Can Two LM334 current sources be used in SERIES? - Page 1
Forum section: electronics / projects

author: MisterHeadache
author_rank: Regular Contributor
timestamp: June 16, 2025, 07:09:43 pm
content: I've got a 60V application that needs a 1mA current source. The LM334 can source that current, but it cannot handle 60V across it, specs say 40V max. So... what about putting two in series? Each one would in theory see 30V, each one would need a 68 Ohm Rset resistor.Would it work? Would I need to put 36V zener diodes across each one, to make sure neither one sees over 40V during transient conditions?

author: IanB
author_rank: Super Contributor
timestamp: June 16, 2025, 07:16:21 pm
content: In general, you would put currents in parallel and voltages in series. Otherwise you would have a constraint on insufficient degrees of freedom.It sounds like you want to source 1 mA with a compliance voltage of 60 V or more?How about if you were to use a current mirror, where the mirrored side can handle the 60 V?

author: TimFox
author_rank: Super Contributor
timestamp: June 16, 2025, 07:23:58 pm
content: Two current sources in series will not split the “compliance” voltage equally, so almost all of it will appear across one of them, to no advantage for voltage rating.

author: MisterHeadache
author_rank: Regular Contributor
timestamp: June 16, 2025, 07:36:40 pm
content: Quote from: IanB on June 16, 2025, 07:16:21 pmYes, that's exactly what I want but the LM334 is only rated to 40V maximum. And a quick search of other single-chip current sources did not turn up anything available at a higher voltage. I suppose I could scratch build a current mirror if the series connection doesn't work like TimFox says.

author: TimFox
author_rank: Super Contributor
timestamp: June 16, 2025, 07:38:48 pm
content: If you are careful on the gate bias network, you can put a JFET or MOSFET in “cascade” to absorb the high voltage while maintaining the current.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: June 16, 2025, 07:52:13 pm
content: Plenty of cascode current source circuits and discussions in diyaudio dot com.Some actually operating from 250 V.

author: TimFox
author_rank: Super Contributor
timestamp: June 16, 2025, 07:55:26 pm
content: IXYS makes some depletion-mode (conducting at 0 V gate-source, normally operated at reverse gate-source bias like a vacuum tube) MOSFETs that simplify the gate bias question.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 16, 2025, 08:13:44 pm
content: Quote from: MisterHeadache on June 16, 2025, 07:09:43 pmHow stable do you need it to be? If a temperature coefficient of ~0.25%/C is acceptable (and I think the LM334 is about that stable anyway) , a single depletion MOSFET, like BSS139 or BSP129 with only one resistor around 1-1.5K between the source and the gate will work up to 200V.CheersAlex

author: Zero999
author_rank: Super Contributor
timestamp: June 16, 2025, 08:45:17 pm
content: Quote from: MisterHeadache on June 16, 2025, 07:36:40 pm

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 16, 2025, 08:57:36 pm
content: My version is somewhat simpler.CheersAlex

author: Zero999
author_rank: Super Contributor
timestamp: June 16, 2025, 09:04:57 pm
content: Quote from: Alex Nikitin on June 16, 2025, 08:57:36 pmYou just have to be lucky with the threshold voltage.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 16, 2025, 09:15:20 pm
content: Quote from: Zero999 on June 16, 2025, 09:04:57 pmYou just have to select the right value for the resistor. And if you get the selectedBSP129H6906orBSS139H6906version, you will have a narrow band for Vth.CheersAlex

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 16, 2025, 09:30:36 pm
content: Quote from: MisterHeadache on June 16, 2025, 07:09:43 pmWhat is the minimum voltage ?You could use a depletion mosfet to increase the voltage range.There are higher threshold fets designed for power droppers, eg DMZ1521 at Lcsc is -5-7v cutoff at 150v rating.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 16, 2025, 09:36:52 pm
content: Quote from: Alex Nikitin on June 16, 2025, 09:15:20 pmFor some values of 'narrow' ...That part specs -2.1 ~ -1vgs, with a typical of 1.4V so that's a 40% tolerance band.LM334 are 6% or 3%, with a defined tempco, which maybe the OP needs.A better Depletion choice for current regulation, with much tighter gate specs might be DMZ6005EHVGS(OFF) Gate-to-Source Cut-off Voltage-2.39 ~ -1.96 V(VDS =3V, ID=8μA)

author: Benta
author_rank: Super Contributor
timestamp: June 16, 2025, 09:40:23 pm
content: How 'bout using a cascode JFET like in Fig. 30 of the data sheet? The FET will take the brunt of the voltage.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 16, 2025, 10:01:00 pm
content: Quote from: PCB.Wiz on June 16, 2025, 09:36:52 pmTheH6906devices are selected in <20% Vth bands, so roughly the same in this respect. From my experience even "non-selected"H6327versions are selected anyway and the distribution of Vth from a single reel could be easily less than 20%.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 16, 2025, 10:04:23 pm
content: Quote from: Benta on June 16, 2025, 09:40:23 pmI would use the BSP129 or BSS139 with the gate to the resistor pin. Such a cascode would also remove a thermal modulation which can be a big problem without it.CheersAlex

author: David Hess
author_rank: Super Contributor
timestamp: June 16, 2025, 10:18:30 pm
content: Quote from: MisterHeadache on June 16, 2025, 07:09:43 pmEach LM334 needs to be clamped with a zener diode below its maximum voltage.Better would be to use an LM334 along with a depletion mode MOSFET to take a majority of the voltage.

author: TimFox
author_rank: Super Contributor
timestamp: June 16, 2025, 10:37:57 pm
content: If you don't need the current source to be compliant below, say 30 V, you could just put a 24 V or similar Zener diode in series, to limit the maximum voltage to 36 V (in that case) across the CC circuit.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 17, 2025, 03:21:08 am
content: Quote from: MisterHeadache on June 16, 2025, 07:09:43 pmAnother solution for 60V / 1mA is Linear LED drivers.eg the PJL8301 has a typical IQ of 72uA and a VSET of 600mV 4%

author: Zero999
author_rank: Super Contributor
timestamp: June 17, 2025, 07:30:23 am
content: Quote from: Benta on June 16, 2025, 09:40:23 pmBecause the original poster needs it to be rated to 60V and there aren't any >60V J-FETs nowadays, hence why I added the potential divider.Quote from: David Hess on June 16, 2025, 10:18:30 pmYes, a depletion mode MOSFET will work. Select one with a threshold voltage has to be below -1V, otherwise the LM334 won't have enough headroom to work.

author: gabiz_ro
author_rank: Regular Contributor
timestamp: June 17, 2025, 08:37:19 am
content: Use LR8 High Voltage Adjustable 3-Terminal Linear Regulator, 450Vhttps://www.microchip.com/en-us/product/lr8

author: MisterHeadache
author_rank: Regular Contributor
timestamp: June 17, 2025, 01:14:05 pm
content: Guys, thanks for the detailed replies and suggestions! Here are more details about the problem I am trying to solve:Application is for providing a minimum load to an LT3083 voltage regulator. Specs say it needs a minimum load current of 1mA.I'm thinking of using the LT3083 for my Heathkit IP-20 power supply project (see separate thread) and applying it using the 'High Voltage Regulator' reference circuit shown on page 20 of the data sheet.The current source would need to work with voltages from 2 volts to just under 60 volts (around 50-55V) because I'm trying to maintain the (near zero) to 50V output of the IP-20.Regarding accuracy: I'd say 1mA to 1.5mA would be fine.

author: David Hess
author_rank: Super Contributor
timestamp: June 17, 2025, 03:00:59 pm
content: I would use a bipolar transistor current sink rather than an LM334, and achieving high voltage with an LM334 will be difficult while working at 2 volts. Example shown below. Dropout voltage is about 0.9 volts.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 17, 2025, 10:36:48 pm
content: Quote from: MisterHeadache on June 17, 2025, 01:14:05 pmFor that min DC load use, you are more interested in getting close to zero, than precision,A simple current mirror using matched pair transistors can manage that easily. (or you can use emitter resistors if you do not have matched)eg a BCM56 is matched to 2mV and 80V, and a current mirror sims to a 10% drop from ~1mA at about 70mVSpice says that has a modest 3% current change over 0-70'C, which can be dropped to < 0.5% with a base resistor on the bias NPN.

author: Zero999
author_rank: Super Contributor
timestamp: June 18, 2025, 07:14:22 am
content: Quote from: PCB.Wiz on June 17, 2025, 10:36:48 pmThen you need something to bias current mirror with.Is the input fairly stable? If so, then an ordinary resistor will do.If not, then you'll need to replace R1 with a current source. It doesn't need as wide compliance voltage as the output, just cover the input voltage range.Perhaps something like this. V2 simulates the load. The current really starts dropping at 58.8V (200mV above 0V) but is probably still acceptable until 59.9V.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 18, 2025, 09:54:30 am
content: Yes, tho the OP does not need high current precision, and note the BC847B is only 45v rated, so a higher V matched pair is needed.

author: Zero999
author_rank: Super Contributor
timestamp: June 18, 2025, 09:08:16 pm
content: Just use David Hess' circuit, but with PNPs, rather than the LM385.

author: David Hess
author_rank: Super Contributor
timestamp: June 18, 2025, 10:41:03 pm
content: Quote from: Zero999 on June 18, 2025, 09:08:16 pmIf you are going to do that, then replace Q4 with a red LED which will temperature compensate the Vbe of Q3.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 18, 2025, 10:43:16 pm
content: Quote from: Zero999 on June 18, 2025, 09:08:16 pmPondering this some more, a classic current mirror has good saturation,but it is sensitive to thermal tracking.You can get matched parts, but mostly in small packages, and the OP's 1-1.5mA does not sound much, until you do that at 60V, when you can need 90mW.On the small packages that will skew the tracking, and worse, as the hot part heats up, it will draw more current.You can use emitter resistors, but that impacts the dropout.Using base resistors is similar and greatly reduces the critical VBE changes, there will still be some HFE changes with warming skews, but they are less severe.One compromise here is the saturation is now worse, (157mV vs 67mV at -10%) because the base is strictly current driven vs a simple current mirror that has a low impedance base drive.A benefit is now any HFE matched parts can be used, so larger packages are possible.Parts like PHPT610035NK 100V have good HFE match (5%) (no VBE spec) and the LFPAK56D has low thermal resistance.It is not 'low cost', but would be good thermally.All of which brings us back to LED drivers - parts like MEL7136B have 100mV sense, and drive external MOSFET for ~ 6c - just add one resistor and 2.7-18V supply.If you set that to 2mA, it looks like 50 ohms + FET RDS to GND below regulation, or 50mV for 1mA, better than NPN's and thermal matching issues vanish and (almost) any MOSFET can be used.The similar ME4315 has 70mV sense 4%

author: MisterHeadache
author_rank: Regular Contributor
timestamp: June 24, 2025, 02:16:55 am
content: Quote from: Zero999 on June 18, 2025, 09:08:16 pmThanks! I built this in LTSpice using the 2SD1782K in place of the BC546B and the complimentary 2SB1198K in place of the BC556B. Circuit works nicely, current does increase with increasing voltage but still well within what I need.I'm playing around with lowering R1 to get more current. Downside quickly becomes heat dissipation for Q2.Which parts would need to be matched - Q1 to Q2 and Q3 to Q4? And to help mitigate thermal tracking, should Q1 and Q2 be thermally coupled together?

author: ledtester
author_rank: Super Contributor
timestamp: June 24, 2025, 02:42:23 am
content: Quote from: MisterHeadache on June 24, 2025, 02:16:55 amOne option is to add another copy of the sink in parallel to increase the current.Or just adding another copy of Q2 in parallel with the original might work, eg. see section "11.4.1 Mirror Gain other than 1" on this page:https://wiki.analog.com/university/courses/electronics/text/chapter-11

author: Kleinstein
author_rank: Super Contributor
timestamp: June 24, 2025, 06:32:22 am
content: For a current mirror with quite some power disipation one would need very tight thermal coupling or emitter resistors. For the not so strict requirements one could even get away with a limited base current to set the collector current. This is however not very stable, as the gain usually goes up with temperature.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 24, 2025, 07:54:49 am
content: Quote from: MisterHeadache on June 24, 2025, 02:16:55 amIf you are only making one, I’d match a couple of TO220F npn’s and bolt them together.That has low dT due to the thermal mass.You can also add base resistors like shown in #29, as that has less effect on the knee voltage than adding emitter resistors.That also means you just need to roughly match HFE, and choose the higher gain one for the active sink.