LED indicator for CC mode on Harrison HP 6201B power supply - Page 1
Forum section: electronics / projects

author: enut11
author_rank: Super Contributor
timestamp: January 24, 2021, 07:25:54 pm
content: The old Harrison (HP) 6201B is still a very good power supply if somewhat lacking in features such as separate volt/amp meters and a visual (LED) indicator when in constant current mode. It comes with coaxial fine/coarse controls for voltage and current - nice!Additional modes of operation include:- Remote programming- Remote sensing- Series for higher voltage- Parallel for higher current- Master/Slave(s)

author: enut11
author_rank: Super Contributor
timestamp: January 24, 2021, 07:40:44 pm
content: There are no opamps in the 6201B power supply. For control it uses differential transistors in the same metal can (6 legged devices) presumably to assist with thermal tracking.I wanted to add an LED as a visual indicator to quickly show when the supply is in constant current (CC) mode.The circuit has an OR GATE summing point at the junction of CR3/CR4 which determines CV or CC operation based on which differential pair is dominant with the given output load.I inserted an LED in series with the CC control line - the upper differential pair in the circuit.

author: enut11
author_rank: Super Contributor
timestamp: January 24, 2021, 07:55:33 pm
content: The LED I used is an RGB multi-leg device. I used the RED pair of leads.It is 'sensitive' as it only requires 0.1mA at about 1.7v to start emitting light. In operation, at the voltage/current control threshold, it quickly emits full brightness with maximum LED current controlled by the impedance of the circuit.So, it seems to work as expected but it does rob the CC control circuit of about 1.7v at the summing point.I am interested in comments on this mod, especially if there is a downside that I am not aware of.enut11

author: blackdog
author_rank: Frequent Contributor
timestamp: January 24, 2021, 11:38:42 pm
content: Hi Bryan,I don't think the LED will be a big problem.You could always short CR4 for an extra 0.7V.Test the power supply both for small currents and also larger currents whether the current limitation is still working properly.Use one scope over the output to see if there is no extra generation phenomena .It is a good / nice power supply, low noise and a relatively low output capacitor of about 100uF.If you use the sense wires the Ri will usually be less than 0.001Ohm for the low frequencies.In normaall use, your connecting wiring will determine the Ri of your test environment if you do not use sense wires.With kind regards,Bram

author: gnuarm
author_rank: Super Contributor
timestamp: January 25, 2021, 03:36:06 am
content: I agree with Blackdog, the added voltage drop from the LED can be a problem in setting the point where current regulation takes over from voltage, but it's hard to say without seeing the entire circuit. I suppose the way the current limit is set is to short the output can adjust the reading on the meters? In that case it won't do much to subtract the extra voltage I think. Still, you can try getting some back by removing CR4, but also you can use another of the LEDs in the package (maybe the green one) in place of CR3. Then you will get a green light when voltage regulating switching to red when current regulating.If you want to try to get closer to original specs, you can leave the existing circuit as is and add two red LEDs with their anodes connected to the anodes of CR3 and CR4 with their cathodes connected to a resistor to the -6.2V rail. This will let the LEDs steer the current for indication without changing the voltage at the CR3 and CR4 summing point. The load resistors driving the diodes are 5 and 15 kohms, so you can't draw too much current without impacting the circuit.A way of doing this with zero impact is to add a comparator with inputs from the anodes of CR3 and CR4. The output can be connected to the LED and a resistor to +6.2V. A SOT-23 package, a resistor and an LED, that's pretty small and simple.

author: enut11
author_rank: Super Contributor
timestamp: January 25, 2021, 08:24:52 am
content: Thank you @gnuarm. The LEDs I have are sensitive enough to have both CC and CV indicators as you suggest without disturbing the circuit operation too much. Tying them to the -6.2v rail with resistors is an interesting idea.These LEDs are great in battery powered circuits due to the very low on-current. Eg using a 9v battery, the limiting resistor can be as high as 39Kohm!This is where I bought them:https://www.ebay.com.au/itm/50Pcs-LED-DIFFUSED-RGB-common-anode-4-PINS-F5-5MM-Super-Bright-Bulb-Lamp-AU/323887940893?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2648@Blackdog, I checked the PS output with a CRO at high and low CC operation. In both cases a very small amount of mains hum, about 3mV PP was impressed on the output when switching from CV to CC. That was with the LED in circuit. I have not checked it without.enut11

author: gnuarm
author_rank: Super Contributor
timestamp: January 25, 2021, 09:30:15 am
content: Quote from: enut11 on January 25, 2021, 08:24:52 amThe concern is not so much the current as the voltage drop. You can try measuring the voltage on two different colors at the same current to see how they vary. Then you could use two colors in the same LED. I've looked at data sheets for red, yellow and green LEDs which seem to be about the same voltage when they have max current through them, but at lower currents the voltage differs somewhat.Given the controls work in a feedback loop the voltage drop may not matter so much. It looks like the driven circuit uses Q17 as an emitter follower at a fixed voltage to act as a current sink, so the base of Q3 remains at a fixed voltage, 2.9V marked on the drawing. So maybe operation is current based. I'm not certain. It looks to me like driving current into node 17 (the "OR gate") must limit the flow of current out of the supply since either of the two control circuits can then limit the drive. Hmmm... I just realized that the sum of the current through the diodes do not flow into the base of Q3. Rather Q3 is a PNP so the base current flows out into the pulldown resistor R7. The OR gate current competes with the base current and so reduces it making Q3 collector voltage more negative. Interesting what people did before there were ICs to do everything.Quote from: enut11 on January 25, 2021, 08:24:52 amOk, but keep in mind R7 is a 61.9K resistor to -6.2V and you can't run too much current through your LEDs or the driving circuits may not work correctly.That's why I suggested a comparator. It would detect which diode was being driven harder and so which circuit is regulating the output without impacting the circuit in any way. There are enough power voltages present to power the comparator easily, ±6.2V are probably good candidates.

author: enut11
author_rank: Super Contributor
timestamp: January 25, 2021, 07:45:08 pm
content: QuoteI measured the voltage drop of the 3 internal LED colours at 0.5mARED: 1.73vGREEN: 2.40vBLUE: 2.54vRED and BLUE were more matched in brightness and the voltage difference might be useful to indicate either CC or CV operation in a circuit. I guess that would involve a voltage reference and an opamp.