.0K
---------------------------------------------------
Capacitor Mallory
LYTIC 100V 10000uF

As a surprise for this newbie at the bridge I am getting 57VDC
That was close to what I was looking for. I then tested the DC
voltage after the capacitor and I`m getting 87 volts. Is this too
high for a Gecko 210. I didn`t realize the capacitor would increase
the voltage to 87 volts. If this is a problem, how do I deal with this
issue.

Thanks Barry

[Al_The_Man]

You have to calculate that the DC will be 1.414 x Acv. There is a method of taking the required number of turns off of the secondary, they are usually wound bi-filar which means wound together so you can reduce both secondaries easily, if you do a search of this forum on this subject you should find it, which includes finding the turns/volt which you will need, otherwise I can point you to the right post.
Al

[ViperTX]

So, the reason is because you're converting AC to DC and as Al mentioned you take the AC voltage secondary times 1.414 and that will be the resulting DC. If you're wondering why you do that....the secondary voltage is measured my traditional voltmeter in what is called RMS (root mean square) and multiplying the RMS by 1.414 gives you the peak AC voltage.

Yes, the voltage is beyond the Gecko Spec's....so you'll need to clamp the output voltage...on another thread I explained to someone how to do this for their Bridgeport Servos & Gecko. Once you calculate the size of the series resistor and the required zener you'll probably want to separate the secondaries and create a supply with two DC outputs.

2nd option is to remove some windings from the transformer secondaries....you can experiment or contact the seller and he may be willing to give you some info or exchange the toroid.

Your options are zener clamp, remove secondary windings, replace the toroid.

[bgolash]

Hi Group
Here is a picture of the wiring of the transformer. Can I combine
a 55 with a 15 to lower the input voltage to the bridge. I`m
not expert in such things ...am wondering if this is possible.

Thanks Barry

[3t3d]

Originally Posted by bgolash Hi Group Here is a picture of the wiring of the transformer. Can I combine a 55 with a 15 to lower the input voltage to the bridge. I`m not expert in such things ...am wondering if this is possible. Thanks Barry

As long as the 15 volt secondary has the current capacity,it will be OK.
They can be wired to either add or subtract.

Another way to reduce the voltage is to add an inductor between the
bridge rectifier, and the capacitor. This will lower the voltage, lower
the ripple, and be easier on the bridge rectifiers. If you have an inductor
you might try it.

Pete

[Al_The_Man]

Originally Posted by 3t3d Another way to reduce the voltage is to add an inductor between the bridge rectifier, and the capacitor. This will lower the voltage, lower the ripple, and be easier on the bridge rectifiers. If you have an inductor you might try it. Pete

I think you will find the whole idea of an inductor (over a resistor say) is to only reduce the ac or ripple content as an inductor has impedance to AC but offers a very low (read neglible) resistance to DC, effectively the resistance of the winding which is typically in a small fraction of an ohm. The result is you still obtain the peak DC with and inductor or smoothing choke, but it smooths out the ripple, if used in a 'pi' filter arrangement with capacitor-choke-capacitor, it is doubly effective. Another name for it is a low pass filter. (Passes very low freq & DC, blocks AC)
Al

[3t3d]

Originally Posted by Al_The_Man I think you will find the whole idea of an inductor (over a resistor say) is to only reduce the ac or ripple content as an inductor has impedance to AC but offers a very low (read neglible) resistance to DC, effectively the resistance of the winding which is typically in a small fraction of an ohm. The result is you still obtain the peak DC with and inductor or smoothing choke, but it smooths out the ripple, if used in a 'pi' filter arrangement with capacitor-choke-capacitor, it is doubly effective. Another name for it is a low pass filter. (Passes very low freq & DC, blocks AC) Al

I specifically said to put the inductor BEFORE the capacitor. NOT as a Pi filter.
The inductor in an inductive input configuration will reduce the peak voltage.

As a Pi filter as you suggest, it will not reduce the peak voltage. Since the
first capacitor (that is the key) will attempt to charge to the peak voltage.

It is an interesting circuit.
Pete

[BobLWeiss]

Couldn't he put the 15v coil and the 55v coil together in opposite directions and effectivly produce 40v AC? Then rectify that to come out with 56vdc? He could do this for both sets of outputs then parallel that output as one?

[Al_The_Man]

As already mentioned, he could put them in the 'buck' mode, however, if the windings are different current rating, the maximum current will be for the lower rating.
Al

[limbo]

Hey 3t3d,

That is really a great trick and an interesting circuit! Have you used it in practice? Is the voltage drop more or less constant with load? One would expect it to be much more constant than with a simple series resistor, because of the diode "on" time decreasing at lower load. Plus you have low losses in the coil. Brilliant.

John

[3t3d]

I wish I was the inventor...
Someone once had a radio transmitter that was eating the power transformers
and input diodes. I suggested this as a fix, the diode peak currents were too
high.
It worked great.

Here is the first hit I got with a google search.

http://www.tpub.com/neets/book7/27f.htm

I learned the circuit many decades ago.
Pete

[ViperTX]

The circuit that Pete pointed us to is excellent if you want to maintain a constant current while the load is fluctuating....the problem that is being addressed is lowering the output voltage of the power supply...economically that is done by lowering the transformer's secondary voltage...any other approach produces a little "burrito" warmer.....at substantial currents it's a good heat source.