Date: Sun, 26 Oct 2003 20:11:35 -0500 (EST)
From: C. Scott Ananian <cscott
cscott.net>
To: Roger Cook <rxcook1hotmail.com>
Subject: Re: EDM Circuit
On Sat, 25 Oct 2003, Roger Cook wrote:
> Your EDM power supply looks like a fine solution for what I want to do. I
> can follow how your circuit works but my electronics experience is limited
> to low power projects and a spot welder.
>
> Have you actually built the EDM power supply circuit? Do you have any
> suggestions for specifications for some of the components, especially:
no, unfortunately i ran out of time to work on it after making pcbs but
before assembling and testing the circuits. I expect some debugging will
be necessary.
[The ICM7555 portion of the circuit is optional, as well; which is why it
is drawn connected via headers to either IR2108 or IR2117. In normal
operation, when Q1 (Q2) is off, C13 (C15) is charged up to 15V via D5
(D10). This provides the gate voltage needed to turn Q1/Q2 on. But if
Q1/Q2 are left on too long, C13/C15 will eventually discharge and Q1/Q2
will switch off (whether you wanted or not). The 7555 pumps C12 high to
generate the necessary gate voltage, but also introduces quite a lot of
extra components and current drain through R23. I suspect that the 7555
will be necessary for testing and debugging, but probably in "real" EDM
operation Q1/Q2 will be "off" often/long enough that the 7555 will not be
necessary. More details on this circuit block are in the IR2108/IR2117
datasheet, IIRC.]
> Q4 Same as Q3?
Q3 is a small-signal mosfet, like digikey 2N7000FS-ND. It only sees a
gate voltage of 15V and a source voltage of 2.2V.
Q4 is optional, but it used to boost the voltage across the workpiece
above the ~300VDC which the capacitive doubler produces. The idea is to
starting current though L1 (the 100uH coil) by shorting the workpiece gap.
Then Q4 is then opened, the coil's desire to "keep current flowing" will
cause it to develop as high a voltage as necessary across the workpiece to
keep current flowing. Z3 limits this voltage to protect the channel of
Q4.
> Z3 A 600 volt Zener seems hard to find.
> Do you know a specific part for this?
> What about the power rating?
Depends on how much current you let flow through Q4 in "boost" mode.
The same amount of current will want to flow through Z3 when Q4 is opened,
if a spark can't be generated across the workpiece.
I had spec'ed either digikey P7268-ND or P7275-ND as Z3, depending on the
selection of Q4. I think I'd already bought a Q4 with a 400V or so
breakdown voltage, which matches the P7275-ND. You'd really want a device
with a 600V breakdown voltage, so that you could get 600V of 'boost'; the
P7268-ND is appropriate for that.
Q4 should be the same as Q2, roughly, but with a higher breakdown
voltage. It should be able to handle the same amount of current.
Note that D1, D2, D8, and D9 should also be high-current diodes. I have
RURG8060-ND specified.
I have attached my digikey order(s) for this project, which completely
specifies the components.
I have also updated the schematic on the website, which was very slightly
out of date.
If you beat me to the actual implementation of this circuit, I would be
very interested to know how you fare!
--scott
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( http://cscott.net/ )
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DigiKey order #1:
qty cat # my desc price each
1 IR2108-ND HIGH-LOW SIDE DRIVER $2.95 U5
1 LM324ANNS-ND QUAD SINGLE-SUPPLY OP-AMP $0.54 U3
1 NJM7815FA-ND 15V REGULATOR $.57 U1
3 1N5245BMSCT-ND 15V ZENER DIODE $.21 Z1,Z2,unused
1 PS2501-2-ND DUAL OUTGOING OPTO $.87 U2
2 TLP550-NS JUSTIN CASE FAST ISO $1.38 unused
1 PS2501-4-ND QUAD INCOMING OPTOISO $1.47 U4
1 Q113-ND 9' POWER CORD $7.56 unused(mistake)
1 Q300-ND POWER INLET MODULE $7.85
1 WK5074-ND 16A 5x20MM FUSE $.47
1 IRG4PSC71KD-ND 60A FAST IGBT $18.90 Q2
2 IRG4PC30KD-ND 17A FAST IGBT $5.38 Q1,Q4(not ideal)
1 P7422-ND POWER CAP 47uF $2.64 C7
3 TMC50-0.15-ND CURRENT SENSE 50W RES. $4.90 R5,R6,R7
1 TMC50-1.0K-ND INRUSH LIMITER 50W RES. $4.90 unused(orig R3)
2 CT94W503-ND CURRENT TRIMMER POT 50K $1.43 R18,R19
1 CT94W504-ND INRUSH DISABLE TRIM 500K $1.43 R11(purpose changed)
1 RB151MS-ND 15V SUPPLY RECTIFIER 1.5A $.52 BR1
2 RURG8060-ND FREEWHEEL/VREG DIODE 80A $5.20 D8,D9
1 GBPC2508A-ND V-DOUBLER BRIDGE 25A $2.84 BR2
5 1N4148FS-ND SMALL-SIGNAL DIODES $.10 D3,D4,D6,D7
2 1N5408DICT-ND HIGH-SIDE ISO DIODE $.41 D5,D10
1 P11727-ND LOWER VOLTAGE DOUBLER $7.49 C3
3 P11926-ND HI-SIDE VOLTAGE DOUBLER $5.80 C4,C5,C6
1 2N7000FS-ND SMALL SIGNAL MOSFET $.26 Q3
total (excluding shipping): $122.09.
DigiKey order #2:
qty cat # my desc price each
1 KC024L-ND INRUSH CURRENT LIMIT $4.18 R3
1 MT2226-ND LOW SIDE TRANSFORMER $7.37 TR1
1 Q116-ND POWER CORD $7.32
1 770-101-R820-ND ISO RESIS $0.30 RA1
2 WK5074-ND EXTRA FUSES $0.47
1 A23305-ND FEMALE 9 DSUB $1.94
1 A23279-ND MALE 9 DSUB $2.33
1 RURG8060-ND EXTRA HI-I DIODE $5.20 D2
1 P815-ND 15V FILTER CAP $0.33 C2
1 P5157-ND 15V FILTER CAP $1.38 C1
2 E1104-ND 100NF $0.34 C13,C15
1 PS1H102J-ND 1NF $0.35 C10
1 PS1H332J-ND 3000PF $0.35 C8
2 PS1H103J-ND 10NF $0.74 C9,C14
Total: $33.41
DigiKey order #3:
qty cat # my desc price each
1 IR2117-ND 2ND HIGH SIDE $1.88 U7
5 1.00MXBK-ND PRECISION 1M $.108 R12,R13
5 30.1KXBK-ND PRECISION 30K $.108 R14,R15
1 P7268-ND PROTECTION FOR Q4 $1.05 Z3
1 P7275-ND PROT FOR SMALL Q4 $1.14 Z3(not ideal)
1 PS1H103J-ND 10NF CAP $0.37 C11
1 E1104-ND 100NF CAP $0.34 C12
Total: $5.86
Purchased from other sources, but also have digikey p/ns:
1 ICMAX7555IPA-ND timer $2.49 U6
Standard parts probably already in parts bin:
Resistors: R1, R2, R4, R8, R9, R10, R16, R17, R20, R21, R22, R23, R24, R25,
R26, R27, R28
LEDs: LED1, LED2, LED3
Built by hand:
L1, L2: 100uH, 60A inductors Wrapped heavy-gauge wire around bolts.
[fixme: update for how many turns, what gauge, what bolts]
Some value experiementation may be needed:
C9, C14, R21, R22, R25
Unknown:
D1. Should be 25A 300V diode.
Not purchased (an oversight?) but should be same as D2/D8/D9.
