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Hi, Getting the Placid to adjust to 350mA seems to be a common(ish) problem. My Placid will only adjust to approx. 250mA. I have read the other posts relating to this issue and am a little confused.
The recommendation appears to be to reduce (or short out) R6 - the 2K resistor. But I have also seen (I think) a reference to shorting out R5 - the 21R resistor.
Which is the correct resistor and why?
Meanwhile, I have experimented with VR1 - I reasoned that if 200 Ohms was not enough - Then try something larger..... So I removed VR1 and fitted a fixed 330R resistor in it's place - Looked good, current now up to 365mA. I am thinking now of replacing VR1 with a 500R pot to give a better range of current. Question is though - Is this a valid mod?
Many thanks - Ken
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I suppose you are referring to v 2.0.1
R5 sets the current of the jfet that powers the LEDs. Shorting it just means that the current through the LED is fixed at the VGS=0 level
R6 is just the resistor of the RC filter in front of the LED reference voltage. IT is there to further clean up the reference voltage of the LEDs.
So in theory, neither R5 or R6 will affect the current through R8.
Your mode is a valid mod.
Also check the voltage at C1 which must be > Vout+3V
Make the adjustments with some resistors connected in the output to simulate your load. For example if you need 300 mA at 5V, you need a resistor=5V/.3A=16 ohm. Make sure it is a power resistor or put a bunch of them in parallel because it will get very hot (and then you will be wondering where is that smell coming from :-) )
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glt wrote:I suppose you are referring to v 2.0.1
R5 sets the current of the jfet that powers the LEDs. Shorting it just means that the current through the LED is fixed at the VGS=0 level
R6 is just the resistor of the RC filter in front of the LED reference voltage. IT is there to further clean up the reference voltage of the LEDs.
So in theory, neither R5 or R6 will affect the current through R8.
Your mode is a valid mod.
Also check the voltage at C1 which must be > Vout+3V
Make the adjustments with some resistors connected in the output to simulate your load. For example if you need 300 mA at 5V, you need a resistor=5V/.3A=16 ohm. Make sure it is a power resistor or put a bunch of them in parallel because it will get very hot (and then you will be wondering where is that smell coming from :-) ) OK glt, many thanks for your help - All understood except why voltage at C1 must be >Vout + 3V - Which in my case would mean >8V. I only have 7.1V across C1, but I have no issues with the output voltage - It's rock steady at 5V. I am using a transformer with 0-6, 0-6 secondary paralleled. Also, why do I need to load the output, surely the shunt transistor will sink the full load current. Cheers - Ken
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The 3V is the recommended headroom (don't know what is the absolute minimum). You are right about the shunt passing the current. Just wanted to simulate actual operating condition (heat, etc). Since your input voltage is ~7v, I would guess that is the reason for needing to increase VR1 Edited by user Monday, May 17, 2010 3:50:05 PM(UTC)
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Hi Ken, Indeed you do want to short R5 to get more current through R8. The reason is that R5 sets the current through the JFET(Q2) based CCS. The current from this CCS runs through the pot to create a reference voltage for the big CCS (QP1/R8). Increasing the current through the JFET CCS gives you a larger range of voltage for the bipolar CCS. You could also simply increase VR1, but that is not as easy. We are planning on changing the value of that pot in new kits to give more headroom without the need to short R5. Still there is absolutely no harm in shorting or reducing R5 which ever you prefer. Edited by user Monday, May 17, 2010 4:34:34 PM(UTC)
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Many thanks for all your help Russ and glt - I will go the VR1 route as I have already removed it, and fit a 500R.
One last thing - Will I be OK with only 7V out of the bridge (across C1) as I assume this headroom is mainly to ensure the opamp has enough voltage?
Cheers - Ken
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I think it will work fine, but I have never tried it. Both the CCS and the Opamp need some headroom. 1.5V is likely enough, if it does not work well then the solution is just to get another transformer. :) The output of the opamp actually sits a bit lower than the output of the regulator. It simulates fine at 7VDC. Let me know how it works out. Edited by user Tuesday, May 18, 2010 5:20:04 AM(UTC)
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Hi Russ, OK no problem - I'm just waiting for the 500R pots to arrive and I'll report back.
Cheers - Ken
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Parts arrived today - Fitted the 500R pot to VR1 and I now have full control over the output current. Following the instructions, everything adjusted perfectly.
Also, I have had the Placid running for some time now and have seen no issues with the 7.1V across C1. I will carry out a few more tests and report back, but it looks like a 6V transformer will be fine for a 5V output.
Cheers - Ken
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Placid is running well - O/P transistors warm to touch, Voltage rock steady, Load current stable, Shunt current stable. Output noise is below what my 'scope can read! I'm happy.
Many thanks to Russ and Brian for giving us a great piece of DIY - Keep up the good work.
Best regards - Ken
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