[Drools]

I have 5 identical NEW 10,000uf electrolitic caps rated at 63 voltes.
I have (45VAC * 1.44 full bridge rectifier) = 64.8VDC no smoothing.
I would like to use the caps I have for my new power supply feeding 1 x G203V and 1 x 6amp Nema34 1200oz stepper from Kelinginc.
I have seen a circuit using capacitors in series here Capacitors - Multiple Capacitors
My question, can I get away with using the caps I have or should I just order 2 x 10,000uf caps at 100VDC.
Basically I forgot about the smoothing caps before I started the upgrade and I have these other ones on hand.

Thanks for the help.

[doorknob]

I have used electrolytic capacitors in series (with the equalizing resistors) in high voltage power supplies before. It does work as advertised. But it can be a pain to connect them up (in the past I have made a custom circuit board to hold them), and you will have to get the resistors. So it may be easier just to get capacitors with a suitable working voltage.

[Drools]

Thanks for the reply dk, I have quite a good assortment of resistors on hand but having the correct wattage might be a problem. I have to read up more to find the value and wattage I need if I actually proceed down this road.

[cnc_4_me]

I will give you a hand calculating the resistors. To start we need the spec sheet of the 10,000 uf caps. What is the full part number of the cap and mfg.

The short version is we need to pick resistors that allow 10x more current than the worst case leakage current of the caps.

[Drools]

Thank you very much for the help!
The caps are nippon, chemicon, 63v@10000uf(M)
KMH 105oC
13U05L 15
diameter 35.17mm, length 51.69mm
I purchased them from digikey. I was hoping that I would not have to use all 5
As I could use one on another project.
http://www.chemi-con.co.jp/e/catalog...g-e-110701.pdf

[cnc_4_me]

From the cap spec sheet.

Leakage current I=0.02CV or 3mA, whichever is smaller.
C = 10,000uF
V = 63V
I is in micro amps.

.02 x 10,000uF x 63V = 12,600uA

12,600uA = .012600 A = 1.26mA

So 1.26mA is smaller than 3mA so you are supposed to use 1.26mA.

Personally i am not a real believer in what the mfg’s say. I think we should use the 3mA number to be safe and see were it leads us in values.

You have a 45 volt ac transformer.

45VAC x 1.414 = 63.63VDC and subtract 2 diode drops for 1.4 volts = 62.23VDC lets just call it 62VDC.

Our 2 resistors in series will carry 3mA across 62VDC. R = E/I with E in volts and I in amps, so 62VDC/.003A = 20,666 ohms or 20.67K.

We have to divide this by 2 because there are 2 resistors in series across the caps. 20.67K/2 = 10.36K and of coarse 10K is a standard and popular value so rounding down gives us slightly more current. We now know we need 10K ohm resistors.

Now we need to know the wattage each resistor will dissipate. P power in watts = I in amps x E in volts across the resistor. .003mA x 1/2 the total voltage 62VDC/2 = .093 watts an 1/8 watt is .125 and 1/4 watt is .250. You are close to 1/8 watt but never run a resistor near its full wattage rating for long life. Use the 1/4 watt at 10K ohms for long life.

That is a very reasonable resistor.


EDIT
OK i made a mistake, i forgot that we needed 10X the current in the resistors $%^&.

so 3mA x 10 = 30ma
R=E/I 62VDC/.03A = 2,066 ohms divided by 2 = about 1000 ohms or 1K.
P=IxE .03A x 62VDC = 1.86 watts.

I would use 1K 3 watt resistors.

[cnc_4_me]

Now to figure out if your cap values are OK.

The Gecko capacitor formula is
(80,000 x I in amps) / V in volts = C in uF

You have a 6 amp motor. Gecko tells us that you only draw 2/3 motor current with their drives. 2/3 x 6A = 4A

(80,000 x 4A) /62VDC = 5161uF

Two 10,000uF caps in series = 5000uF, close enough.

Your capacitors are rated at 4.40 amps max. So you can just get by with 2 caps in series since each cap will see the full current.

Or you can make up 2 sets of capacitors in series and then parallel them, you will then have 10,000uF and 8.8 amps capacity.

[Drools]

thanks again cnc_4_me, I didn't consider the idea of 2 sets of 2 in parallel. I have some 1/2 watt 10K resistors in my tray which should work well.
This has saved me a digikey order, I'm very happy.

[cnc_4_me]

We could also check to see if your transformer has enough capacity. Do you have the specs for it.

[doorknob]

Leakage current I=0.02CV or 3mA, whichever is smaller.
C = 10,000uF
V = 63V
I is in micro amps.

.02 x 10,000uF x 63V = 12,600uA

12,600uA = .012600 A = 1.26mA

So 1.26mA is smaller than 3mA so you are supposed to use 1.26mA.

Personally i am not a real believer in what the mfg’s say. I think we should use the 3mA number to be safe and see were it leads us in values.

I think that you had a misplaced decimal point there, as 12,600uA would be 12.6mA, however since you did the calculation at 3mA anyway, it looks like you're good to go...

Thanks for slogging through the detailed analysis. I'm sure that it will be helpful to lots of folks facing similar design decisions...

[Drools]

I may have to dig a little deeper for the larger resistors , if I remember correctly I might have a few 1K@5watt ceramic resistors in the tray, maybe a little closer to the back and collecting dust.
Here is the twist, I have a G540 and 1 x G203V.
I have about 10 of these:
Amp Transformer Mid West Co. 430-9001 (8) Class 130B
Model 430-9001, Transformer
Input: 125 VAC black/white - 115 VAC brown/white
Output: 14 VAC 15 Amp ( terminal .250” )
On the brown/white inputs the output is 15VAC while the black/white inputs gives you 14VAC output.

My plan is to use 3 of these in series, take a tap off the first 2, to provide the voltage for the G540 and use all 3 for the G203v.

In my current setup I have 2 of the 430-9001 transformers in series running the G540 with 3 nema23 steppers, XY-axis are using CNCRP [email protected] amd stepper and the Z-axis uses a Tormach 280oz@3amp stepper. The G540 current resistor is set at 2.8 amps.
The bridge rectifier I'm using is the MB251 100V@25amp. (I also have 10 of these)

In the new setup I'm upgrading the X-axis to a Kelinginc(automationtechnoligies) Nema34 1200oz@6amps with the G203v. The Y-axis will stay the same and the Z-axis will get the stepper from the X-axis. This leaves me 2 free drivers on the G540 to maybe use the new plastic extruder I seen in this thread. http://www.cnczone.com/forums/diy-cn...d_printer.html As well I have always had my mind set on a 4th axis.

[cnc_4_me]

I think that you had a misplaced decimal point there, as 12,600uA would be 12.6mA, however since you did the calculation at 3mA anyway, it looks like you're good to go...

Thanks for slogging through the detailed analysis. I'm sure that it will be helpful to lots of folks facing similar design decisions...

Good catch, it is 12.6mA

[cnc_4_me]

Drools said:
“In my current setup I have 2 of the 430-9001 transformers in series running the G540 with 3 nema23 steppers, XY-axis are using CNCRP [email protected] amd stepper and the Z-axis uses a Tormach 280oz@3amp stepper. The G540 current resistor is set at 2.8 amps.
The bridge rectifier I'm using is the MB251 100V@25amp. (I also have 10 of these)

In the new setup I'm upgrading the X-axis to a Kelinginc(automationtechnoligies) Nema34 1200oz@6amps with the G203v. The Y-axis will stay the same and the Z-axis will get the stepper from the X-axis. This leaves me 2 free drivers on the G540 to maybe use the new plastic extruder I seen in this thread. CNC router to 3D printer As well I have always had my mind set on a 4th axis.”

I did not understand if all G540‘s were set to 2.8 amps or just the the Tormach 280oz@3amp stepper. What G540 current setting are you using for the CNCRP [email protected] amd steppers,

[Drools]

I thought that might raise an eyebrow, yes the G540 is currently set to 2.8amps for all axis. This was the setup for my original build which used 3 x Tormach 280oz steppers. I just unplugged the control box from that machine and plugged it into the new machine. My intension all along was to add a heat-sink to the G540 and increase the current resistor to 3.5 when I build the new controller for the Nema 34 upgrade. I have been able to cut at 100ipm no problem with the way it is setup now, so I just left it.
I found a good sized heat-sink to be used with the G540 so I’m comfortable upgrading the current to 3.5amps. I’m just finishing up the wiring for the 3 transformers which in testing produced the exact voltages I listed above. I have yet to go hunting for the resistors I need for the caps.
I understand running the ~600oz steppers at 2.8amps effectivly detuned(lowered) the output.

[Drools]

One question I have while building the power supply section is this, the MB251 bridges have a metal casing. Since I'm using 2 of them does the casing have to be isolated from each other or can I bolt them to the same heat-sink?
Note: not the heat-sink used on either the G540 or the G203V, I have a seperate heat-sink for the bridges.

[cnc_4_me]

One question I have while building the power supply section is this, the MB251 bridges have a metal casing. Since I'm using 2 of them does the casing have to be isolated from each other or can I bolt them to the same heat-sink?
Note: not the heat-sink used on either the G540 or the G203V, I have a seperate heat-sink for the bridges.

I am fairly certain the metal case on the heat sink is isolated but it would be easy to check with an ohm meter. Set the meter to 1x or 10 ohms and connect one lead to the case and probe all 4 of the terminals to see if any conduct to the case.

I think you may have a problem wiring up the transformers as you want. Now that i know the current you are using I can run some calculations.

[cnc_4_me]

I thought that might raise an eyebrow, yes the G540 is currently set to 2.8amps for all axis. This was the setup for my original build which used 3 x Tormach 280oz steppers. I just unplugged the control box from that machine and plugged it into the new machine. My intension all along was to add a heat-sink to the G540 and increase the current resistor to 3.5 when I build the new controller for the Nema 34 upgrade. I have been able to cut at 100ipm no problem with the way it is setup now, so I just left it.
I found a good sized heat-sink to be used with the G540 so I’m comfortable upgrading the current to 3.5amps. I’m just finishing up the wiring for the 3 transformers which in testing produced the exact voltages I listed above. I have yet to go hunting for the resistors I need for the caps.
I understand running the ~600oz steppers at 2.8amps effectivly detuned(lowered) the output.

It looks like you will just squeak by with your transformer current rating. As close as can be calculated your bottom two transformers will have to supply 15.6 amps AC to the bridge to run all 3 motors at the same time. Assuming you will be doing that.

[Drools]

Hi cnc_4_me, if I understand correctly the transformers will still only product 15amps even though I have 3 in series. 2 transformers will supply 30VAC to one bridge, running the 2 nema 23 motors. The third transformer will add another 15VAC for 45VAC to run the Nema 34 stepper. If I only use 2/3 of the a motors current then 3.5 + 3.5 + 6 = 15, and 2/3 of 15 is 10. So if I'm correct I will only need 10 amps. Please say I'm right
I hope it will work.
I uploaded a very simple schematic of what I'm wanting to do.

[Drools]

Transformer calculations pending, I found 2 x 5700uf@70VDC in my drawer they are used but still good. Is 70VDC cutting it too close for for the 64 volts we calculated?

[cnc_4_me]

Your schematic looks OK, i assume that the green is DC- and red is DC+. You just have to connect you 2 green wires from the bridges together or you will have a “floating” power supply’s. Best case it would not work, worst case you would start blowing motor drives. The point were the two green wires connect will be called your DC ground, this is the point that will connect to your motor drivers GND connection. From GND to your first bridge is your low voltage output 38VDC and from GND to your 3rd transformer and second bridge is your high voltage output 58VDC.

It might be best if you add that to your schematic with DC GND and + labeled and add your capacitors with polarity marked on them.

64 Volts on a 70 volt capacitor is to close. Remember that there can be variations in electrical power to your house that could raise the transformer voltage up a little and when you decelerate a stepper that loss in mechanical energy goes on to making voltage like a generator and gets put back in the caps, so the voltage on the caps goes up with deceleration. 20 volts is considered the minimum safety margin from power supply voltage to capacitor rating, at these voltage levels. and 30VDC is a good conservative design.

By the way as you are finding out my calculated DC voltage is low compared to your measured values. That is because a transformer has loose regulation. With no load the 14 volt output could be a couple of volts higher or so. The transformer mfg only guarenties that you have 14 volts with a 15 amp load.

One last comment on your schematic, while it is valid, by convention you usually draw a totem pole device like you are making with GND at the bottom and plus at the top. Your top 2 transformers should be at the bottom with the DC- terminal on the bridge labeled as DC GND. Then you have your low voltage plus and your high voltage plus on top of that. That makes for a easy to read schematic.

For the transformer calculations the thing to understand here is that the AC RMS current in to a bridge rectifier with a capacitor on the DC side is 1.8 times the DC output current. So 1 amp of DC costs 1.8 amps of AC.

Your Y and Z motors on the G540 will be set to 3.5 amps each = 7 amps, but they only draw 2/3 that because of the PWM’ing of the gecko drives. 2/3 is .667 so 7 x .667 = 4.67 amps DC from the bridge. 1.8 x 4.67 amps = 8.4 amps AC from transformer. Your transformers are in series so this current flows through the two transformers connected to the bridge across them.

The third transformer is also in series with the other two transformers but it is not in the loop of the bridge on the other transformer so it does not supply any current for them. The other bridge for 58VDC is connected across all three transformers so they will all see this bridges current draw.

The bridge for the 58 volt supply will be connected to a 6 amp motor, .667 x 6 = 4 amps DC from that bridge. 4A DC x 1.8 = 7.12 amps AC from all transformers.

So now add up these currents. If all motors are running the first two transformers will see 8.4 amps AC from Y and Z motors and 7.12 amps AC from the X motor = 15.52 amps AC. The third transformer only sees the 58 volt bridge and only has to supply 7.12 amps AC.
If you truly ran all 3 axis for hours at a time then i would say put a fan on the transformer.

I made a rough hand sketch of the schematic. When my camera is done charging i will take a pic of it and try to post it.

BTW, Mouser electronics has no minimum order. A quick look shows the cheapest cap with just enough UF and a good voltage rating is this one, to use on your 58VDC (measured voltage 63) X axis.

Mouser Part #: 667-ECO-S2AP562FA
Manufacturer Part #: ECO-S2AP562FA
5600uF 100VDC at 5.75 amps max.
$5.11 plus shipping

You need a minimum of 4 amps for the cap. The current is what heats up the cap and causes short life. 5.75 amps is a good de-rating for longer life.